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Controls on Water Quality in the

New Croton Reservoir/Turkey Mountain Watershed

Final Report for New York State Department of Transportation

March 2006

Controls on Water Quality in the New Croton Reservoir/Turkey Mountain Watershed

Final Report

March 2006

Charles T. Driscoll1, Donald W. Lake2, Shobha K. Bhatia1, Douglas DeKoskie3, James Buchanan3, Joel DuBois3 and Kimberley M. Driscoll1

1Department of Civil and Environmental Engineering

Syracuse University 151 Link Hall

Syracuse, NY 13244

2361 Funk Road Erieville, NY 13061

3Integrated River Solutions, Inc. 9 River Road

Ulster Park, NY 12487

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Table of Contents

List of Tables ...................................................................................................................... v List of Figures.................................................................................................................. viii Executive Summary.......................................................................................................... xii Introduction......................................................................................................................... 1 TASK 1: Development of a Geographic Information System............................................ 1

Location ......................................................................................................................... 1 Objectives and Methodology........................................................................................ 1 Description of Coverages in the Geographic Information System ........................... 2

Relief ........................................................................................................................... 2 Bedrock ....................................................................................................................... 2 Surficial Geology ........................................................................................................ 2 Steep Slopes ................................................................................................................ 3 Hydrography ............................................................................................................... 3 Land Use ..................................................................................................................... 3 Population................................................................................................................... 3 Transportation ............................................................................................................ 4 Soil Distribution.......................................................................................................... 4

Results and Discussion of Geographic Properties of the Turkey Mountain/Sawmill Creek Watershed.......................................................................... 4

TASK 2: Location of Sites of Environmental Degradation................................................ 6 Method ........................................................................................................................... 6 Data Summary .............................................................................................................. 7

TASK 3: Quantitative Stormwater Quality Modeling........................................................ 8 Water Quantity ............................................................................................................. 8

Background ................................................................................................................. 8 Approach..................................................................................................................... 8 Results ......................................................................................................................... 9

Water Quality................................................................................................................ 9 Background ................................................................................................................. 9 Approach................................................................................................................... 10 Results ....................................................................................................................... 10

TASK 4A. Water Quality Analysis .................................................................................. 12 Objectives..................................................................................................................... 12 Background: Dynamics of Major Non-point Source Pollutants............................. 12

Oxygen Demand........................................................................................................ 12 Dissolved Organic Carbon Dynamics ...................................................................... 12 Nutrients in Streams.................................................................................................. 13 Bacteria..................................................................................................................... 14

Field and Laboratory Methods.................................................................................. 14 Sampling Procedures ................................................................................................ 14 Analytical Procedures............................................................................................... 15 Hydrology and Annual Loads ................................................................................... 15

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Results .......................................................................................................................... 16 Hydrology ................................................................................................................. 16 Water Quality............................................................................................................ 16

Temperature, pH, Turbidity, DO, TSS ................................................................. 16 Nutrients................................................................................................................ 18 Bacteria ................................................................................................................. 20

Multivariate Statistical Analysis ............................................................................... 20 Factor Analysis ..................................................................................................... 20 Cluster Analysis .................................................................................................... 21

Watershed Loadings.................................................................................................. 21 Stormwater Events .................................................................................................... 22 Toxicity Testing ......................................................................................................... 23 Comparison of Surrounding Waters ......................................................................... 23

Summary...................................................................................................................... 23 TASK 4B: Habitat Analysis ............................................................................................. 25

Fishery Assessment ..................................................................................................... 25 Findings – Fishery Assessment ................................................................................. 25 Findings – Habitat Assessment ................................................................................. 26 Results and Discussion ............................................................................................. 27

Macroinvertebrate Assessment.................................................................................. 29 Methods..................................................................................................................... 29

Physical Habitat .................................................................................................... 29 Water Quality Analysis......................................................................................... 29 Macroinvertebrate Collection ............................................................................... 30 Laboratory Methods.............................................................................................. 30 Sorting Quality Assurance .................................................................................... 31 Sample Identification ............................................................................................ 32 QA/QC of Taxonomic Identifications .................................................................. 32

Biological Assessment ................................................................................................. 32 Results and Discussion ............................................................................................. 33

Physical Habitat .................................................................................................... 33 Water Quality Results ........................................................................................... 35

Conclusions .................................................................................................................. 36 TASK 5: Fluvial Geomorphic Inventories and Assessment............................................. 37

Watershed Characterization...................................................................................... 37 Valley Classification and Valley Slope ..................................................................... 38 Level I – Stream Classification ................................................................................. 40 Historical Trends and Aerial Photograph Interpretation......................................... 42 Summary of Watershed Characterization ................................................................. 44

Hydrology and Flow Regime Analysis ...................................................................... 45 Regional Regression Equations ................................................................................ 45 Bankfull Regional Curve Data.................................................................................. 46 Regional USGS Gage Stations Analysis ................................................................... 46

Bankfull Calibration.............................................................................................. 46 Timing and Magnitude of Major Flood Events .................................................... 47

Field Evaluations of Bankfull Indicators .................................................................. 47

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Hydraulic Modeling .................................................................................................. 48 Stream Assessment and Morphological Description ............................................... 48

Corridor Walkover Inventories................................................................................. 48 Bed Scour and Bank Erosion ................................................................................ 49 Riprap and Revetment........................................................................................... 49 Bedrock ................................................................................................................. 50 Natural and Man Made Grade Controls................................................................ 50 Debris Blockages .................................................................................................. 50 Stream Classification ............................................................................................ 50 Tributary Confluences .......................................................................................... 51 Monitoring Station................................................................................................ 51 Reference Reaches ................................................................................................ 51 Clay Exposures ..................................................................................................... 51 Notable Channel and Floodplain Conditions........................................................ 51

Level II Stream Classification................................................................................... 52 Level III Stream Classification ................................................................................. 54

Riparian Vegetation .............................................................................................. 55 Flow Regime......................................................................................................... 55 Stream Size and Order .......................................................................................... 55 Depositional Patterns ............................................................................................ 56 Meander Patterns .................................................................................................. 56 Debris and Channel Blockages ............................................................................. 57 Streambank Erosion Potential............................................................................... 57 Channel Stability Evaluation – Pfankuch ............................................................. 58 Altered Stream Channels ...................................................................................... 58 Channel Sediment ................................................................................................. 58

Erosion Assessment and Monitoring......................................................................... 59 Stream bank failure mechanisms .............................................................................. 59 Cross Section Monitoring ......................................................................................... 60

Bankfull Area........................................................................................................ 60 Bankfull Width...................................................................................................... 61 Bankfull Depth...................................................................................................... 61 Width to Depth Ratio ............................................................................................ 62 Entrenchment Ratio .............................................................................................. 62 Stream Classification ............................................................................................ 62 Channel Deposition............................................................................................... 62

Channel Erosion ....................................................................................................... 63 Stream Bank Erosion Modeling ................................................................................ 64

The Stepwise Multivariate Regression Model...................................................... 64 The Bank Stability and Toe Erosion Model ......................................................... 65

Stream Bank Erosion Prediction .............................................................................. 66 TASK 6: Mitigation and Restoration Plan........................................................................ 68

Stream Stability and Erosion ..................................................................................... 68 Unit 1 ........................................................................................................................ 69

Description............................................................................................................ 69 Recommendations................................................................................................. 70

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Unit 2 ........................................................................................................................ 70 Description............................................................................................................ 70 Recommendations................................................................................................. 72



Unit 5 ........................................................................................................................ 79 Description............................................................................................................ 79 Unit 5 - Upper Segment ........................................................................................ 80 Unit 5 – Lower Segment ....................................................................................... 81 Revetment Summary............................................................................................. 82 Erosion Summary.................................................................................................. 82 Upper Segment Recommendations....................................................................... 83 Lower Segment Recommendations ...................................................................... 84

Unit 6 ........................................................................................................................ 85 Description............................................................................................................ 85 Unit 6 - Upper Segment ........................................................................................ 87 Unit 6 - Middle Segment....................................................................................... 87 Unit 6 - Lower Segment........................................................................................ 89 Revetment Summary............................................................................................. 90 Erosion Summary.................................................................................................. 90 Upper Segment Recommendations (Station 86+45 to Station 90+80)................. 91 Middle Segment Recommendations (Station 90+80 to 98+45)............................ 92 Lower Segment Recommendations (Station 98+45 to 108+70)........................... 93

Unit 7 ........................................................................................................................ 93 Description............................................................................................................ 93 Recommendation .................................................................................................. 94

Stream Bank Erosion Modeling and Prediction ....................................................... 94 Water Quality............................................................................................................ 95

Unit 1 .................................................................................................................... 96 Unit 3 .................................................................................................................... 96 Unit 4 .................................................................................................................... 96 Unit 5 .................................................................................................................... 97 Unit 6 .................................................................................................................... 97 Unit 7 .................................................................................................................... 97

General Habitat Recommendations .......................................................................... 97 References......................................................................................................................... 99

TABLES..................................................................................................................... 108 FIGURES................................................................................................................... 136

Appendix Summary ........................................................................................................ 189

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List of Tables

Table 1. Data layers for the Turkey Mountain/Sawmill Creek Watershed………......... 109 Table 2. Soil units and their percentage composition in the watershed………….......... 110 Table 3. Environmental degradation features and the number of occurrences along Sawmill Creek……………………………………………………………………......... 111 Table 4. Summary of total length and percent of stream length where bank erosion and bank revetment occur along Sawmill Creek…………………………………......... 111 Table 5. Summary of parameters for the hydrologic sub-area in Sawmill Creek watershed…………………………………………………………………………......... 111 Table 6. Rainfall and peak discharges for 1, 2, 5, 10, 25, 50 and 100 year storm events……………………………………………………………………………........... 112 Table 7. A comparison of estimated discharges under flood frequency for Sawmill Creek in 1000 m3/day for this study and FEMA (1993)………………………............. 112 Table 8. Sawmill Creek Basin - sub-area hydrologic parameter summary………......... 112 Table 9. Sawmill Creek pollutant load summary - total suspended solids (in kg/yr)..... 113 Table 10. Sawmill Creek pollutant load summary - total phosphorus (in kg/yr)…........ 113 Table 11. Sawmill Creek pollutant load summary by major land use categories…....... 114 Table 12. Summary of analytical methods…………………………………………...... 115 Table 13. Mean temperatures for the wetlands sites (1, 2, 3) and freely draining sites (4, 5, 6)………………………………………………………………………........ 116 Table 14. Spearman Rank Order Correlation for pH and temperature measurements with the major solutes measured. Significant at p<0.05……………………………...... 116 Table 15. Summary statistics of concentrations of total nitrogen and nitrogen species in stream water by season. All units are in mg N /l……………………………………..... 117 Table 16. Spearman Rank Order correlations for total nitrogen and nitrogen species. (significant at p< 0.050)……………………………………………………………...... 117 Table 17. Summary statistics of total phosphorus and phosphorus species in stream water by season. All units are in µg/l………………………………………………...... 118

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Table 18. Spearman’s Ranked Correlation of total phosphorus and phosphorus species to the other water quality parameters………………………………………...... 118 Table 19. Principal factor scores……………………………………………………..... 119 Table 20. Comparison of total phosphorus and total suspended solids loss estimated for different locations in Sawmill Creek with values estimated using the Simple Method. Data are shown by hydrologic sub-areas (and water quality sampling sites) and for the entire watershed……………………………………………………………………...... 119 Table 21. Comparison of water quality for Sawmill Creek (this study), unnamed watershed (adjacent to Sawmill Creek; citation) and the New Croton Reservoir (after Effler et al. 2003)…………………………………………………………………….... 120 Table 22. Percent oxygen saturation of water at Sawmill Creek, sample sites from October 2002 to September 2003…………………………………………………….... 121 Table 23. Physical habitat features at macroinvertebrate sampling stations…………... 122 Table 24. Water quality data (10/18/02-9/25/04) with regard to macroinvertebrate sampling……………………………………………………………………………….. 123 Table 25. Valley segments determined by valley slope for Sawmill Creek…………... 124 Table 26. Percentage of undeveloped areas in Sawmill Creek watershed in 1974, 1985, and 2004……………………………………………………………………………...... 124 Table 27. Regression equations for hydrologic region 3 or Sawmill Creek at 1.5mi2 ... 124 Table 28. Full regression equations for Sawmill Creek at 1.5mi2, 1.0mi2, 0.5mi2 ….... 125 Table 29. Regional hydraulic geometry and bankfull discharge data for selected areas of New York and Eastern U.S…………………………………………………………..... 125 Table 30. Summary of site information for selected USGS gauging stations……….... 126 Table 31. Log Pearson – Type III Distribution of Gauging Sites……………………... 126 Table 32. Annual peak stream flow recorded at two regional USGS gauging stations.. 126 Table 33. Location and type of debris inventoried along Sawmill Creek……………... 127 Table 34. Rosgen Level II classification for 28 cross sections on Sawmill Creek……. 128 Table 35. BEHI scores for 18 banks along Sawmill Creek……………………………. 129

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Table 36. Pfankuch stability ratings for seven reaches along Sawmill Creek................ 129 Table 37. Particle size analysis for 34 locations along Sawmill Creek........................... 130 Table 38. Summary of failure mechanisms on the Sawmill Creek................................. 131 Table 39. Independent variables considered in the stepwise multi-variate regression model (SMRM)............................................................................................................... 131 Table 40. Engineering properties for different bank materials (Source: USDA-ARS 2003)............................................................................................ 132 Table 41. Critical shear stress and erodibility for bank, bank toe, and channel bed materials (Source: USDA-ARS 2003)............................................................................ 132 Table 42. Annual stream bank erosion estimated by the stepwise multi-variate regression model (SMRM) and the USDA-ARS Model (stationing 83+50 – 98+00)..................... 132 Table 43. Location and length of delineated units at Sawmill Creek............................. 133 Table 44. Stream types inventoried in Unit 5 at Sawmill Creek..................................... 133 Table 45. Summary of stream types in Unit 6 in Sawmill Creek................................... 134 Table 46. Summary of stormwater problems and potential mitigation for Sawmill Creek............................................................................................................................... 135

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List of Figures

Figure 1. Topographic map of Turkey Mountain Watershed.......................................... 137 Figure 2. Westchester County, showing drainage divides and Turkey Mountain Watershed. ...................................................................................................................... 138 Figure 3. Bedrock geology of Turkey Mountain Watershed.......................................... 139 Figure 4. Surficial geology of Turkey Mountain Watershed.......................................... 140 Figure 5. The distribution of steep slopes and a 100m stream buffer in Turkey Mountain Watershed....................................................................................................... 141 Figure 6. Hydrological features within Turkey Mountain Watershed............................ 142 Figure 7. Land use distribution of Turkey Mountain Watershed.................................... 143 Figure 8. Population density within Turkey Mountain Watershed................................. 144 Figure 9. Roads within Turkey Mountain Watershed..................................................... 145 Figure 10. Distribution of soil units within Turkey Mountain Watershed...................... 146 Figure 11. Hydrography of Turkey Mountain Watershed showing sampling sites and road networks.................................................................................................................. 147 Figure 12. 30m buffer around Sawmill Creek. Shown are Route 118 and stream sampling sites.................................................................................................................. 148 Figure 13. Distribution of major land-use class within Turkey Mountain Watershed.... 149 Figure 14. Proportion of land use of Turkey Mountain Watershed in and near 100m buffer around wetlands.................................................................................................... 150 Figure 15. Critical undeveloped and residential areas within Turkey Mountain Watershed, including: a) underdeveloped areas that are on steep slopes, and b) residential areas that are on steep slopes........................................................................................................... 151 Figure 16. Estimated of mean daily discharge values for Sawmill Creek predicted from Angle Fly Brook using the proportional area discharge relationship. Sample collection dates and shown. Monthly water samples were collected under a range of flow conditions........................................................................................................................ 152

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Figure 17. Variation of selected surface water characteristics of Sawmill Creek. Temporal patterns (a) are the mean value monthly of the six sampling sites. Spatial patterns (b) are illustrated by the mean of monthly observations for each of the longitudinal sites........ 153 Figure 18. Spatial variation of dissolved oxygen concentrations at Sawmill Creek. Shown are mean and standard deviation based on monthly collections. The NYS water quality standard for dissolved oxygen is 5 mg/L........................................................................ 154 Figure 19. Temporal variation of dissolved oxygen deficit for Sawmill Creek. Shown are mean and standard deviation of the study sites......................................................... 155 Figure 20. Seasonal dissolved oxygen and temperature variations for Sawmill Creek. Shown are mean and standard deviation of sampling sites. The NYS standard for dissolved oxygen is 5 mg/L............................................................................................. 156 Figure 21. Spatial variation of dissolved oxygen deficit for sampling sites along Sawmill Creek. Mean and standard deviation are shown............................................... 157 Figure 22. Concentrations of total suspended solids (TSS) as a function of turbidity for the sampling sites along Sawmill Creek......................................................................... 158 Figure 23. Total suspended solids as a function of turbidity in stream water of Sawmill Creek with values shown by season................................................................................ 159 Figure 24. Temporal and spatial patterns of dissolved organic carbon (DOC) in Sawmill Creek. Mean and standard deviation of study sites are shown....................................... 160 Figure 25. Temperature and dissolved organic carbon variation over the study period in Sawmill Creek. Mean and standard deviation of study sites are shown......................... 161 Figure 26. Mean concentrations (and standard deviation) of total nitrogen (TN) and its species for all samples collected at Sawmill Creek......................................................... 162 Figure 27. Proportion of nitrogen species for sampling sites of Sawmill Creek............ 163 Figure 28. Seasonal variation of the means and standard deviations of total nitrogen and nitrogen species for samples collected at Sawmill Creek............................................... 164 Figure 29. Monthly mean and standard deviations of total nitrogen concentrations at sampling sites of Sawmill Creek..................................................................................... 165 Figure 30. Spatial variation of mean and standard deviation for total nitrogen concentrations for sampling sites at Sawmill Creek....................................................... 166

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Figure 31. Monthly variation of mean and standard deviation for total phosphorus concentrations of stream sites in Sawmill Creek. Note that there is a New York Department of Environmental Protection advisory level of 20 µg/L.............................. 167 Figure 32. Mean total phosphorus and standard deviation by site. Values represent mean of monthly samples. Note that there is a New York Department of Environmental Protection advisory level of 20 µg/L............................................................................... 168 Figure 33. Mean and standard deviation of total phosphorus and phosphorus species seasonally. SRP is soluble reactive phosphorus. DOP is dissolved organic phosphorus. PP is particulate phosphorus................................................................................................. 169 Figure 34. Distribution of parameters in the plane defined by principal components 1 and 2............................................................................................................................. 170 Figure 35. Linkage diagram showing similarities in water chemistry between sampling sites.................................................................................................................................. 171 Figure 36. Estimated monthly loads of total phosphorus, total nitrogen and total suspended solids (TSS) transported from Turkey Mountain Watershed. Estimated flow is also presented as a reference. ......................................................................................... 172 Figure 37. Flow, turbidity, total suspended solids (TSS), dissolved organic carbon (DOC), total phosphorus (TP) and nitrogen species of upper Sawmill Creek site (site 3) during January storm event............................................................................................. 173 Figure 38. Flow, turbidity, total suspended solids (TSS), dissolved organic carbon (DOC), total phosphorus (TP) and nitrogen species of lower Sawmill Creek site (site 5) during January storm event............................................................................................. 174 Figure 39. Flow, turbidity, total suspended solids (TSS), dissolved organic carbon (DOC), total phosphorus (TP) and nitrogen species of the upper site (site 3) of Sawmill Creek during October storm event.................................................................................. 175 Figure 40. Location of habitat analysis sampling locations in Sawmill Creek............... 176 Figure 41. Water quality rating for Sawmill Creek sites................................................ 177 Figure 42. Hilsenhoff Biotic Index values in Sawmill Creek......................................... 177 Figure 43. Taxa richness in Sawmill Creek.................................................................... 178 Figure 44. EPT taxa richness in Sawmill Creek............................................................. 178 Figure 45. Percent model affinity in Sawmill Creek...................................................... 179

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Figure 46. Mean community similarity values in Sawmill Creek.................................. 180 Figure 47. Bank height measurements along eroded banks in Sawmill Creek............... 181 Figure 48. Bank length measurements along eroded banks in Sawmill Creek............... 182 Figure 49. Length and location of stabilized streambanks along Sawmill Creek........... 183 Figure 50. Length and location of stabilized and eroding streambanks along Sawmill Creek............................................................................................................................... 184 Figure 51. The failure mechanisms of the stream bank in the Turkey Mountain Watershed........................................................................................................................ 185 Figure 52a. Active bank erosion near cross-section 10 (St. 97+00) Picture taken in 2004 (notice the tree was on the stream bank)...................................... 186 Figure 52b: Active bank erosion near cross-section 10 (St. 97+00) Picture taken in 2005 (notice the tree fell into the stream channel)................................ 186 Figure 53: Erosion at cross-section 10.2 (a) and cross-section 12 (b) during 2004-2005 monitoring season........................................................................................................... 187 Figure 54. Estimated daily mean discharge for Sawmill Creek (1/1/2004–5/30/2005).. 188

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Executive Summary

There is considerable interest and concern over the water quality and stream bank stability of Sawmill Creek, near Yorktown Heights in Westchester County, NY. Sawmill Creek drains the Turkey Mountain Watershed (41o 15’ N, 73o 48’ W) and is a tributary to the New Croton Reservoir, a water supply for New York City. The project team established six research objectives for this study. The first objective was to develop a geographic information system (GIS) of relevant data layers, including land-use and hydrography. The second objective was to develop GIS coverages for existing sites of environmental degradation from field observations. The third objective was to apply a storm water model to the Turkey Mountain Watershed to obtain stormwater discharges for Sawmill Creek. The fourth objective was to collect field data to support the watershed study, including the collection of water quality data and the characterization of aquatic habitat. The fifth objective was to conduct a fluvial geomorphic analysis to assess the stream stability of the Creek. Finally, the project team developed a prioritized storm water and stream stability mitigation and habitat restoration plan to address the appropriate control of non-point source pollutants and improvement of habitat quality in the Turkey Mountain Watershed. We established six tasks to accomplish these project objectives.

Our research team used an integrated approach of: 1) GIS, 2) field studies of water quality under baseflow and during hydrologic events, fluvial geomorphic conditions, and analysis of aquatic habitat, and 3) storm water modeling to quantify existing conditions within the Turkey Mountain Watershed. We acquired relevant data layers, including a digital elevation model, land-use and hydrography (Task 1) and developed site-specific data layers (Task 2) that comprise a GIS for Turkey Mountain Watershed. The development of a GIS facilitated the selection of sites within the watershed for water quality sampling, the interpretation of water quality data and application of the storm water model. We quantified storm discharge and nutrient and solids loss from the watershed (Task 3). Stormwater discharges for the 1-year to the 100-year frequency rainfall events were calculated for the Sawmill Creek watershed using the USDA-NRCS WIN TR-20 computer model. These discharges were then incorporated into the USACOE HEC-RAS computer model to generate water surface profiles and stream segment velocities for use to evaluate potential streambank erosion and flooding. Our results showed peak discharges at the downstream end of the study to be 592.7 thousand m3/d for a 10-year storm and 1,200 thousand m3/d for a 100-year storm. Overall, the discharges computed in this study averaged 32 % higher than the discharges calculated as part of a FEMA Flood Insurance Study conducted in August 1993. This difference is largely due to land use changes over past 20+ years, the use of regression analysis in the FEMA study, and the physical limit of that study which only included detailed data from the outlet upstream to Locke Avenue. An evaluation of the Sawmill Creek Watershed was conducted using the Simple Method to assess and prioritize potential pollutant loads and their respective sources. The results of this analysis showed that the largest contribution of total suspended solids originates with residential land use; almost double that from commercial/industrial areas and roads and highways. Residential land use is also responsible for over 56 % of the load calculated for total phosphorus. Based on this analysis, stormwater outlets and the infrastructure in the watershed were

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located, inspected and evaluated for potential retrofit and/or incorporation of stormwater management practices to reduce pollutant discharges by employing source controls. Thirty-eight storm outfall locations were identified in the watershed. A table summarizing the recommendations for these locations is included in the mitigation section of this report.

We established and implemented a program to monitor water quality that enabled us to estimate nutrient and solids loads within the watershed and to the New Croton Reservoir (Task 4a). Starting in October 2002 water samples were collected at six sites along Sawmill Creek monthly for two years. These samples were analyzed for major solutes, species of nutrients (i.e., nitrogen, phosphorus), suspended solids, and coliform. These data demonstrate marked temporal and spatial patterns in water quality. The water quality of Sawmill Creek is typical of a stream draining an urban and suburban area. Concentrations of nutrients and fecal coliform were elevated and during summer, dissolved oxygen concentrations were low in the upper reaches of the creek. The temporal patterns varied from analyte to analyte. However, the spatial patterns generally indicated a decrease in concentration from the headwater sites adjacent to Yorktown Heights to the New Croton Reservoir. Concentrations of water quality constituents observed for Sawmill Creek were comparable to values recently observed for other tributaries to the New Croton Reservoir. Discharge data for Sawmill Creek were estimated using the Discharge–Area Ratio Method. Angle Fly Brook watershed (41°16' N, 73°43 W) was used as the index watershed (~ 4.4 km (2.7 mi) from Sawmill Creek). This information, in turn, enabled us to determine site-specific areal pollutant loads within and from the watershed. Our watershed specific nutrient and solid loads for Turkey Mountain watershed showed elevated values at the headwaters, with values decreasing somewhat with increasing drainage area to the New Croton Reservoir. Our watershed specific suspended solids and nutrients were considerably greater than values estimated for runoff from impervious surfaces using the Simple Method (Task 3) but were well within the range of values reported for urban and suburban watersheds in the literature. As part of this study we collected water samples before, during and following selected storm events. The storm sampling revealed order of magnitude increases in suspended solids and turbidity shortly on the rising limb of the hydrograph. These conditions did not persist for the duration of the event, but rather decreased shortly after this initial pulse. In contrast, total phosphorus, dissolved organic carbon and dissolved organic nitrogen concentrations increased by a factor of about two during events studied and elevated concentrations persisted for the duration of the event. In contrast, concentrations of nitrate decreased in response to high flow conditions. As part of this task we sampled sediments in the headwaters of Sawmill Creek (site 1) and at the base of the watershed (site 6). These sediments were tested for toxicity using the Microtox procedure. Although sediments adjacent to drainage from Yorktown Heights exhibited greater toxicity than those in the lower reach, neither site had values greater than values reported in the literature thought not to exhibit any toxic response for this bioassay procedure. Discharge of elevated loads of phosphorus, dissolved organic carbon, suspended matter and coliform have important implications for the water quality management of the New Croton Reservoir.

Fishery and macroinvertebrates sampling were performed to characterize the aquatic habitat of the Sawmill Creek (Task 4b). Based on the initial physical assessment

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of the watershed, the project team selected two locations for sampling macroinvertebrates and fish populations. One location was selected based on observable features, which indicate a relatively stable form, and the other based on observable signs of degradation from upstream reaches including significant erosion and multiple stormwater and drainage outfalls. Both sites were subjected to macroinvertebrate sampling, physical stream habitat survey measurements, and a fishery survey done with a battery-powered backpack electroshocker. Data were subjected to statistical review to determine how closely the samples represented the population in the sample sites. Although physical habitat is generally similar at both sites, the macroinvertebrate and fish communities were different. This pattern indicated there were potential differences in water quality that may be driving the biological variation between sites. Population estimates of the two most abundant fish species in the collections, standardized by number of fish per unit area, differed greatly between the two sites, as did the biomass estimates for the two species.

Multiple fluvial-geomorphic assessments were conducted between November 2003 and May 2005 to evaluate the physical condition of Sawmill Creek (Task 5). Initial field observations, inventories, and measurements taken in 2003 were used to determine the location, magnitude, and extent of instabilities as well as provide the location for more detailed assessment and monitoring. These locations and a number of other relevant corridor features were collected using global positioning system (GPS) receivers. GPS inventories were performed in 2004 and 2005 and the data were used in the spatial analysis of stream features. Based on Rosgen Level I (1996) stream channel classification, 42% of the stream lengths are classified as C- type, 19% as G-type, 18% as B-type, 16% as E-type, and 5% as F-type. Rosgen (1996) Level II analyses confirmed and refined reach classifications incorporating channel and floodplain geometry and channel sediment characteristics. Rosgen (1996) Level III classifications performed at seven reaches provided detailed characterization of Sawmill Creek. Additionally, detailed monitoring was conducted at 35 cross sections that confirmed the type and rate of stream channel and bank erosion processes. The integrated results of these assessments delineated Sawmill Creek into seven units that provided the framework for the development of the proposed mitigation restoration plan component of this study. The stream bank erosion assessment was conducted along Sawmill Creek from Yorktown Heights to the confluence of New Croton Reservoir. The assessment consisted of a site walkover and cross-section monitoring of 35 sections. During the walkover assessment, all eroding stream banks along the Creek were identified and the locations of the banks were recorded using GPS equipment. The length and height of the eroding banks were measured and the exposed areas were calculated. Monitored cross-sections were used to observe changes in the stream reach morphology, verify stream conditions, and develop stream bank erosion rates.

The primary failure mechanisms along Sawmill Creek were planar failures, although areas with cantilever, rotational, and piping failures were also identified. The bank materials of the failed banks consisted of considerable amounts of clays and silts. Although most failed banks were local and small in scale, for example, the heights of 90% of the failed banks were lower than 5 m (16.4 ft) in height, there were several significant bank failures in the lower part on the stream reach where the failed banks were more than 10 m (32.8 ft) in height and 50 m (164 ft) in length.

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To predict the stream bank erosion of Sawmill Creek, two different models were used. The first model was the Stepwise Multivariate Regression Model (SMRM) developed by Chen (2005) and the second model was the Bank Stability and Toe Erosion Model (BSTEM) developed by the United States Department of Agriculture, Agriculture Research Service (USDA-ARS 2003). The independent variables needed for the models were collected from various sources such as cross-sectional surveys, walkovers, and other public databases. The estimated soil erosion using BOTH the SMRM and USDA-ARS models was close to the measured average erosion. Based on these results, stream bank erosion prediction can help analyze the physical conditions of a watershed, and thus prioritize the mitigation methods to effectively manage the watershed.

Approximately 20% of the stream length in Sawmill Creek currently contains exposed and/or actively eroding stream banks. These unstable stream reaches are located throughout the watershed, but are more concentrated within specific segments of the stream. In contrast to typical urban watersheds, several factors have helped to minimize the extent of channel erosion and related stream impacts in Sawmill Creek. These factors include the relatively large wetland areas located within the upper watershed, significant undeveloped areas and a wooded riparian corridor. Active floodplains also serve to protect downstream areas from increased sediment loading by providing opportunities to redistribute the sediment loads generated during large storm events. Notable contrasts were observed throughout Sawmill Creek where the loss of either rooting stability or floodplains has resulted in considerably degraded conditions.

Recommendations were developed for the seven units of Sawmill Creek, though significant units include two physical stable reference segments, and two physically impaired units (Task 6). The reference units located in the upper watershed consisted of wetland complexes, exhibiting a general enhanced physical condition, where preservation was recommended. Evaluation of the two impaired units located in the middle of the watershed revealed increased rates of erosion and stream channel process, exacerbated by high bank erosion potential and mass wasting, and risk of private and public infrastructure. Restoration or stabilization were prescribed for these units.

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Introduction

There is considerable interest and concern over the water quality and stream bank stability of Sawmill Creek, which drains from Yorktown Heights in Westchester County, NY. Sawmill Creek is in Turkey Mountain Watershed (41o 15’ N, 73o 48’ W) and a tributary to the New Croton Reservoir, a water supply for New York City. The project team established six research objectives for this study. First, we developed a geographic information system (GIS) of relevant data layers, including land-use and hydrography. Second, from field observations GIS coverages were developed for existing sites of environmental degradation. The third objective was to apply a storm water model to Turkey Mountain Watershed. The fourth objective was to collect field data to support the watershed study, including the collection of water quality data and the characterization of aquatic habitat. Fifth, an assessment of fluvial geomorphic stability of Sawmill Creek was conducted. Finally the project team developed a prioritized storm water mitigation and habitat restoration plan to address the appropriate control of non-point source pollutants and improvement of habitat quality in the Turkey Mountain Watershed. We established six tasks to accomplish these project objectives. The following is the final report for this study.

TASK 1: Development of a Geographic Information System

Location The Turkey Mountain Watershed is a small watershed located within the New

Croton Watershed in Westchester County, New York. The New Croton Watershed covers 458 km2, and represents 39% of the total area of Westchester County. Turkey Mountain Watershed accounts for less than one percent of the total area (0.8%) of the New Croton Watershed. Sawmill Creek, which drains Turkey Mountain watershed, (41o 15’ N, 73o 48’ W) is a tributary to the New Croton Reservoir. Sawmill Creek originates from the town of Yorktown Heights and flows southwards into the reservoir. Objectives and Methodology

A geographic information system (GIS) was developed as a tool for analyzing and

interpreting the physical and chemical processes relevant to areas of environmental degradation occurring in the Turkey Mountain watershed. The GIS includes coverages representing land-use, soils, bedrock, watershed delineation, hydrography, study sampling points, slope, surficial geology, population, transportation network, and relief.

The coverages for this task were obtained from government and commercial sources, for which there is a specific interest in the watershed. Coverages were also developed by the project team for this study (see Task 2). The largest source of GIS data was the Westchester County GIS database (www.Westchestergov.com). Westchester County provides an online interactive data management tool for producing and downloading data for use in spatial analysis. The New York State GIS Clearinghouse (http://www.nysgis.state.ny.us/) and the Cornell University Geospatial Information Repository (http://cugir.mannlib.cornell.edu/) were other important sources of geographic

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data for the Turkey Mountain/Sawmill Creek GIS/dataset. These databases are a compilation of geographic data from other organizations (e.g. USGS, STATSGO, U.S. Census Bureau). In addition to these online sources, the town of Yorktown Heights provided the current topographic map of the Sawmill Creek Watershed and the 2000 Orthographic Images of the area. Description of Coverages in the Geographic Information System

The GIS for the Turkey Mountain Watershed includes eighteen relevant data layers. The combination of these different geographic covers allowed us to interpret physical, chemical and biological processes in the watershed (Table 1). Relief

Turkey Mountain Watershed is elongated and covers an area of 4.2 km2 (Figure 1). The northern portion of the watershed is in the town of Yorktown Heights and the southern portion is in the New Croton Reservoir Valley. On the west side of the watershed is Turkey Mountain, the highest elevation within the watershed (250 m above sea level). The lowest point of the watershed is at the mouth of Sawmill Creek (60 m above sea level). Sawmill Creek is a small stream with a length of 1.4 km from source to mouth. Its source area is the town of Yorktown Heights and it flows through the watershed as the arterial drainage mechanism for the basin. The Turkey Mountain/Sawmill Creek Watershed drains into the New Croton Reservoir and is a sub-watershed of the larger New Croton Watershed (Figure 2). Bedrock

The bedrock geology of Turkey Mountain Watershed is largely the Manhattan Formation (Figure 3). This bedrock material is mainly of schistose gneiss with local interlayer of amphibolites and marble. This formation covers 3.70 km2 in area (i.e. about 95% of the watershed area). The only other bedrock type, Inwood Marble, is located in the northern portion of the watershed.

Surficial Geology

Within the watershed there are three classes of surficial geological materials: outwash sand and gravel (og), glacial till (t), bedrock (r; Figure 4). Outwash sand and gravel (og) is coarse to fine gravel with sand, of pro-glacial fluvial deposition, well rounded and stratified and could be 2-20 m in thickness. Glacial till (t) is the most common surface geological material. It is relatively impermeable (loamy matrix) with variable clay content. Glacial tills range from abundant, well-rounded diverse lithologies in valley tills to relatively angular, more limited lithologies in the uplands. Glacial till tends to be sandy in areas underlain by gneiss or sandstone, which creates potential land instability on steep slopes. It has a variable thickness. Exposed bedrock (r) is generally within one meter of surface and is found on the top of Turkey Mountain. The watershed is approximately 13% outwash sand gravel, 83% till and 4% bedrock.

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Steep Slopes

The slopes of the watershed were grouped into three categories, below 15%, 15-25%, greater than 25% (Figure 5). Slopes greater than 15% are considered steep slopes. Steep slopes make up 35% of the land area (1.41 km2). Turkey Mountain has the steepest slopes within the watershed (i.e. greater than 25%). Ten percent of the area within the watershed has slopes greater than 25%. Another location of steep slopes is at the southern end of the basin, along the banks of the stream. These steep slopes intersect with the stream channel and are critical environmental locations, with respect to landslides and housing developments. Twenty-five percent of the watershed contains slopes between 15 and 25 percent. The remainder of the watershed has slopes below 15%.

Hydrography

There are a number of hydrological features within the watershed, including Sawmill Creek, ponds/small lakes, wetlands, flood zone and an aquifer (Figure 6). These hydrologic features exhibit a pattern whereby the flood zone, which covers an area of 0.375 km2, transects the basin from the upper reaches in the north to the south at the base of Sawmill Creek. Wetlands are evident in the upper reaches, in two different unconnected locations that cover 0.16 km2. The wetlands and small ponds are located in the flood zone. An aquifer (0.56 km2) is located in the northern portion of the watershed which together with the wetland provides much of the source water for the watershed.

Land Use

The northern portion of the watershed is developed with respect to commerce and industry. Within the manufacturing zone there are two hazardous waste sites (Figure 7). A large portion of the land area is residential housing. Parks, open spaces, and recreational areas are also a substantial portion of the watershed; parks and recreational areas alone account for 20% of the total land area of the watershed. A private recreational area (i.e, golf course) is found in the southern portion of the watershed. About 19% of Turkey Mountain Watershed is undeveloped. These underdeveloped areas are largely in the western portion of the watershed. Agriculture is not evident within the watershed.

Population

The population within the watershed is moderate and unevenly distributed (Figure 8). The population distribution coverage is described as very low density, low density, and mixed density. The bulk of the population resides in the eastern part of the basin. Most of the population within the watershed is immediately south of the village of Yorktown Heights. The watershed has approximately 1800 inhabitants. The population is represented with respect to total population according to U.S. Census Bureau in 2000.

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Transportation

There are few roads in the western portion of the watershed. The road network follows the general pattern of the population coverage and most of the roads are found on the eastside of the basin. The one major road is Sawmill River Road (NYS Route 118; Figure 9). This road runs through the basin from the town of Yorktown Heights southwards, and parallel to Sawmill Creek. Sawmill River Road merges with I-129 that runs perpendicular to the base of the watershed. All the other roads within the watershed simply link the housing developments to Sawmill River Road. There is a total of 21 km of roads within the watershed.

Soil Distribution

The soils dataset is from the digital survey of Putman and Westchester Counties, published by the National Cooperative Soil Survey, a joint effort of the United States Department of Agriculture in cooperation with Cornell University Agricultural Experimentation Station (1994). Soil map units by symbol, name and percentage for the Turkey Mountain Watershed are shown in Figure 10 and are listed in Table 2. There are 28 different soil map units within Turkey Mountain Watershed. The Charlton Chatfield complex accounts for about a third of the area and its properties affect the watershed drainage. These soils are very deep, well–drained and medium textured. They have permeability of 0.0-6.0 in/hr throughout their profiles, thus resulting in moderate available water capacity property. Unlike the Charlton soil, the Chatfield soils have a low available water capacity. Since these soils are deep and well drained they limit water available for surface runoff. Most of the soil units have a moderate rate of surface runoff, especially those along the stream and flood zone region. These soil units will have an effect on the quality of the stream waters, as well as on stream flow. Results and Discussion of Geographic Properties of the Turkey Mountain/Sawmill Creek Watershed Six water-quality monitoring sites were established for this study on Sawmill creek, located along the direction of flow (Site 1 is at the headwaters adjacent to Yorktown Heights and Site 6 at the base of the watershed prior to discharge to the New Croton Reservoir; Figure 11; see Task 4a). The sampling sites have a close proximity to Route 118 (Sawmill Creek Road). Due to continuous vehicle traffic along the road, chemicals, gasoline, antifreeze, and road salt applications during winter (Mehaffey et al. 1999) are deposited on Route 118. Some of these chemicals associated with road use may ultimately be transported to Sawmill Creek; making Route 118 a potential source of contamination that would influence water quality. The potential of Route 118 as a source of contamination was observed in the ratio of total length of roads in meters within a 30m buffer, this index is reported as length of road in meters per kilometer of stream length within the basin. For every 1.0 km of river distance within the 30m buffer around Sawmill Creek there was 550m of road length within the buffer (Figure 12).

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Turkey Mountain watershed has been extensively developed with 65% (Figure 13) of the watershed area as urban land (i.e., residential, commercial). The northern and western portions of the watershed are the most highly developed areas in the watershed. The eastern portion is the main residential quarters and the northern portion is mixed with commercial and industrial activities (Figure 13). The amount of road networks (Figure 11) coincides with the developed areas within the watershed. The topography in the watershed is largely responsible for this development pattern. The steeper slopes are found on the western side of the watershed (and Sawmill Creek) and on the southeastern portion of the watershed. The critical land use features are at a close proximity to the wetlands, especially the wetland in the upper reach (Figure 14). Urban land (residential and commercial) constitutes a significant proportion of the buffer area around the wetlands in the upper reach of the watershed. Therefore site 1 was expected to show distinct water chemistry characteristics than sites 2 and 3 which are within the wetlands in the middle reach with almost 80% of the land use near the buffer around the wetland classed as undeveloped or forested. Undeveloped land and open spaces are generally in the western section of the watershed where the topography is predominantly steep (>15%) slopes. Critical environmental areas that are on steep slopes are shown on Figures 15 a,b. The proportion of the watershed identified as critical underdeveloped and open space (Figure 15a) is approximately equal to the underdeveloped and open space areas combined within the basin. Some residential units are located on slopes greater than 15% (Figure 15b). Approximately 13% of the basin area was categorized as “critical developed areas” and a significant proportion of these critical residential areas are located in the southern end of the basin. The geographic information database described in Task 1 is provided in Appendix 1.1. Appendix 1.1 Sawmill Creek Geographic Information Database

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TASK 2: Location of Sites of Environmental Degradation

The objective of Task 2 was to locate the existing sites of environmental degradation using Global Positioning System (GPS) equipment, and incorporate these findings into the Geographic Information System (GIS). Sites of environmental degradation, as collected as part of this assessment, included the following features:

• Aggradation/Degradation (incision/deposition), • Bank Erosion, • Bank Revetment (riprap, stacked rock walls, etc.), • Bedrock, • Grade controls (natural, manmade), • Stream crossings (bridge and culvert), • Direct stormwater/culvert outfalls, • Debris blockages, • Changes in stream type (Rosgen Classification), • Confluences with tributaries, • Monitoring stations, • Reference areas, • Clay exposures, and • Channel and floodplain conditions.

The inventory was conducted along the main stem of Sawmill Creek from

Yorktown Heights to the New Croton Reservoir. Additionally, the inventory was used as a reconnaissance by the study team to become more familiar with the physical character, degree of degradation, and scale of the corridor. Sites for aquatic sampling, as well as the locations for more detailed inventories, were selected based on the results of this initial reconnaissance. Method

Preparation for the assessment included the development of a data dictionary to streamline the GPS data collection procedure. The dictionary was developed to apply line, point, and area measurements to the specific features. Attribute data was generated to assist describing and quantifying the respective characteristics of each feature. Generic features were also developed in order to capture unexpected physical features of the corridor. A summary of the features and attributes is included in Appendix 2.1.

Trimble GEOEXPLORER 3c handheld GPS receivers were employed for the inventory initiated in November of 2003. A two-person field team utilized the GPS equipment to locate the positions of each observed feature. The units provided one-meter horizontal accuracy for individual positions. In addition, digital photographs and written field notes were recorded at each feature. The images were collected and indexed in a numbered serial file system, for correlated to the GPS positions. Field notes were taken to supplement description of the GPS features.

The data were processed using differential correction of the raw GPS field data. The feature positions and attributes were exported to the GIS and edited to reduce any

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significant error. The data were projected into the New York State Plane coordinate system (NY State Plane, NAD 83, East Zone, Ft) and combined with various base maps for further assessment and presentation.

Two additional inventories were conducted in May 2004 and May 2005, utilizing Trimble Pro XR and Trimble GEOEXPLORER 3c handheld GPS receivers respectively. These inventories were performed to provide comparative data for spatial and temporal trend analysis and included with more detailed assessments in Task 5. Data Summary A summary table of the collected data is presented in Table 3 and 4. The spatial location and frequency of sites of environmental degradation have been included in the GIS database and are represented in Appendix 2.2. A summary of the collected data, inventory field notes and photographs have been included in Appendix 2.3. A detailed comparative analysis is presented and discussed within Task 5 – Fluvial Geomorphic Inventories and Assessment. Appendix 2.1 GPS Data Dictionary Appendix 2.2 Map of Environmental Degradation Appendix 2.3 Walkover Notes 2003

Walkover Notes 2004 2004 Photographs Walkover Notes 2005

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TASK 3: Quantitative Stormwater Quality Modeling

Water Quantity Background

For the purposes of this planning task the watershed was subdivided into four smaller catchments for the hydrologic stormwater quality modeling. This approach provides closer inspection and evaluation of the hydrologic parameters in the basin. The watershed has all four hydrologic soil groups, however “B” and “C” soils are dominant. The land use in the watershed is widely varied (see Task 1). Hydrologic sub-area 1 (37.8 ha), located in the upper part of the watershed in the Village of Yorktown Heights, is predominantly residential of mixed density with an about 19% commercial area near its outlet at Underhill Avenue. Hydrologic sub-area 2 (154.3 ha) is comprised of mainly residential land use on the southeast section of the village. It also includes the industrial section along Front Street and approximately 28 acres of wetland. The outlet for hydrologic sub-area 2 is the bridge/culvert under Old Country Way. Hydrologic sub-area 3 (161 ha) contains a large percentage of woods, brush, and wetlands, and about 22% medium density residential area. It begins at Old Country Way and outlets at Revere Drive. Hydrologic sub-area 4 (65.7 ha) begins at Revere and outlets at River Road to the New Croton reservoir. Its land use is predominantly woods and open land with about 15 % of the area residential. Approach

Each hydrologic sub-area was evaluated for its runoff curve number and time of concentration necessary for developing the runoff hydrographs for rainfall events. Table 5 summarizes their data. These watershed parameters were used to generate peak discharges and runoff volumes that were flood routed through the watershed system to the outlet at NYS Route 129. USDA-NRCS TR-55 methodology (Natural Resources Conservation Service, 2005a) was utilized to compute sub-area runoff curve numbers and their corresponding times of concentration. The U.S. Army Corps of Engineers HEC-RAS 3.1.2 computer program (2005) was utilized to develop hydraulic rating curves for selected stream and valley cross-sections in the Sawmill Creek watershed beginning in the north wetland just south of Underhill Avenue downstream to just above the storage basin at NYS Route 129. These rating curves were then incorporated into the USDA-NRCS WIN TR-20 (Natural Resources Conservation Service, 2005b) watershed hydrology computer program along with the watershed sub-area parameters to compute peak discharges at specified locations within the watershed. There are two significant wetland areas located along the stream system with culverts at their outlets. The upper wetland, approximately 11.3 ha, extends from just below Underhill Avenue south to Old Country Way. Its outlet is controlled by a 1.52 m x 2.13 m structural arch corrugated metal pipe. The lower wetland, approximately 17 ha, extends from the pond west of NYS Route 118, south to a concrete box culvert outlet, 1.22 m high and 1.52 m wide passing under NYS Route 118.

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These wetlands were modeled in WIN TR-20 as structures. Each of these areas has significant storage during storm flows, so stage-discharge-storage relationships were developed for these wetlands so they could be flood routed as structures in the program. Although there are other culverts on the main stream in the watershed, they exist in areas of insignificant storage and were thus ignored for this planning level study. Design storms were then selected to rout through the watershed model. These were the 1 year, 2, 5, 10, 25, 50, and 100 year frequency, 24 hour duration, type III rainfall distribution storm events.

Results

The peak discharges for these storms at the watershed outlet are summarized in Table 6. These discharges were then input into HEC-RAS 3.1.2 (U.S. Army Corps of Engineers, 2005) to determine the water surface profiles and associated channel and floodplain velocities based on the cross-sections previously inputted to the model. These cross-sections were surveyed in the field and supplemented with topography provided by the Town of Yorktown Heights. Summary Table 1 in the HEC-RAS 3.1.2 output in the supporting documentation for this report, summarizes discharges and water surface elevations for each channel cross-section and each storm event. Comparisons were made between the results of this study with those of the FEMA Flood Insurance Study (1985) and revised later in August 1993 (Table 7). The FEMA (1993) study incorporates generalized field data from the outlet at NYS Route 129 upstream to Locke Avenue; or approximately one third the length of the watershed. In addition, their hydrologic analysis was accomplished using a regression analysis forwarded by the Federal Highway Administration. The “Report No. FHWA-RD-77-159” was published in 1961; revised in 1963. In contrast, our analysis begins with a full watershed evaluation for drainage areas, site specific soils data for runoff determinations, detailed development of flow regimes for the times of concentration based on the detailed topographic maps of the watershed, and development of the channel cross-sections by field surveys and mapping. Thus, it seems likely that our model is more representative of the watershed runoff response to rainfall events. Also, these two basic models (WIN TR-20 and HEC-RAS 3.1.2) can be utilized to further breakdown sub-areas to obtain more refined results at particular locations. Full documentation for this hydrologic study of the Sawmill Creek Watershed is included in the support documentation appended to this report. Water Quality Background

There is concern regarding water quality in the Sawmill Creek Watershed since it directly discharges to the New Croton Reservoir, one of the New York city water supply distribution reservoirs. Also of concern is the quality of the water in various stream segments to support fish habitat (see Task 4b). In order to initially identify and quantify pollutant sources and loads, the watershed was divided up into four major sub-areas (Table 8). Area 1 was the drainage

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area above Underhill Avenue. The land use in this area was dominated by low density residential and commercial development. Area 2 comprised the drainage to the main creek stem from Underhill Avenue to Old Country Way. Of the approximate 154.3 ha (381 ac) in this area, 53.4 ha (132 ac) were residential development of low, medium and high density (34.7 %). There was also a significant amount of impervious areas from commercial, industrial and highway uses (8%) adjacent to the wetland and water courses. Area 3 extended from Old Country Way to Locke avenue. This area had approximately 28 % residential area. Area 4, a sub-area of 65.6 ha (162 ac), had 18.7 % land use in area from roads and residential development in its long and narrow shape to the outlet at NYS-Route 129. A complete breakdown of land use for each sub-area is provided in the tables 8, 9, 10 and 11. Since the New Croton Reservoir is a phosphorus restricted basin for the New York City drinking water supply, total phosphorus (TP) and total suspended solids (TSS) were selected for evaluating potential annual wash off pollutant loads in the watershed for this task. Approach

The Simple Method (Schueler 1987) was selected to estimate pollutant loadings for major land use categories. It utilizes pollutant event mean concentrations obtained from research done during the US EPA-National Urban Runoff Program (US EPA 1983a) and updated by researchers leading to values published by the Terrene Institute in 1996. The Simple Method is primarily intended for use on developed sites no larger than a square mile in area. It provides a fairly rapid and versatile means to approximate pollutant loads within watersheds. Although some accuracy is sacrificed for ease of use, it is considered precise enough for planning level non-point source pollutant management decisions (NYS-DEC, 1992) for site development. Note that for this study we also calculate site specific loads in Task 4 and compare these values with the approximations based on the Simple Method. This procedure utilizes a runoff coefficient (Rv), based on the percent of impervious in the area being evaluated, and the climatic rainfall for the site. The formula is: L = [(P) (Pj) (Rv)/12] (C) (A) (2.72) Where: L = the storm pollutant in pounds, P = selected rainfall depth (in.) (annual or storm), Pj = correction factor, 0.9 for annual P, 1.0 for storms, Rv = 0.05 + 0.009 I, where I = % impervious, C = flow weighted mean concentration, mg/l, and A = area in acres. Note 12 and 2.72 are unit conversion factors. Results

The computation of the pollutant loads exported from specific areas based on land use and sub-area are detailed in Table 9, 10 and 11. The loads of TSS by land use and

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sub-area are detailed in Table 9. The largest load of TSS is derived from sub-area 2, with the major source being the residential land use. The loads for TP are detailed in Table 10. Similarly the largest load is from sub-area 2 with about 57% originating with the residential land use. Drainage loads for both pollutants by land use without regard to sub-area are summarized in Table 11. These values are expressed in kilograms/yr as well as a percentage of the total. According to model calculations residential land use provides almost 45% of the TSS load, almost double that of the commercial and industrial category and the roads/highways category (Table 11). This land use is also responsible for over 56% of the TP load and greater than three times that for the commercial and industrial category and the roads/highways category. Stormwater discharges outlets were identified in the Town of Yorktown Heights sub-division map. In addition, potential locations for elevated discharge were located. The majority of these locations were verified and photographed in the field January 6, 2005. Existing stormwater management facilities were inventoried and located on the aerial map of the project in the appendix 2.2. Their impact on pollutant reduction and potential mitigation opportunities in the watershed are discussed in Task 6 – Mitigation.

Appendix 3.1 Supporting Documentation for Hydrological Modeling

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TASK 4A. Water Quality Analysis

Objectives The objective of this task was to collect stream field data to support the identification of areas of degradation in the watershed. This task includes the collection of water chemistry data (Task 4a) and the characterization of aquatic habitat (Task 4b). The water quality monitoring program enabled us to estimate nutrient and solids loads within the watershed and to the New Croton Reservoir. Background: Dynamics of Major Non-point Source Pollutants Oxygen Demand

The availability of dissolved oxygen (DO) in water reflects atmospheric transfer, as well as processes (autotrophic and heterotrophic) that produce and consume oxygen, (Odum 1956; Schurr and Ruchti 1977; Parkhill and Gulliver 1999). Dissolved oxygen concentrations have been used as a primary indication of water quality standards in aquatic environments for decades (Wang and Lyons 2003; Wang and Kanehl 2003). Higher life forms (i.e. fish) in aquatic ecosystems need an adequate oxygen supply and this supply is primarily regulated by the amount of algae present (Cox 2003). When oxygen is depleted in aquatic ecosystems, the survival of fishes and other aquatic organisms are threatened (Cox 2003). To maintain the integrity of streams and other aquatic organisms regulating agencies make recommendations on the concentration of DO in water. New York State has recommended daily average DO should not be below 5mg/l with minimum values not to exceed 4 mg/l. The availability of dissolved oxygen in water bodies involves complex interactions among physical, chemical, and biological processes. The determination of DO in aquatic ecosystems for water quality purposes is important as it gives insight into the biochemical reactions occurring in a water body. This information does not only make it an indicator of stream metabolism (Young and Huryn 1999), but also used in interpreting the general health of aquatic ecosystems (Wang and Lyons 2003; Wang and Kanehl 2003). Dissolved Organic Carbon Dynamics

Dissolved organic carbon (DOC) is important in the realm of water quality because it affects a wide range of properties of freshwater systems. It attenuates light and provides protection for aquatic organisms from radiation. Concentrations of DOC also have important implications for water supplies. DOC imparts color to a water supply (Smith and Davies-Colley 1992), which need to be removed through water treatment. High concentrations of DOC react with chlorine in water treatment to produce trihalomethane, a known carcinogen (Vogt and Regli, 1981).

Biological activities are influenced by the availability of DOC; such as primary productivity in lakes and reservoirs (Schindler et al. 1996; Schindler and Curtis 1997; Jackson and Hecky 1980), secondary production (Moran and Hodson 1990) and the

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availability of certain phosphorus species to phytoplankton (Steinberg and Muenster 1985). DOC concentrations in natural waters range from 2-15 mg/l, with a mean of about 4-6mg/l (Degens 1982) and in water draining wetlands values range from about 5-60 mg/l with a mean of 25 mg/l (Thurman 1985).

Dissolved organic carbon (DOC) plays an important role in a number of chemical processes within streams (Perdue et al. 1976). Hydrology and source (terrestrial or algae) play an important role in the temporal and spatial variability of DOC and UV attenuation. DOC enters streams through different pathways. These paths include surface runoff, soil water flushes, groundwater inputs, leachate from transported litter to streams and from algae secretions (Wetzel 1990; Kowalczewski 1978; Meyer et al. 1998; Strauss and Lamberti 2002). Most studies conducted to examine the controls on the concentration and fluxes on DOC in stream water have shown that DOC is influenced by watershed physiography, precipitation, vegetation and wetland cover (Eckhardt and Moore 1990; Dalva and Moore 1991; Clair et al. 1994; Guyot and Wasson 1994; Gergel et al. 1999)

Dissolved organic carbon concentrations in streams have also been known to be influenced by land use (Likens et al. 1977; Soulsby and Reynolds 1995; Burns 1996; Schlesinger and Melack 1981; Eckhardt and Moore 1990). But the strongest relationship has been found to be between the soil carbon pool and that of DOC concentration in streams for small catchments (< 5km2; Aitkenhead et al. 1999). The relationship between landscape position and land-use on the export of DOC and eventual concentration in streams is not fully understood (Allan et al. 1993).

Nutrients in Streams

Chemical speciation, the periodic application of fertilizers and the vulnerability of waste disposal systems together with seasonal variations in hydrologic conditions are major factors that determine nutrient loss from catchments (Puckett 1995). The loss of nutrients, especially nitrogen has been known to show spatial and temporal variability depending on the heterogeneity of the catchment’s landscape (e.g. uplands, wetlands, lakes and streams; Inamdar et al. 2000). A modeling approach that linked hydrological and biogeochemical processes for watersheds in Ontario, Canada showed the importance of topographic features in explaining the variability of NO3

- export among small watersheds (Creed and Band 1998).

It has been suggested for the Catskills in New York State, that ground water with high concentrations of NO3

- may be an important source of nitrogen in surface waters during base flow when ground water is the major contributor (Burns et al. 1998). On the other hand Lovett et al. (2000) suggested that for this region (Catskills) the variability in NO3

- concentrations is due to differences in forest species composition and forest history. The role of nitrogen and phosphorus in the eutrophication of fresh and coastal

waters is recognized and non-point sources of nitrogen and phosphorus pollution to rivers and lakes have attracted widespread attention (Arheimer and Liden 2000). Nutrient concentrations are relevant to the health of streams and interconnected reservoirs, wetlands and coastal waters. There are various factors such as flow, light intensity, and the presence of other nutrients that determine the nutrient that limits plant growth. The nitrogen to phosphorus ratio (N: P) is used to assess the nutrient that is limiting plant and algae growth in aquatic ecosystems. As a rule of thumb (Horne and Goldman 1994),

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nitrogen is the limiting nutrient if N: P is less than 10 and phosphorus limits if N: P is greater than 10. Either or both can limit plant growth if N: P is close to 10. One of the major nutrients in short supply in temperate rivers and fresh water ecosystems is phosphorus and thus is this nutrient that is most likely to limit plant growth (Mainstone and Parr 2002).

Phosphorus is transported to rivers from various sources. The phosphorus can originate from point sources (e.g. sewage treatment works) and through diffuse/non-point sources which are a more complicated and highly seasonal (Bowes et al. 2003) pathway. In the absence of sewage discharge points within Turkey Mountain Watershed, diffuse/non-point sources of phosphorus into Sawmill Creek are the major pathway.

Bacteria

Bacteria (and viruses) include infectious agents and disease-producing organisms normally associated with human and animal wastes. The survival rate of these bacteria in the stream column is limited to a few days, but within sediments the survival rate is markedly extended to 30 days or more (Sherer et al. 1992). High bacteria populations within streams and water supplies indicate the possible presence of pathogens, which are responsible for water borne diseases such as bacillary, gastroenteritis, dysentery, typhoid fever and cholera (Myers and Sylvester 1997; Dufour 1984). Fecal coliforms (FC) and E. coli (EC) are extensively used as indicators of fecal contamination in water as they are bacteria that reside in the intestinal tract of humans.

Human waste can contaminate streamwater and potentially drinking water supplies through a number of pathways, including direct discharge, surface runoff, and seepage through groundwater from faulty septic tanks and sewer systems. Urban, developed and agricultural areas produce the highest bacterial export (Hyland et al. 2003; Smith et al. 1993; Wilhelm and Maluk 1998), with urban areas posing the greatest threat to water supplies, due to sewer overflows and septic system failures. Bacterial survival is highly dependent on temperature and bacteria multiply faster during warm weather. Field and Laboratory Methods Sampling Procedures

Water sampling was conducted at six longitudinal sites within the Turkey Mountain Watershed to identify patterns of water quality in Sawmill Creek (Figure 11). An initial survey of water quality was conducted to determine locations for permanent sampling stations. After the permanent water quality stations were established, each site was monitored monthly for 24 months. The first three sites (i.e., 1, 2, 3) are located in the headwaters of the watershed. These sites are impacted by drainage from Yorktown Heights and a wetland in the upper reaches of Sawmill Creek. The lower three sites (i.e., 4, 5, 6) reflect more freely draining water and drainage derived from residential and undeveloped areas of the watershed. One hundred and ninety-two samples (including triplicates) were collected during this period. All sample collection bottles were acid-washed and thoroughly rinsed with de-ionized water before sample collection. Dissolved oxygen was collected in a 300ml glass BOD bottle. Samples were fixed on site, before

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transport to the laboratory for measurement of dissolved oxygen. Samples for phosphorus analysis were collected in a 250ml borosilicate glass bottle. Turbidity was measured with a HACH turbidimeter in the field. One liter of water was sampled for total suspended solids, total volatile solids and total dissolved solids. Sample for all other analysis were collected in high-density polyethylene Nalgene bottles. Samples were transported from the field to the laboratory at Syracuse University in coolers within approximately 5 hrs of collection. All water samples were analyzed for a suite of water quality measurements, including turbidity, total suspended solids (TSS), total dissolved solids (TDS), total volatile solids (TVS), total phosphorus (TP), total dissolved phosphorus (TDP), soluble reactive phosphorus (SRP), total nitrogen (TN), ammonium (NH4), nitrate (NO3), dissolved organic carbon (DOC), dissolved oxygen (DO), pH, specific conductance, chloride (Cl), fecal and total coliform and other analytes using standard methods (APHA 2005). Immediately after arrival to the laboratory, following sampling, pH, fecal coliform, total coliform and DO measurements were conducted. Sediment were sampled (using a PVC coring device) for potential toxicity from site 1 and site 6 using the Microtox assay (Microbics Corporation, 1992 ) and analyzed by Aquatox Inc of Syracuse, NY..

Automatic stormwater samplers were deployed to two sites in the watershed to collect samples (Site 3 and 5). Following events samples were returned to the laboratory and measured for pH, turbidity, total suspended solids (TSS), total phosphorus (TP), total nitrogen (TN), ammonium (NH4), nitrate (NO3), dissolved organic carbon (DOC), specific conductance.

Analytical Procedures

After the samples were transported to the laboratory, they were stored under dark and cool conditions (4oC) to prevent chemical and biological activity within the sample bottle. Prior to analysis, samples were allowed to come to room temperature. Analyses were conducted in a time-sensitive manner for each particular parameter (Standard Methods; i.e. the very sensitive analytes such as pH, phosphorus and ammonia were all analyzed within the first 48 hours).

All analytes were measured using standard laboratory methods (Table 12). Nitrite (NO2

-) and NO3- (nitrate) were measured as NO3

-. The sum of measured ammonia and nitrate is referred to as dissolved inorganic nitrogen (DIN). Organic nitrogen was determined as the difference between total nitrogen (TN) and DIN. The phosphorus species analyzed included soluble reactive phosphorus (SRP), which incorporates both orthophosphate and other polyphosphates; unfiltered, digested phosphorus (TP); and filtered digested phosphorus, or total dissolved phosphorus (TDP). Dissolved organic phosphorus (DOP) was defined as the difference between TDP and SRP. Hydrology and Annual Loads

Hydrologic studies to determine runoff and peak discharges from a watershed are based on long-term stationary stream flow records for the watershed. Like most small drainage basins, Sawmill Creek is un-gauged. Therefore in order to calculate chemical loads from the watershed estimates of flow were obtained. Drainage-area ratio method

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and multiple linear-regression analysis are two methods used to estimate stream flow for un-gauged watersheds (Ries and Friesz 2000; Vogel and Kroll 1990; Wandle and Randall 1994). Discharge data for Sawmill Creek were estimated using the Discharge–Area Ratio Method. Angle Fly Brook watershed (41°16' N, 73°43' W) was used as the index watershed. Angle Fly Brook is approximately 4.4 km East of Sawmill Creek. Angle Fly Brook has an approximate area of 7.80 km2 about twice the area of Turkey Mountain Watershed, with 60% percent open space (parks and recreational areas, water bodies, and underdeveloped area) and 40% developed (residential, and commercial areas). Discharge values from Angle Fly Brook were obtained from USGS real time discharge values. Due to proximity, aspect, size and land-use, we felt that Angle Fly Brook was the best site to estimate discharge for Sawmill Creek. Results Hydrology

Estimated daily stream flow for Sawmill Creek from October 2002 through September 2004 is shown in Figure 16. The estimated mean stream flow is 8458 m3/day. The highest estimated daily mean discharge (243941 m3/day) occurred in September 2003 and the lowest estimated mean daily discharge of 106 m3/day occurred in July 2004 (Figure 16).

Water samples collected during December, 2003 were made under conditions of highest discharge of all samples collected during this study. Estimated flow from Sawmill Creek was highest in March and April during spring snowmelt and rain events, lowest during the warm, dry summer months July, August and September except for September 2004 when there was a large rainfall event.

Water Quality

Temperature, pH, Turbidity, DO, TSS Surface water chemistry along Sawmill Creek showed both temporal and spatial

patterns for most of the solutes measured (Figure 17). The values of pH along the stream transect from Site 1 through 6 lie within the range 7.0 –8.8 (Table 8), which is typical of surface waters within the New Croton System. The waters draining Turkey Mountain watershed are neutral with annual mean pH of 7.7. The lowest pH value observed was 7.0 at site 1 in August, 2003. A strong positive correlation (r=0.78) with sites was observed for pH, increasing from Site 1 through Site 6 (Figure 17). On average the lowest pH values were observed in the upper reaches (sites 1, 2 & 3) of the watershed in waters draining the wetlands. A large increase in pH values above 8.0 were observed at site 5 and site 6 at the mouth of the stream. Other parameters, that showed statistically relevant correlations with pH are listed in Table 8, with the exception of sampling location, all the other parameters were negatively correlated suggesting a decrease in the concentration of water quality parameters down stream.

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Surface water temperatures, as expected, reflect the extremes of winter and summer seasons. The coldest water temperatures (0°C), were recorded in January and February, and the highest in August (24°C). The wetland sites (1, 2, 3) were on average, warmer than the freely draining sites (4, 5, 6) during the period of study (Table 13). Temperature also showed strong correlations with most stream nutrients measured, with the exception of NO3

- (Table 14). Spring was the period of highest average pH along Sawmill Creek, while the

lowest seasonal average and least variable period occurred in winter (Figure 17). Stream pH increased down stream along Sawmill Creek (Figure 17), with the most variability observed at site 2.

The upstream and downstream DO averages were 7.7 mg/l and 9.7 mg/l respectively. The variability between sites was such that DO decreased from S1 to S2 and then increased gradually from S2 to S6 in the downstream direction (Figure 18). About 10% of the samples analyzed had DO concentrations below the 5 mg/l critical level for aquatic life (EPA, Figure 19). These low DO values were restricted to sites S1 and S2 (Figure 18). Dissolved oxygen decreased with increasing in-stream temperatures, reflecting the temperature-dependent solubility (Figure 20). High dissolved oxygen (DO) values were measured during periods of extreme lows in temperatures. The DO values ranged from 2.0 mg/l in August to 13.6 mg/l in January reflecting the extremes of the summer and winter temperatures. The dissolved oxygen deficit was greatest in the wetland sites and decreased downstream. The magnitude of the dissolved oxygen deficits at the upper sites (S1 and S2) are reflective of lower turbulence, lack of reaeration and higher metabolism of this reach compared to the downstream sampling sites (Wang and Lyons 2003; Wang and Kanehl 2003; Figure 21). The oxygen deficit was consistent over the sampling period, except for lower values which occurred during the months of March -May (Figure 19).

The total suspended solids and turbidity (Figure 22) showed a weak relationship, with data clustering near the origin and higher values showing some variation, which is similar to patterns observed from other studies (Packman et al. 1999). TSS concentrations ranged from 0.12 to 15.7 mg/l, with a mean of 2.20 mg/l. Sites S1 and S2 had the highest mean TSS concentrations. Sites 1 and 2 also had the highest mean turbidity values. Turbidity has been used as a surrogate for TSS in streams and aquatic ecosystems (Gippel 1989; Halfman and Scholz 1993; Mitsch and Reeder 1992; Packman et al.1999). Turbidity measurements do not necessarily correspond with TSS values, as observed for sites S1 and S2 (Figure 22). The regression model gives a statistically weak R2 value (0.10) for the relationship between turbidity and TSS.

Although the regression model for TSS and turbidity has a weak predictive power (i.e., low R2), there were strong correlations with statistically significant predictive powers for the summer period (R2= 0.71 respectively). From Figure 23, it is evident that the spring and summer data are biased slightly towards the turbidity axis (i.e. the high turbidity values are not necessarily a result of the presence of particulate or suspended matter). Other in stream processes may be responsible.

The major source of DOC to streams is through leached leaf litter (Qualls et al. 2003; McDowell 1985). DOC concentrations in Sawmill Creek were highest in October (6.1 mg/l) and lowest in January (2.6 mg/l). The spatial variability was the lowest during January, February, March and April (Figure 24). As a whole, except for September, DOC

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did not exhibit large spatial variations (SD=1.09mg/l), compared to temporal variations (SD=1.30 mg/l). The increase in DOC was observed to coincide with increases in temperature, showing a distinctive seasonal trend (Figure 25).

Concentrations of DOC in Sawmill Creek ranged from 1.4 to 8.2 mg/l, with a mean of 4.5 mg/l. Natural waters typically have observed concentrations between 2-15 mg/l (Degens 1982). A decreasing gradient in DOC concentrations was observed along the stream transect (Figure 24). DOC concentrations were higher in the upper reaches of the watershed (within and around the wetlands), and decreased with increasing distance downstream. DOC concentrations within the Sawmill Creek wetlands were lower than typical values reported for wetlands. Typical stream values have an observed DOC range of 5 to 60 mg/l, with a mean of approximately 25 mg/l (Thurman 1985).

Nutrients

The organic fraction was about one third of the total dissolved nitrogen concentrations (Figure 27). Organic nitrogen concentrations ranged from less than 0.01 mg N/l to 1.01 mg N/l, with a mean concentration of 0.31 mg N /l.

The inorganic fraction of nitrogen constituted almost two thirds (65%) of the total nitrogen in Sawmill Creek water, with NO3

- and NH4+ concentrations varying by site

(Figure 27). The headwater site (S1) had the highest NO3- concentration compared to the

other sites. The NH4+ fraction was equal to the NO3

- for the winter season and slightly lower than NO3

- for the fall spring and summer seasons. NO3- constituted just a little less

than 30% of total nitrogen load but increased to approximately 40% in the summer. Ammonia-N was a significant fraction of the total nitrogen concentration in Sawmill Creek (Figure 26). NH4

+ concentrations ranged less, 0.02 to 1.02 mg/l with a median concentration of 0.31mg/l.

Speciation of nitrogen changed slightly with season. The NH4+ fraction was

greatest in winter when all three species were about one third of the total nitrogen concentration (Figure 28). Nitrate was the largest fraction of the total nitrogen in the fall, and the concentration of organic nitrogen species was the largest fraction in the summer when biological activity is the greatest (Figure 28).

The concentration of NO3- never approached the NYDEC drinking water standard

for NO3- of 10 mg N/l during the period sampled. The mean TN concentration was 0.92

mg N/l, with a standard deviation of 0.30 mg N/l. The highest single concentration (2.29 mg N/l) of total nitrogen was observed in January, 2004; with the highest seasonal average observed in winter (Figure 28). The largest variations in concentrations of total nitrogen in Sawmill Creek were observed from April through June (Figure 29). Spring was the most variable period of total nitrogen concentrations, suggesting the hydrological factors (i.e., snowmelt, increased discharge) had a role in the dynamics and transport of nitrogen and nitrogen species in Sawmill Creek. The highest mean concentration of total nitrogen was observed during winter, with no significant difference in mean values for the other seasons (Figure 28).

Total nitrogen concentrations decreased markedly in a downstream direction (Figure 30), with the upstream sites showing higher mean values and variability than the

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downstream sites. The mean concentration of total nitrogen for the upper reaches (wetland region) was 1.17 mg N/l and for the lower reaches was 0.68 mg N/l.

Nitrate (NO3-) and dissolved organic nitrogen did not show distinct temporal

patterns. The highest concentration of nitrate (1.25 mg N/l), was observed in December 2003 at site 2. The average NO3

- concentration for the study was 0.34 mg N/l. The upper sites (1, 2 and 3), which are within and close to the wetlands, showed higher mean NO3

- composition than the lower sites (Table 15). This pattern is suggestive of a substancial source of NO3

- to the stream in this reach, as wetlands are very effective sinks for inputs of NO3

-. Total nitrogen showed statistically significant correlations with site, temperature,

pH Turbidity, DIC, Mg, Na, K and Cl (Table 16). All nitrogen species exhibited negative correlations to sampling site. Both NO3

- and the organic fraction of nitrogen (OrgN) showed statistically significant positive correlations with dissolved inorganic carbon (DIC), with NO3

- more positively correlated than organic nitrogen (Table 16). Stream concentrations of nitrogen were higher than phosphorus for Sawmill Creek, N: P ratios were routinely above 10 which is considered the criteria for determining the nutrient limitation of a system (Horne and Goldman 1994). The highest and lowest ratios occurred during winter and summer, respectively.

Total phosphorus showed a marked variation over the period investigated. There was an increase in the mean values during spring, with high values observed from June through August then declines from September through January (Figure 31). The maximum concentration of total phosphorus was detected in November 2002 (30.9 µg/l) and the minimum value in November 2003 (7.5 µg/l). Seasonally, the highest concentrations of total phosphorus were detected in the spring and fall months, with seasonal means of 18.0 µg/l and 20.2µg/l respectively (Table 17). These values are above the proposed Phase II TMDL for phosphorus in the New York City watersheds. This condition places Sawmill Creek in violation of water quality with respect to phosphorus during spring and summer. Total phosphorus did not show a clear spatial pattern along the stream transect (Figure 32). Site S1 had the highest annual mean concentration. There were appreciable variations of TP concentrations at all sites during the period of study. For overall Total phosphorus composition, 48 % was in particulate form and 52 % was as total dissolved phosphorus (TDP). Dissolved inorganic phosphorus (DIP; SRP) made up about 29% of total phosphorus (TP) .The mean dissolved organic phosphorus (DOP) and dissolved inorganic phosphorus (DIP) concentrations were found at comparable values of about 46% to 54% of the total dissolved phosphorus (TDP), respectively. The highest concentration of dissolved inorganic phosphorus (DIP) was observed during February 2004, accounting for two thirds of the phosphorus concentration and over 98% of the dissolved fraction (Figure 33). Marked increase in particulate phosphorus was observed from spring and throughout the summer (Figure 33). The increase was probably due to flushing associated with high flow during snowmelt in spring and low flow with greater temperatures and subsequently, higher biological activity in the summer.

The correlation between total phosphorus and phosphorus species to other water quality parameters are listed in Table 18. Temperature, DOC, DO, NH4

+, DOC, DIC, SO4 and Cl- are water quality parameters to which most phosphorus species were positively correlated. Temperature was the parameter that had the highest significant

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correlation to total phosphorus and phosphorus species suggesting that temperature was a major factor regulating phosphorus dynamics. Dissolved organic phosphorus (DOP) was the only species that did not show any significant correlation to the other water quality parameters (Table 18).

Bacteria Total and fecal coliform exhibited predictable patterns during the study period and across the watershed. Concentrations of both fecal and total coliform were lower in the winter (fecal = 23 colonies/100ml, total = 191 colonies/100ml) and spring (fecal = 24 colonies/100ml, total = 206 colonies/100ml) periods than in the summer (fecal = 229 colonies/100ml, total = 703 colonies/100ml) and fall (fecal = 246 colonies/100ml, total = 961 colonies/100ml). Average concentration of fecal and total coliform decreased with downstream distance from the headwaters to the stream outlet draining into the New Croton Reservoir. Mean concentrations in the headwaters (site 1) were 231 colonies/100ml for fecal and 953 colonies/100ml for total coliform. Mean concentrations at the base of the watershed (site 6) were 89 colonies/100ml for fecal and 353 colonies/100ml for total coliform. Although the average concentrations for site 6 are below the New York State water quality standard of monthly geometric mean of 200 colonies/100 ml for fecal coliform, seven out of the twelve sampling dates in the spring and fall had values greater than this value. Fecal bacteria concentrations in streams are often correlated with the amount of developed land and impervious area within a watershed (Mallin et al. 2000). Multivariate Statistical Analysis

The major nutrients (TN and TP) all show temperature-dependence, suggesting that biological activity had a major role in regulating their concentrations. Temperature was the parameter that appeared most in the models, although not always being the parameter with the most influence on individual p-values.

Factor Analysis Since a number of variables were well correlated, there is a possibility of further simplifying the regression model (Parinet et al. 2004). Factor analysis, principal component analysis and principal factor analysis was used to further classify and explain the data. Principal component analysis showed that the eigen values of the first three principal components represented approximately 62% of the total variance of the observations. The first principal component explained 33% of the variance in the water quality of Sawmill Creek. It represents the chemical gradient along the stream and the variation in ionic solutes. The second component explained about 22% of the variance in the data and explains the nutrient group of phosphorus and nitrogen species. The third factor explained just 8% but is the only factor that explained the variance in suspended

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solids and the organic fraction of nitrogen. Most water quality parameters were explained significantly by the first two factors, but parameters such as turbidity and NH4

+ were significantly explained by the three selected factors. The cations were explained by the first factor only. Because of the large number of variables, the factor loadings of the variables were plotted to provide clarity (Figure 34). Together with the rotated factor loadings for each variable (Table 19), the graph (Figure 34) can be interpreted accurately and a representation of water quality for the period studied can be deduced.

Cluster Analysis In an attempt to identify the sampling sites with similar water chemistry, the multivariate technique of cluster analysis was used on the stream water quality data. Not surprising, the sites with the most similar water chemistry were site S5 and S6. These sites were positioned with the least distance between them. These sites showed similarity according to their location along the stream reach. Site S1, the first upstream site was not similar to any of the other sites. The nearest site in water quality to S1 was site S2 (Figure 35), which was positioned immediately down stream. Sites S1 and S2 were not only significantly different from all the other sites but were not similar to each other. Sites S3 to S6 had comparatively similar in water chemistry, even though there is a considerable distances between these sites. Watershed Loadings

With monthly measurements of stream chemistry concentrations and estimates of daily discharge, we calculated watershed material loads. Total phosphorus (TP) loads varied throughout the study with peaks in April, August, December 2003 and September 2004 (Figure 36). These increases coincided with the high monthly discharge. The annual estimated TP transported from Turkey Mountain Watershed was 1.71 kg/yr. The areal loading of TP was greatest in the upper urban reach of the watershed (site 1: 460 µg/m2-yr). Loads decreased somewhat at site 2 (390 µg/m2-yr) and remained constant throughout the study reach, except for an additional increase just prior to discharge into the New Croton Reservoir (408 µg/m2-yr) and Total nitrogen (TN) load was also variable throughout the study period. The highest loads were observed in April, July, December 2003 and October 2004, again largely driven by discharge (Figure 36). Transport of TN was lowest generally lowest during July and August of both years of the study period. The annual estimated TN transported from Turkey Mountain Watershed was 69,700 kg/yr (16.6 g/m2-yr). The monthly loading of TSS also followed variations in discharge. The annual TSS load from the watershed was 6,370 kg/yr. Areal TSS loss from the watershed decreased with decreasing elevation in the watershed, from 90 g/m2-yr at site 1 to 23 g/m2-yr at site 6. Note that based on our routine monthly measurements, we did not observe an increase in TSS in the reach with the bank failure (between sites 4 and 5).

Estimates of loading of TSS and TP from runoff events from impervious surfaces of the watershed were calculated through the hydrologic modeling (Task 3) and loading model (Simple Method; Schueler 1987). We compared these estimates with the loads calculated from field measurements of water quality measurements and our estimates of

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flow for watershed increments between our sampling stations (Table 20). In general the actual site load estimates of TP and TSS were considerably greater than calculations based on literature from the Simple Method. Material loadings from the Simple Method are not directly comparable to the site specific watershed flux estimates, but some insight might be obtained viewing these values in the context of one another. The fact that impervious surface runoff estimates were an important but generally, not dominant component of the flux suggest the importance of other sources in the watershed for material supply. For TP important sources would likely be the wetland and non-point leachable associated with residential areas. For TSS likely additional sources are the wetland and bank erosion.

Stormwater Events

Data are shown for two selected storm events (January 13-15 2005; October 7-8 2005). For the January event, precipitation late on 13 January resulted in a marked increase in flow followed by a slight recession and a second peak in discharge. At both site 3 and site 5 the initial increase in flow resulted in an approximately order of magnitude increase in turbidity and TSS (Figure 37, 38). Concentrations of TP, DOC and DON also increased at both sites in response to the increase in flow, although increases were much more modest than observed for TSS and turbidity, and concentrations remained elevated throughout the duration of the sampling period. In contrast to the other species, NO3

- concentrations decreased with increasing flow most probably as a result of dilution. The event occurred in January when nitrogen was not being utilized effectively by vegetation and biota in the watershed. Elevated concentrations of nitrate likely represent base flow supply by groundwater. During the event a large volume of water from shallow flow paths or of discharge from impervious surfaces dilutes this base flow NO3

-. The marked rapid increases in TSS and turbidity are likely due to erosion occurring in the stream channel. Total phosphorus and DOC increases are likely from flushing from shallow flow path sources in the watershed (such as the riparian wetland and impervious surfaces). Note we did not observe significant differences in water chemistry measurements between site 3 and 5 during events.

During the October storm event (Figure 39) there was an initial small increase in discharge followed by a much larger increase in flow later during the sampling interval. Concentrations of TSS and turbidity increased initially during the rising limb of the hydrograph, followed by a short period of dilution and then marked increases in particulate matter (Figure 39). There were minor increases in TSS and turbidity later associated with the highest flow conditions during the event. The initial flush resulted in nearly an order of magnitude increase in suspended matter. There were slight increases and fluctuations in TP with the initial increase in flow of the event. Later in the event dilution of TP was evident followed by a second peak. High concentrations NO3

- during baseflow prior to the event initially were diluted during the initial peak in the hydrograph. Concentrations of NO3

- remained constant during the remainder of the event except for a large peak during the highest flow at the end of the sampling period.

These event data illustrate the limitations of monitoring for TSS and turbidity during fixed interval sampling. Very large increases in suspended matter occurred on the rising limb of hydrologic events. Without capturing this portion of the flow regime one

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has a poor understanding of the loss of suspended matter. Much of this particulate matter is probably suspended by the initial increase in flow and is readily redeposited to the stream bed. Toxicity Testing

Solid-phase microtox toxicity tests were conducted on sediments collected from site 1 and site 6. The 30-minute EC50 was 2.92 mg/ml (SD 2.38-3.59) and 1.05 mg/ml (SD 0.80-1.37) for site 1 and site 6, respectively. Both sites had values less than 3 mg/ml, which suggests that their toxicity is no different than values reported for control sediments (Johnson and Long 1998). Note the toxicity values were greater in sediments adjacent to Yorktown Heights than in the lower reaches of the watershed. Comparison of Surrounding Waters

The Stroud Water Research Center (http://www.stroudcenter.org/index.htm) is conducting a six-year study to monitor and evaluate water quality and sources of pollution in the streams, rivers, and reservoirs that provide New York City's (NYC) drinking water. Although there are no plans to sample Sawmill Creek, during phase one of the study an unnamed tributary adjacent to the Sawmill Creek watershed was sampled (Table 21). Upstate Freshwater Institute (2003) conducted a study of the water quality of the New Croton Reservoir (Table 21). Our results are comparable to the unnamed tributary from the Stroud Water Research Center study. The unnamed tributary is largely forested (76%), with a significant component of agriculture land cover (19%). Although it is difficult to compare watersheds, in general concentrations of nutrients, DOC and TSS were somewhat greater in Sawmill Creek, undoubtedly due to the greater urban and suburban land cover. Likewise concentrations of nutrients, DOC and TSS were greater in Sawmill Creek than the New Croton Reservoir. This pattern is undoubtedly to in-lake retention of these materials in the reservoir. Our analysis suggests that Sawmill Creek contributes to water quality problems of the New Croton Reservoir. Summary

The water quality of Sawmill Creek is typical of urban and suburban watersheds.

Stream water exhibits elevated concentrations of nutrients, particularly phosphorus and fecal coliform. Dissolved oxygen concentrations in the upper reaches were low during the summer period. Spatial variations in water quality reflect the source area of the watershed in the urban area of Yorktown Heights. With increased drainage through undeveloped portions of the lower watershed, there is a general improvement in water quality. Estimates of site specific nutrient and TSS loads demonstrated the greatest mass flux of materials at the upper reach of the watershed with diminished flux toward the New Croton Reservoir. Our site specific fluxes for Sawmill Creek were considerably greater than values using the Simple Method. Storm sampling demonstrated approximately order of magnitude increases in TSS and turbidity on initial rising limb of the hydrographs. These spikes in particulate matter persisted for short periods of the beginning of events. This pattern suggests that routine monthly grab samples would

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underestimate the total solids load within and from the catchment. Increases in TP, DON and DOC also were evident during events, although the magnitude of these increases was more modest, typically a factor of two. In contrast, concentrations of NO3

- decreased during events, suggesting the mixing of high NO3

- base flow with dilute water draining impervious surfaces or shallow flow paths. Our observations for Sawmill Creek were comparable to an adjacent watershed and other watersheds draining into the New Croton Reservoir. Our water quality observations were also comparable, although generally somewhat greater than, values reported for the New Croton Reservoir. Appendix 4.1 Water Quality Monitoring Chemistry Data

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TASK 4B: Habitat Analysis

Based on the initial assessment of the watershed, the project team selected two locations for sampling and testing of macroinvertebrates and fish populations. One location was selected based on observable features which indicate a relatively stable form, and the other based on observable signs of degradation from upstream reaches including significant erosion and multiple stormwater and drainage outfalls. The locations of the two sampling sites are presented in Figure 40. The upper sampling station (Sawmill Creek – Upper) was a reach located between a large wetland and a large constructed pond in a relatively stable section of channel (between water quality sampling site 2 and site 3). The lower sampling station (Sawmill Creek – Lower) was a reach starting immediately below several large bank failures (near water quality sampling site 5). Fishery Assessment

On June 29, 2004, each of two sites on Sawmill Creek was subjected to stream

habitat survey measurements and a fishery survey using protocols developed by USGS (Mulvihill et al. 2003; Baldigo et. al. 2004). The lengths of the stream reaches were measured to be about 20 times the average width of the stream at the site. Fishing was done with a battery-powered backpack electroshocker and stunned fish were collected by netting. To ensure that fish were neither entering nor leaving the study reaches during sampling, the reaches to be fished were isolated by blocking nets at the upstream and downstream ends, those nets set roughly perpendicular to the stream banks and anchored at the bottom with rocks and stones. Fish from each of three passes were held separately and processed separately, by species, before being returned to the stream, most alive, after sampling and processing. Fish data were subjected to statistical review to determine how closely the samples represented the population in the sample sites.

Surface water temperatures were read and water samples were collected at six sites (Figure 12) on one day each for the last three months of 2002, each month in 2003 and for the first nine months in 2004. Water sampling procedures and analysis of samples is described in Task 4A of the report. Findings – Fishery Assessment

Five species of fish were collected during sampling, four species at the lower reach and three at the upper reach. Creek chubs (Semotilus atromaculatus) and blacknose dace (Rhinichthes atratulus), both minnow species, were the predominant species at both stations. One common shiner (Luxilus cornutus), another minnow species, was also captured at the lower site. All three species are native to the watershed, are most commonly found in streams and commonly occur together. One specimen each of two sunfish species, pumpkinseed (Lepomis gibbosus) and bluegill (L. machrochirus), were caught in the upstream reach. Pumpkinseeds are native to the watershed, while bluegills have been introduced. Neither sunfish is typical of small streams. Both are normally pond

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inhabitants and are thought to have been introduced into the creek from either the wetland at the source of the stream or the online pond area located between the sampling sites.

Population estimates of the two most abundant species in the collections, standardized by number of fish per unit area, differed greatly between the two sites as did the biomass estimates for the two species. Creek chubs were 3.6 times more abundant at the lower site where they averaged about 1.6 fish per square meter of habitat. Lower site creek chubs averaged 5.2 times heavier and 1.9 times longer than their upper site counterparts, indicating that they were generally older. Fifteen of the 58 measured lower site creek chubs were longer than 102 mm (4 inches). All of the creek chubs measured from the upper site were less than 102 mm long. Scott and Crossman (1973) reported that in Michigan, female creek chubs mature during their third year of life and males during their fourth year at which time fish of both sexes were 102-178 mm (4 to 7 inches) long.

Creek chubs were larger than blacknose dace, on average, at both sites. Blacknose nose dace were the most abundant fish species at both sites. About 2.3 times as many blacknose dace were estimated for the upper site as the lower site, about 5.1 fish per square meter of stream area versus about 2.2 for the lower site. Blacknose dace at the upper site averaged about 56.8 mm, roughly 6 mm longer than those at the lower site.

A fish survey done by staff of the New York State Department of Environmental Conservation (DEC) on 11 July, 2000, resulted in the capture of two brook trout (Salvelinus fontinalis), one white sucker (Catostomus commersoni), eight blacknose dace and one creek chub. The location for that fish collection was 0.4 km (0.24 miles) upstream from the mouth of the creek, below the Route 118 bridge. Findings – Habitat Assessment

The lower site was calculated to be 305.3 meters square in area, 75 meters long, about 12 meters longer than the upper site and about one meter wider than the upper site, on average. Habitat suitability data describe the upper site as being mostly pool and less riffle than the lower section where none of the sample transects intersected pool habitat. Pool habitat in each section was rated as “C” signifying that less than 60% of the reach consisted of pools with good and or moderately good habitat. The average water velocity in upper site riffles was less than 0.17 m/s (0.5 fps) while that in the lower section was nearly half again faster at 0.26 m/s (0.78 fps).

Substrate characteristics were also very dissimilar between sites with comparatively greater percentages of gravel and cobble in the lower section and proportionally higher percentages of silt/sand and boulder substrate at the upstream site. About 39% of the substrate particles sampled in the spawning area(s) of the upper site were within a range of 3-80 mm compared to 52% in the lower site. Embeddedness was more prevalent in the riffles of the lower site but there was a greater percentage of silt/sand, 11% versus 8%, in the riffles of the upstream site.

In-stream trout cover was about 8% greater in the lower section than the upper section, 28% versus 20%, on average for juvenile and adult cover. Average thalweg depth was about 4cm greater in the lower site, 17.9 cm versus 13.9 cm. Shade from trees was greater at the upstream site, owing to both greater over-story and less light penetration per area of over-story.

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Bank stability was inventoried solely by the percentage of vegetation along the creek as well as the average percentage of rooted vegetation and rock. In general, the vegetation at the downstream site averaged 72% versus 36% at the upstream reach. Rooted vegetation and rock percentage was similar at 53% for the downstream reach and 55% at the upstream reach.

The five habitat suitability index variables not specifically measured during the assessment were the average yearly maximum water temperature, the average maximum water temperature for the time period trout embryos are developing, the average minimum dissolved oxygen, the average maximum or minimum pH and the average annual base flow as a percentage of the average annual daily flow. The habitat suitability index for the lower site was 0.71 while that for the upper was 0.64, both on a scale of 0–1.

Results and Discussion

Water temperature readings and findings from water sampling and analysis are described in Task 4A. Surface water temperatures were measured in degrees C (converted to degrees F for this report) and surface water samples were taken at six sites one day each month from October 2002 through September 2004. Although the sampling sites and sampling dates were different from the fish/habitat sites and sampling day and did not fit the description of the USGS Habitat Suitability Index variables for brook trout, the findings from analysis of those water samples and temperature data may provide some insight into the absence of brook trout from the fish surveys of 2004.

The highest water temperature measured was 24 C (75.2 F) recorded for site 2 during August of 2003 and sites 5 and 6 during August of 2004. August water temperatures were the highest for all sites for all months they were measured. August water temperatures were at least 22 C (71.6 F) for all sites except site 1 in 2003 where the water temperature was 21 C (69.8 F). Measured July water temperatures did not exceed 68.0 and September water temperatures were all 18 C (64.4 F) or less.

Dissolved oxygen (DO) was below 5 mg/l for readings during three months at site 1 and 12 months at site 2. The lowest DO value for the entire study period was 1.59 mg/l from site 2 in October of 2003. Water that contains all the oxygen that it is able to hold at varying atmospheric pressures and temperatures is said to be saturated. Saturation also varies with salinity. DO saturation is an important facilitator for the respiration of aquatic organisms including fish and invertebrates that need to extract oxygen from the water to breath, usually by gills. Percent oxygen saturation varied from a low of 15% at site 2 in December of 2003 to 117% (oversaturation) at site 5 in March of 2004, Table 22. Oversaturation results from surface water turbulence and synthetic oxygen inputs. Except for site 2, DO saturation was generally above 70% and it improved at sites downstream (see Task 4a). DO oversaturation was mostly evident in samples from sites 5 and 6 and for some months during 2004. All water samples tested on the alkaline side of neutral for pH. pH ranged from a low of 7.01, nearly neutral, to a high of 8.75. Findings from measurements of 12 trout habitat suitability index variables, measured for Sawmill Creek during 2004, did not account for the absence of trout in the study reaches. brook trout have been evident in Sawmill Creek from the findings of the DEC fish survey in the year 2000. Sawmill Creek is not a stocked trout stream, at least it

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is not stocked by the DEC (personal communication with Ron Pierce, DEC Fisheries Biologist, DEC Region Three, New Paltz, NY). The fish collected during 2000 were probably stream fish since brook trout normally occur in tributary waters rather than large standing water bodies in this part of New York State. The absence of brook trout from the study reaches may be due to unsuitable water temperatures at the time the collections were made or perhaps isolation by in-stream barriers to fish passage. During periods of elevated water temperatures, brook trout seek out spring seeps or cool water near mouths of tributaries as refugia and they may have been located at such refugia at the time the collections were made. The fish species associations are not out of the ordinary for fish collections from New York streams of the same size and all the species collected are found throughout the state. The absence of white suckers in the 2004 fish samples is surprising. Suckers normally migrate upstream to spawn in the spring and at least young of the year would expected to be found at one or both sample sites if spawning had occurred there or upstream. Adult suckers would probably return to the reservoir subsequent to spawning and would not be expected in fish collections made at the end of June.

A field inspection was made to the confluence of Sawmill Creek and the reservoir. Although not visible, a subsurface outlet structure exists under Route 129. Fish passage upstream from New Croton Reservoir into Sawmill Creek may not occur due to the structure. In all likelihood, the only fish species that would access the creek from the reservoir would be white suckers attempting to migrate upstream during their spring spawning. It is thought that there is a resident population of white suckers. Nevertheless, barring suckers access to the creek from the reservoir deprives creek resident fishes and macroinvertebrates of the forage that additional sucker eggs and young suckers would provide. The competition for food and space that additional suckers would exert on the creek fauna would expected to be negligible since suckers occupy a demersal niche, inhabiting the stream bottom, and generally feed low on the food chain. Likewise, the loss of Sawmill Creek as sucker spawning habitat lessens the abundance of suckers in New Croton Reservoir. Suckers are forage for predators that feed on larger fishes in New Croton Reservoir.

Probably the most important habitat variable among the 17 that constitute the brook trout habitat suitability index is water temperature. Water temperature data are essential to clarifying brook trout presence as a function of habitat. The associations of the other fish species lend nothing to understanding their impacts on each other but do show that suitable habitat exists for their presence and for their propagation. Although the water sample and water temperature sites were different from the fish/habitat collection sites, and water was sampled on a different day then fish, the water quality findings do provide some insight into the absence of brook trout in the fish collections. Smith (1985) reported that brook trout are limited to water with temperatures below 23.9 oC (75 F). Water quality of Sawmill Creek, as described by temperature and DO values, is not inconsistent with the absence of brook trout at both fish/habitat sampling sites for this study, or the capture of two brook trout during a June fish survey by DEC in the year 2000.

Baird and Krueger (2003) reported on the behavior of radio tagged brook trout and rainbow trout (Oncorynchus mykiss) in an Adirondack river with summer water temperatures similar to those measured in Sawmill Creek during 2003. Hatchery fish of

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both trout species and two wild brook trout were marked with surgically implanted temperature sensitive radio transmitters and released into the study river. Trout of both species were observed to use two of five tributary or groundwater discharges in the 12 km (7.2 mile) study site as thermal refugia. During periods when the average daily surface water temperatures were 20 oC (68.0 oF) or warmer brook trout body temperatures averaged 4 oC (7.2 oF) cooler than the river surface temperature. Those trout were cooled by the cooler subsurface water at the refugia. The authors also discuss habitat improvement strategies, both instream and riparian, that might be used to protect or enhance cold water fish refugia. DO readings for water samples taken at site 2, where 20 of 24 values were below 75% of saturation and 12 of 24 were below 50%, indicate reduced water quality at that site. Findings from macroinvertebrate collections made at one upstream site and one downstream site and documented elsewhere in this report reinforce the fact that the upstream section has impaired water quality. The pH values for all sites, from 7.01 to 8.75, are suitable for aquatic life. Macroinvertebrate Assessment

On May 28, 2005 a baseline inventory of benthic macroinvertebrates was

conducted at two stations on Sawmill Creek according to NYSDEC biomonitoring protocols for wadeable streams (Figure 40). The lower sampling station (Sawmill Creek – Lower) was a reach starting immediately below several large bank failures and continued downstream a distance of 100 meters. The upper sampling station (Sawmill Creek – Upper) was a reach located between a large wetland and a large constructed pond in a relatively stable section of channel. The reach continued downstream for a length of 100 meters. Both sampling reaches coincided with previous fish sampling efforts. The complete macroinvertebrate assessment report, to include photographs of both sites, is presented in Appendix 4.1. Methods

Physical Habitat

Physical habitat features were collected at each station according to NYSDEC biomonitoring protocols (Bode et al 2002). General habitat descriptors, such as temperature, stream width and depth, velocity, substrate composition and embeddedness, canopy cover and algae cover were visually estimated and recorded. The downstream station was sampled first (10am – 11:50am) while the upstream station was sampled later in the day (12:00 noon – 1:18pm).

Water Quality Analysis

Water quality data were collected at six sites in the watershed on a monthly basis from October 2002 through September 2004. Although these collections were not

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spatially or temporally matched with the May 2005 macroinvertebrate collections, the data nevertheless are valuable in characterizing the stability of Sawmill Creek regarding its seasonal water quality. Water quality sites 1 and 2 were located upstream of the Upper Station, sites 3 and 4 were upstream of the Lower Station and below the pond, and sites 5 and 6 were below both macroinvertebrate sites.

Macroinvertebrate Collection

Benthic macroinvertebrate samples were collected using a traveling kicknet sampling technique according to NYSDEC biomonitoring protocols (Bode et al 2002). Three replicate samples were collected at each sampling station. Each sample was preserved in 95% ethanol, placed in a 500 ml plastic jar and labeled appropriately. Samples were shipped to the EcoAnalysts, Inc. bioassessment laboratory in Moscow, Idaho for sorting and identification.

Laboratory Methods

Upon arrival at the laboratory all samples were inventoried and inspected for leakage, damage, and adequate preservative. The samples for this project were all in good condition and none were desiccated or damaged.

For each sample, the entire contents (detritus, inorganic substrata, and other materials) were placed in a sorting tray and the larger pieces of debris were rinsed and discarded after any attached invertebrates were returned to the sample. The remainder of the sample was washed with water to help separate invertebrates and organic matter from inorganic sediments. The organic material was drained through a 500-µm sieve; the remaining inorganic sediments were spread throughout the sorting tray and were inspected for any invertebrates too heavy to have been floated off (e.g. mollusks, snails, stone-cased Trichoptera). All invertebrates were placed into the organic fraction of the sample and the inorganic material was discarded.

The organic portion of the sample was evenly distributed throughout a Caton (1991) sorting tray and inspected to estimate the total abundance of organisms in the sample. If the sample contained <100 invertebrates, the entire sample was sorted. If the abundance of invertebrates was >100, the contents were removed from a randomly selected grid and sorted under a dissecting scope. All organisms in the selected grid were saved for identification by taxonomists. If the total number of organisms in the grid was <100, the process was repeated until the total number of organisms removed from the sample exceeded 100. Each grid-square was sorted completely. That is, if the target count was reached “mid-square,” the square was sorted to its completion. This technique often results in >100 invertebrates being extracted and subsequently identified, but it is important because it eliminates any bias toward larger organisms, which may skew the results of a bioassessment.

The proportion of the sample sorted to attain 100 organisms was recorded and used to estimate total invertebrate abundance for the entire sample. The samples were sorted under a dissecting microscope (6X minimum magnification). After sorting was

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complete, both fractions of the sample, (1) organisms and (2) processed detritus, were retained in labeled vials containing 70% ethanol for further analysis. Taxonomists identified the organisms removed from the sample and the processed detritus was re-examined to ensure the sorting efficiency was sufficiently high (see Sorting Quality Assurance below).

Sorting Quality Assurance

Every sample was checked to ensure at least 90% efficiency was maintained in the sorting process. After a lab technician exceeded the target count of 100 or the sample was completely sorted (whichever occurred first), the processed detritus portion from the sorted portion (fraction-2 above) was redistributed into a Caton (1991) sorting tray. The sorted material was evenly distributed and a second technician re-sorted a randomly selected 20% portion of the sample and estimated the total number of organisms missed by the primary technician. The calculations were conducted as follows:

1. Estimating the number of organisms missed: e = (a/b) c

Where: e = estimated total number of organisms missed by sorter, a = the number of organisms found in the 20% resort, b = the number of grids resorted (usually 6), and c = the total number of grids in the Caton tray (usually 30).

2. Estimating the actual total count:

c = a + b Where: c = the estimated total number of bugs in the sorted portion of the original

sample, a = the number of macroinvertebrates picked by the first sorter, and b = the estimated number of macroinvertebrates missed (this is the value for “e” in equation #1).

3. Estimating the percent sorting efficiency:

e = (a/b) 100 Where: e = the estimated percent sorting efficiency, a = number of macroinvertebrates picked by the first sorter, and b = the estimated total number of macroinvertebrates (the value of “c” in equation #2).

If the estimated percent sorting efficiency was greater than or equal to 90%, the

sample passed the QC check. If the estimate was < 90%, the sample failed the sorting efficiency check and was completely resorted. If this occurred, the re-sorted sample underwent the QC process again until it surpassed the 90% efficiency level.

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During the re-sorting process, the secondary sorter examined the sorted invertebrates to determine if any “reject taxa” were present among the sorted organisms. Reject taxa are organisms routinely excluded from specific bioassessment programs. For example, some states exclude groups such as ostracods, mites, or nematodes from the analysis and they are not counted towards the sub-sampling target. Similarly, most states omit terrestrial invertebrates from the samples. Because the removal of these specimens could affect the amount of sample processing required, this evaluation must be conducted before taxonomic analysis to ensure additional sorting is not required. If “reject-taxa” were found, they were removed and discussed with the primary sorter to help them recognize reject taxa in future work. The secondary sorter also inspected labels to ensure all necessary information had been recorded and all the labels matched.

Sample Identification The sciences underlying the process of identifying organisms are taxonomy and

systematics. Taxonomy is the science of assigning correct names to organisms and systematics focuses on the developmental relationships and organization among species and species-groups. Traditional aquatic invertebrate taxonomy uses morphological characters as the primary means of identification. Therefore, an extensive library of taxonomic literature, as well as a reference collection of specimens verified by nationally known taxonomists was used to determine the identity of invertebrates for this project.

Species limits of invertebrates are often defined by the adult male, which has distinct morphological and genitalia characters. Reliable species-level identification of immature stages is often impossible, as larvae of several species within a genus can be physically indistinguishable. Therefore, genus-level determinations are common in macroinvertebrate datasets. Some taxonomists use distributional data in order to put a species name on a specimen; however, it is discouraged because many historical distributions are now outdated and may cause considerable error. Where possible, identifications were made to the genus/species level, including Chironomidae and Oligochaeta.

QA/QC of Taxonomic Identifications

The project taxonomist established a voucher collection consisting of at least one

good specimen (preferably 3-5 specimens) of each taxon encountered. This collection was reviewed by a second taxonomist and both taxonomists compared and discussed their results. Biological Assessment

The NYSDEC biological assessment profile condenses a sample of benthic macroinvertebrate taxa and counts down to a single numerical rating of biological condition. This water quality scale ranges from zero to 10, with 10 indicating excellent

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water quality. Four biological metrics are included in this assessment process, and after conversion to a 10-scale value, the average score for the metrics is calculated to generate a final water quality score for the sample or site. Since 3 samples were collected at each site, mean values for each of the four metrics were calculated before converting to 10-scale values.

The metrics used in the biological assessment profile include total species richness, EPT richness, Hilsenhoff Biotic Index (HBI), and Percent Model Affinity (PMA). Species richness, which is the total number of identifiable taxa in the sample, increases when water quality increases. The relationship is not necessarily linear, however, as there can be species replacement in incidences of mild disturbances. Tests of significant difference between the means were calculated for each metric using a t-test assuming unequal variances.

EPT richness is the number of all taxa in the sample belonging to the orders Ephemeroptera, Plecoptera, and Trichoptera. These orders are generally sensitive to pollution and disturbance, so a reduction in the EPT value is an indicator of impairment. This index is particularly good at detecting severe impacts such as toxic effects and low dissolved oxygen, but in nutrient-limited streams the index can actually increase when small to moderate amounts of nutrients are added to a stream.

The HBI is a measure of community tolerance to organic pollution. Each species is rated on a scale from zero (intolerant) to 10 (highly tolerant). A weighted average of all organisms in the sample is calculated to generate the HBI value. Higher values indicate the site has higher organic loading. Headwater streams (first and second order) will naturally have lower HBI values than middle order (third and fourth) streams. Larger streams and rivers will have even higher HBI values. Addition of nutrients at any point along a stream can increase the HBI values above background levels.

PMA is a measure of similarity to a model community that is non-impacted (Novak and Bode 1992). The model community is based upon the relative contribution of seven major taxonomic groups: Ephemeroptera (40%), Plecoptera (5%), Trichoptera (10%), Coleoptera (10%), Chironomidae (20%), Oligochaeta (5%), and Other (10%). By performing a percent similarity (Washington 1984), the test sample is compared against the model. Low similarity is indicative of impairment. Results and Discussion

Physical Habitat Physical habitat characteristics showed some differences and similarities between

the two sites. Water temperature was over 4 degrees warmer at the upstream station, even though air temperature remained constant between the two sampling stations. Water depth was similar, while wetted width at the upper station was nearly half that of the lower station. Average current velocity was slower at the upper station. Canopy cover was high at both stations, ranging from 75% at the lower station to 90% at the upper (Table 23). Percent embeddedness was similar between stations, averaging 40% and 50% at the upper and lower stations, respectively. Substrate composition was similar, with rubble

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dominating the mix, followed by rock gravel, and sand. Very little filamentous algae were observed in the lower reach, while none were present in the upper reach. Diatoms were present on approximately 40% of the substrate in the lower reach and they were rare/absent in the upper (shadier) reach (Table 23).

Upper Station

Macroinvertebrate samples at the upstream station indicated the biological condition at this site was slightly impaired. Although total taxa richness indicated the site might not be impaired, EPT richness, HBI and Percent Model Affinity all indicated the site was in less than optimal biological condition. The average (overall) water quality score for this site was 6.40 (Figure 41).

The upper station was dominated by organisms that were moderately tolerant to organic enrichment. The dominant three taxa at this site were the facultative midge Tvetenia bavarica gr., the tolerant worm Nais variabilis, and the facultative midge Polypedilum aviceps. Collectively, these three organisms accounted for 36% of the total community (when pooling the three replicates at this site). Dominance by these three taxa indicated the site was experiencing some moderate amount of organic loading and dissolved oxygen at this site was somewhat depressed. In addition, the pooled data also indicated a greater percentage of filterers at the upper versus lower site (12% vs. 4%) and 7% of the community composition were the filter feeding black flies Simulium sp.

The HBI values at the upper station ranged from 5.8 – 6.25, with an average value of 6.00 (Figure 42). These values are above the range expected from non-impaired, small order streams, which further indicated the site was affected by organic loading. Mean HBI values were significantly different between the upper and lower stations (p=0.00439, Appendix 4.1).

Total taxa richness at the upper station averaged 32.33 taxa per replicate sample (Figure 43). After pooling all three replicates, a total of 57 taxa were found at this site. Taxa richness was not significantly different between the two sites (p=0.2043, Appendix 4.1). Of these taxa, there was an average of 6.33 EPT taxa per replicate and a total of 11 EPT taxa (Figure 44). EPT taxa richness was significantly different between the sampling stations (p=0.02860, Appendix 4.1). Taxa richness and EPT richness values fell within the range of non-impaired wadeable streams. Of all the individuals in the samples, an average of 14.32% was EPT taxa.

The PMA values ranged from 45.3 – 55.8%, and indicated the site was either slightly or moderately impaired (Figure 45). PMA values were significantly different between the upper and lower stations (p=0.00496, Appendix 4.1). The model riffle community from non-impaired wadeable streams normally consists of 55% EPT individuals. As stated above, the upper station had only 14.32% EPT individuals, which is a significant deviation from the model. Variability among sample replicates indicated the field sampling methods yielded samples that were representative of the benthic community. Percent similarity between the replicates ranged from 63 – 68% with a mean of 65%. These values indicated there was a high degree of similarity in the taxonomic composition of each representative sample. Values less than 50% would indicate a need to resample the site

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(Bode et al 2002). In contrast, the similarity of replicates between sites was 24 – 34%, indicating the two stations were biologically distinct (Figure 46). The difference between the sites was statistically significant (p<0.0001, Appendix 4.2).

Lower Station

Biological condition at the lower station fell into the non-impaired condition category. Although EPT richness was less than optimal, total species richness, Hilsenhoff’s Biotic Index and Percent Model Affinity all indicated the site was not impaired. The average water quality score for this site was 7.86 (Figure 41).

The lower station was dominated by organisms less tolerant to organic pollution than the upper station. The dominant three taxa at this site were the facultative mayfly Baetis tricaudatus, the intolerant stonefly Leuctra sp., and the facultative caddisfly Hydroptila sp. These three organisms accounted for 53% of the community (when pooling the three replicates at this site). The presence of Leuctra sp., which requires cool water and high dissolved oxygen to survive, as one of the dominant taxa indicated organic loading was not significantly limiting the macroinvertebrate community at this site.

The HBI values at the lower station ranged from 4.41 – 4.53 with an average value of 4.45 (Figure 42). These values were within the range expected from non-impaired, small order streams. The HBI values provided further evidence that organic loading was not a significant issue at this site.

Total taxa richness at the lower station averaged 27 taxa per replicate (Figure 43). After pooling all three replicates, a total of 43 taxa were found at the site. Of these taxa, there was an average of 9.33 EPT taxa per replicate and a total of 15 EPT taxa (Figure 44). Taxa richness and EPT richness values were slightly less than those found in non-impaired wadeable streams. Of all the individuals in the samples, an average of 64% was EPT taxa.

The PMA values at the lower station ranged from 78.8 – 86.9%, and indicated the site was non-impaired (Figure 45). Percent EPT individuals were somewhat higher than the model value of 55%.

Variability between replicates indicated the field sampling methods yielded samples that were representative of the benthic community. Percent similarity between the replicates ranged from 54 – 78% with a mean of 64%. These values indicated there was a high degree of similarity in the composition of each representative sample (Figure 46).

Water Quality Results From the 28 original water quality parameters collected monthly, 10 parameters

were selected to better define possible impacts upon the macroinvertebrate communities at the two sites. Even though the water quality and macroinvertebrates were not sampled concurrently (10/2002-9/2004 vs. 5/2005) nor at the same locations, the water quality data are valuable in characterizing the entire watershed. A summarization of the 10 parameters and their mean, minimum, and maximum values are presented in Table 24. Five of these (temperature, turbidity, dissolved oxygen, pH, and total suspended solids)

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are basic water quality measures and five are nutrient- based (total nitrogen, organic nitrogen, nitrates, ammonia, total phosphorus). All of these are known to influence macroinvertebrate community structure.

A fairly constant temperature and pH condition based on the monthly collections was observed (Table 24). Most notable are the dissolved oxygen concentrations at sites 1 and particularly 2 where excursions below 5 mg/l occurred in the summer months of 2003, with values falling below 2 mg/l. Turbidity concentrations were greater than 5 NTU above the man-made pond but were reduced to ~3 NTU below, likely due to settling of solids. The total suspended solids data also reflected the presence of the pond but site 5 showed a marked increase of over 200 mg/l in August and September 2003. The nutrient related parameter values were basically reflective of an upstream-downstream reduction in concentration. The headwater wetland and the pond were likely responsible for this condition.

Conclusions

Although physical habitat is generally similar at both sites, the macroinvertebrate communities at each site were different from each other. This pattern indicated there were potential differences in water quality that may be driving the biological variation between sites.

Since the upper stream site was likely affected by organic loading, one would assume a lower stream station so close would also be affected. However, the macroinvertebrate community data suggest organic loading at the lower station diminished from that of the upper station. One potential cause for this difference is the presence of a large man-made pond located between the two sites, as well as a wetland complex located upstream of the upper sampling reach. It is possible the wetland complex delivers increased organic matter to the upper reach, of which a portion is removed by the pond through simple settling. The selected water quality data for many of the 10 parameters also support these conclusions even though collected in different temporal and spatial settings.

Another potential cause for the differences in biological community structure is water temperature. The upper stream site was warmer than the lower stream site, even though the canopy cover was denser. It is possible a source of groundwater may be entering the stream between the sites and cooling the lower reach. The presence of stoneflies and other sensitive organisms indicated the water temperature tended to be cooler at the lower site. Appendix 4.2– Macroinvertebrate Bioassessment of Sawmill Creek (Complete Report)

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TASK 5: Fluvial Geomorphic Inventories and Assessment

It has been demonstrated that the morphology, or shape of alluvial streams is largely an expression of a dynamic equilibrium between the tendency of the stream to maintain a stable form, and the evolution of the stream channel form in response to changes in the sediment load and streamflow. To understand the form of alluvial streams, fluvial geomorphology considers three primary categories of stream form which include, the streams dimension (cross sectional relationships), plan form (pattern) and profile (slope).

Geomorphic assessments provides the process-based framework to define past and present watershed dynamics, develop integrated solutions, and assess the consequences of restoration activities (FISRWG, 1998). A geomorphic assessment generally includes data collection, field investigations, and channel stability assessments.

A comprehensive and thorough evaluation of conditions of stream bank erosion lies at the heart of stream reconnaissance and stream bank management strategies because it is essential that, prior to the engineering work, all pre-existing instabilities are identified. The stream bank erosion assessment can provide stream managers current erosion information along the entire stream length and the means to prioritize stream reaches for restoration. In addition, the stream bank erosion assessment can provide information on the existing stream structures, function and disturbance factors, and serve as a reference point to compare historical changes with current conditions.

In the Turkey Mountain watershed, the stream bank erosion assessment is conducted in the way of corridor walkover and cross-section monitoring survey. The corridor walkover documents the existing erosion conditions of the stream from Yorktown Heights to the confluence of the New Croton Reservoir. In the cross-section survey monitors morphological changes of specific cross-sections setup on the Sawmill Creek are monitored. The existing stream bank erosion conditions and bank failure mechanisms were documented during the erosion assessment. The analysis of stream bank erosion conditions can help stream managers better understand the pattern of watershed stability and adopt the best strategy to mitigate the existing erosion problems and to prevent possible bank failures.

The objective of the fluvial geomorphic inventory and assessment for Sawmill Creek ass to characterize the current physical condition and develop predictions of future response of the morphology where applicable. In developing the results, findings, and recommendations of this report, a multi-phased and iterative approach was used to inventory and characterize the morphology of Sawmill Creek. This portion of the report was divided into several sections, which correspond to general methods of assessment and analysis used to describe the current state of Sawmill Creek. Major divisions in data collection and organization include watershed characterization, hydrology and flow regime analysis, stream assessments and morphological description, and erosion assessment and monitoring. Watershed Characterization

A watershed characterization of the Sawmill Creek watershed was the initial step in the fluvial geomorphic assessment. The purpose of the characterization was to

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inventory and document the existing conditions of the basin and to use the inventory to refine more intensive future investigations. The watershed characterization has been divided into four main sections to include Valley Classification and Valley Slope, Level I Rosgen Stream Classification, Corridor Walkover, and Historical Trends and Aerial Photograph Interpretation. Assessments were completed using numerous data sources including a combination of remote sensing, field reconnaissance and general field observations. Valley Classification and Valley Slope

The classification of valley type and broad determination of various valley profile slopes can give an initial assessment of stream morphology by the analysis of the landform characteristics. In general, headwaters streams are steeper with a narrower valley floor, and moving downstream, the slope decreases and the valley floor widens. The overall longitudinal profile of most streams can be roughly divided into three zones (Schumm 1977). The three primary zones are summarized in FISRWG (1998) as: Zone 1, or headwaters, often has the steepest gradient. Sediment erodes from slopes of the watershed and moves downstream. Zone 2, the transfer zone, receives some of the eroded material. It is usually characterized by wide floodplains and meandering channel patterns. The gradient flattens in Zone 3, the primary depositional zone.

Rosgen (1996) identifies ten different valley types based on slope, confinement, sediment supply and whether the soils are colluvial, alluvial and/or glacial. Valley morphology although at a broad level, can provide information on typical erosion processes, channel resilience, and recovery potential of the channel after a disturbance. These interpretations can assist in refining general management objectives and planning.

Since stream channel types are closely related to valley morphology, an analysis was performed to delineate the basin into separate areas by slope class, Rosgen valley classification, and the associated or typical Rosgen stream types found within the zones.

Method

A series of GIS data, obtained from numerous sources (Task 1) to include base mapping of aerial photography, hydrography, road maps, wetlands, and topography were utilized in the evaluation of valley type and valley slopes of the basin. A valley alignment was created using 10’ contour interval base mapping obtained for Westchester County. The alignment begins at the confluence of Sawmill Creek and the New Croton Reservoir (Station 0+00) and continues upward through the valley to the watershed divide in Yorktown Heights (Station 147+00).

Numerous digital plannimetric base maps, containing 2’ contour intervals, were obtained from Yorktown in January of 2005. The individual maps were combined to create a single digital plannimetric base map of the entire Sawmill Creek watershed. The existing contour lines were then used to produce a 3-dimensional surface model of the entire watershed.

The valley alignment was applied to the surface model and used to generate a profile of the Sawmill Creek valley in order to assess the general slope characteristics. Distinct changes in slope were noted along the valley alignment and used to divide the

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valley profile into segments. Valley cross sections were then generated to represent each major valley segment.

Summary and Discussion The general shape of a watershed can describe how water and sediment are

transported through the basin. Valley profiles and cross sections can describe natural fluvial processes and provide evidence of more recent accelerated fluvial process through modification of the drainage flow regime.

The watershed has similar terrain characteristics to Valley Type V in the Rosgen (1996) system which he describes a “U-shaped” valley, with valley slopes less than 4%. The stream types most often seen in Valley Type V are C, D, and G.

The valley profile divides the Sawmill Creek watershed into seven distinct segments based upon valley slope (Table 25).

The Sawmill Creek watershed is generally long and narrow, which typically results in an extended natural hydrograph with a lower peak, as compared to similar sized basins of differing shape. The valley slope of the basin, depicted in the Table 25, describes general trend of increasing slope, moving down valley. This pattern is non-typical of lower order drainage basins, where slope generally decreases and the valley floor widens moving down-valley.

In general as stream flows down a watershed, characteristics related to slope, velocity and bed materials decrease, while discharge, sediment storage all increase. In contrast, the Sawmill Creek slope, bed materials, discharge and velocity increase toward its confluence with New Croton Reservoir. Sediment storage is reduced through its length. The rate of change in these features is representative of watershed factors to include hydrologic regime, soils, land use and valley morphology.

The valley cross-sections reveal varying degrees of confinement along the valley floor and channel floodplain. This pattern is displayed through multiple remnant terraces in the lower basins cross sections. The slopes of these terraces are steeper and expected to be more recent in origin.

The upper watershed from the Yorktown Heights to Croton Heights Road has a “U- shaped” valley with a wider valley floor and generally milder slopes. The wider valley floor and gentle slope typically assumes lower erosional forces and increased potential for channel stability. Below the Croton Heights Road bridge, the valley floor narrows considerably, constricting the streams floodplain and belt width. The down valley slope increases dramatically, generating higher energy and the potential for vertical erosion into the landform.

It is suspected that the impingement of the valley walls in the lower basin creates a greater potential for bank failure, where mass wasting potentially results from a relatively small adjustment in channel plan form. These vertical and lateral adjustments are natural process where the stream is attempting to regain the balance between waters energy and channel bank and bed resistance. Typically this process is accelerated by increased urbanization and floodplain modification, and halted when the stream base level meets confining levels of bedrock or the stream develops adequate floodplain and channel dimensions.

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Level I – Stream Classification

Level I classification provides an initial sorting of stream types within a study area based primarily upon remotely sensed data. The results of a Level I classification can be subsequently verified by a site reconnaissance, whereby select portions of a stream system are investigated for further evaluation and refinement. The Level I classification may be thought of as a preliminary application which, in addition to providing the foundation for subsequent levels of evaluation, also prepares and familiarizes the investigator with the landforms and stream types to be encountered. The Rosgen classification scheme is a hierarchical based assessment of the basin and stream channel morphology, which can be used to: 1. Predict a river’s behavior from its appearance;

2. Develop specific hydraulic and sediment relationships for a given stream type and its state;

3. Provide a mechanism to extrapolate site-specific data to stream reaches having similar characteristics; and

4. Provide a consistent frame of reference for communicating stream morphology and condition among a variety of disciplines and interested parties (Rosgen, 1996).

Method Aerial photography and topographic data were utilized in the evaluation of Level I

stream typing of the basin. USGS topographic data were used to sample slopes, sinuosity and valley morphology for use within Rosgen Level I classification keys. Additionally the Level I classification was refined during subsequent site visits and GPS field reconnaissance.

Analysis Sinuosity of the Sawmill Creek reflects the general slope of the valley constraints

as well anthropogenic impact. The current stream alignment contains three generic plan-form morphologies; relatively straight, and meandering with high and low sinuosity.

Remote sensing and field reconnaissance determined 24 divisions of stream type, which include 5 major categories of stream type. The spatial extents of Rosgen B, C, E, F, and G types have been provided within the GIS database and summarized as follows:

The “B” Stream Type:

The “B” stream type exists predominantly in moderate relief landforms with narrow, gently sloping valleys. “B” stream types are moderately entrenched; display a low channel sinuosity and exhibit bed morphology dominated by rapids with infrequently spaced pools. Typically the dominant channel slope ranges between of 2 - 4% (Rosgen, 1996).

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The “C” Stream Type:

The “C” stream types are generally located in broad valleys with floodplain terraces constructed from alluvial deposition. Their channels have a well-developed floodplain, are slightly entrenched, are relatively sinuous with a channel slope of 2% or less, and possess bed morphology indicative of a riffle/pool configuration (Rosgen, 1996).

The “E” Stream Type:

The “E” stream types are generally located within broad flat valleys composed of alluvium. “E” channels are primarily found in low gradient meadow and wetland features and tend to have meandering riffle/pool morphology with low width/depth ratio and little deposition. They tend to be very efficient and stable when banks are well-vegetated (Rosgen 1996).

The “F” Stream Type:

The “F” stream types are the classic entrenched and meandering channels that are evolving towards reestablishment of a functional floodplain inside the confines of the existing channel. These stream types are incised in gentle gradient valleys containing highly weathered rock and/or erodible materials and are generally laterally unstable. “F” channels typically contain riffle/pool sequence channels with a high width to depth ratio (Rosgen 1996).

The “G” Stream Type:

The “G” or gully stream type is an entrenched, narrow, and deep, step/pool channel with a low to moderate sinuosity. Channel slopes are generally steep, although “G” channels may be associated with gentler slopes where they occur as down-cut gullies in meadows. Although these stream types can occur in a variety of land types to include alluvial fans, debris cones, meadows, or channels within older relic channels, typical landforms include narrow valleys or deeply incised alluvial or colluvial materials. “G” stream types are generally unstable, with grade control problems and high bank erosion rates (Rosgen 1996).

The determined Level I stream types were consistent with the landform and slope

ranges inventoried. Historical aerial imagery and field reconnaissance provided detailed evidence that anthropogenic actions influenced the current stream channel types inventoried. Several locations containing “artificial” floodplain fill were inventoried, which was a common practice within historic development. The effects result in channel evolution generating accelerated vertical and lateral erosion. The result is entrenched, unstable stream types, as typical “F” and “G” type.

The Sawmill contains approximately 725 m (2,380 ft) of “F” type channel and 131 m (430 ft) of “G” type. These sections comprise 24% of the total length of Sawmill

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Creek. These reaches potentially have negative physical and biological impacts to both upstream and downstream reaches in comparison to more ideal channel types.

Several sections of “E” stream channel were identified in the corridor, existing in the wetland complexes located in the upper portion of the valley. Three separate “E” type reaches were identified, which represent 19% of the Sawmill Creek length. As discussed, the “E” type channels are typical stable, when stream banks are well-vegetated, and can function to provide substantial physical, chemical and ecological benefits to the resource. Protection of the “E” channel types and adjacent associated floodplain areas can be a viable fundamental management approach, which will be discussed in future tasks.

The “C” stream type is a sinuous low relief channel containing characteristic point bars within the active channel. The overall stability of “C” type channels relies heavily on the density and vigor of riparian vegetation and the stability of neighboring reaches. General riparian management and stormwater controls are necessary for stability of sediment and water regimes. “C” type channels occur in 11 different reaches making up the majority of Sawmill Creek. Nearly 47% of the stream and approximately one-mile of its length includes “C” type stream channel.

“B” stream types are typically located in narrow valleys that limit the development of a wide floodplain. Streambank erosion rates are typically low in the “B” stream type as are rates of aggradation and degradation. The “B” type comprises 10% of the length along Sawmill Creek, and was identified within two reaches.

A number of Level I divisions were identified as having both “B” and “C” type channel characteristics which will be refined during the Level II classification. Historical Trends and Aerial Photograph Interpretation

The transition of a watershed from the natural, forested state to a predominantly urban condition includes the removal of vegetation and canopy, compaction of soils, creation of impervious surfaces, and alteration of natural drainage networks. Depending on the degree of watershed impervious cover, the annual volume of storm water runoff can increase by 2 to 16 times its predevelopment rate, with proportional reductions in ground water recharge (Schueler 1995). These actions result in increased surface runoff and sediment supply. Typically these changes stimulate a geomorphic response, commonly resulting in enlarged, unstable channels. In many regions of the country, as little as 10% watershed impervious cover has been linked to stream degradation, with the degradation becoming more severe as impervious cover increases (Schueler 1995).

Aerial photographs provide an excellent resource to help document and understand watershed and landscape interaction. Comparisons between a series of historical and recent imagery can be used to investigate the changes and impacts relating to land use, vegetation, and general development, as well as correlate changes in stream channel morphology. They are especially well suited for studies of change in stream channel plan form because the stream banks are usually reasonably clear, even on single aerial photos or orthophotos.

Two assessments of aerial imagery were initiated for the Sawmill Creek watershed. The first assessment attempted to compare stream channel plan form alignment over a series of historical imagery to assess changes in alignment and

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document lateral rates of erosion. The second assessment included an assessment of major watershed changes to document changes in land use and development.

Method

A series of historic and aerial photography was acquired for the Sawmill Creek watershed, containing photographs from 1974, 1985, and 1994. The photographs were correlated to an April 2000 series of high resolution digital orthoimagery by Westchester County, set in the base projection of: NAD 83 New York State Plane, East Zone, U.S. Foot. In addition, a 1995 digital orthographic quarter quad was transformed into the working projection for use in the analysis.

Digital orthographic imagery combines the image characteristics of a photograph with the geometric quality of a map. Unlike a standard aerial photograph, relief displacement in the in the orthophoto has been removed so that ground features are displayed in their true ground position. Correlating refers to the process of positioning an image to a known coordinate system so that the scale, rotation, and coordinates match a defined set of units with position.

Image distortions are typically present in aerial photography and caused by aircraft tilt, camera distortion, and unevenness of terrain. Additional distortion can occur during the image scanning process. In order to reduce the amount of distortion in the aerial photographs each image was independently matched using two fixed points that were represented on both the historical image and the 2000 digital orthoimagery. This procedure was used to match each historical photograph to the scale and rotation of the digital orthoimagery.

The individual images were then correlated using a 3rd degree polynomial transformation so that a group of control points existing on the source image matched the corresponding control points on the destination image, which effectively “warp” the scanned images to match the control image. A minimum of fifteen points were used in the transformation and were selected from matching areas which immediately surround the Sawmill Creek and its watershed. This process typically corrects most of the multiple distortions and results in a final image set that is effectively correlated to the existing digital orthoimagery. The images were then “cropped” around the watershed area and exported with the corresponding base projection of: NAD 83 New York State Plane, West Zone, U.S. Foot, for common use among GIS programs (Task 2).

An attempt was made to digitize the stream channel alignment on each image using the visible water surface edge as a boundary, but the size of the stream channel and the dense vegetative overstory did not allow for completion of this assessment.

Analysis

A comparative analysis was performed using historic and recent aerial imagery for the Sawmill Creek watershed. Images from 1974, 1985, and 2004 were compared for changes in general land use and vegetation, Appendix 5.1. Major features on the 2004 imagery were inventoried to include the utility crossing, golf course, railroad bed (bike path) and “developed areas” which included residential, commercial and industrial areas.

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The utility crossing and golf course were both present on the earlier imagery, indicating no change over the past 30 years. The railroad bed, running along the eastern side of Sawmill Creek, was converted into a multiple use bike path. Vegetative apparently remained unchanged over the series, except in areas where land use changed through development, converting open space areas to developed areas.

A more comprehensive comparison was made between developed and open space areas. A boundary was created surrounding all of the developed and open space areas on each of the aerial images and areas were calculated for each distinct region, Appendix 5.2.

In 1974, open space accounted for more than 227 ha (560 ac) of the watershed area, which reduced to approximately 170 ha (420 ac) in 1985 and 154 ha (380 ac) in 2004 (Table 26). Between 1974 and 1985, a significant increase in watershed development resulted in a decrease in open space of 13.7%. Likewise between 1985 and 2005, watershed development resulted in an additional 4.2% decrease in open space.

In the thirty year period covered by the aerial imagery, total open space was reduced by 75 ha (186 ac), 17.9%. The corresponding average annual increase in developed areas totals 2.5 ha (6.2 ac) per year or 0.6% per year. Summary of Watershed Characterization

The Turkey Mountain watershed has similar terrain characteristics to Valley Type V in the Rosgen (1996) system. The watershed is generally long and narrow, with a pattern of increasing valley and channel slope to its confluence with New Croton Reservoir. The watershed has varying degrees of confinement along the valley floor and channel floodplain. The upper watershed from the Yorktown Heights to Croton Heights Road has a “U shaped” valley with a wider valley floor and generally milder slopes. The wider valley floor and gentle slope assumes lower erosion forces and increased potential for channel stability. Below the Croton Heights Road Bridge, the valley floor narrows considerably, constricting the streams floodplain and belt width. The down valley slope increases dramatically, generating higher energy and the potential for vertical erosion into the landform. The impingement of the valley walls in the lower basin assumes a potential for bank failure, and mass wasting by relatively small adjustment in channel plan-form.

The Sawmill Creek was delineated into twenty-four individual Level I stream reaches, including “C”, “B”, “E”, “G” and “F stream types. The majority of the stream length was classified as stream types as “C” and “B” type channels with “F” and type “G” type comprising 24% of the total length of Sawmill Creek. Several sections of “E” stream channel were identified in the corridor, predominately existing in the wetland complexes located in the upper portion of the valley. Stream typing correlated with valley typing, and identified natural and historic anthropogenic valley and floodplain constraints. Aerial assessment characterized 63% the Sawmill Creek watershed as developed. A time series comparative analysis of aerial imagery determined total open space was reduced by 75ha (186 ac), or 17.9% in the past 30 years. Developed areas averaged 2.5 ha (6.2 ac) per year. A reduction in the rate of conversion of open space to developed area was noted in recent years. The reduction is assumed to have occurred for numerous

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reasons including existing limitations on available developable areas and economics trends. Hydrology and Flow Regime Analysis

A hydrological analysis was conducted to assess the likely flow conditions within areas of Sawmill Creek for use in the fluvial geomorphic assessment, as well as to assist with future management and planning efforts. Because measured stream flow data does not exist for Sawmill Creek, several alternative sources of information were used in the analysis, including regional regression equations; regional USGS gauging stations with similar drainage areas, field evaluations of bankfull indicators, bankfull regional curve data, and hydrologic and hydraulic modeling.

Although multiple discharge values provide the necessary range of information for design purposes, specific discharge values have proven useful for geomorphic assessments and stream classification. Channel morphology and sediment transport have been shown to be highly correlated with bankfull flow. Long term bed load and flow measurements have shown that it is the bankfull flow that transports the greatest amount of material over time (Leopold, 1994). Bankfull flow is considered to be the channel forming discharge, which relates to the prominent development and maintenance of a streams morphologic form, and generally corresponds to a recurrence interval between one and two years. For the purposes of the geomorphic assessment, the hydrologic assessment is primarily focused on bankfull flow and corresponding hydraulic geometry parameters.

Regional Regression Equations

Regional regression equations, created for New York State by theUSGS (Lumia, 1991), were used to generate estimates of flood peak discharges for various return intervals in Hydrologic Region 3. The drainage area (A) at several locations long Sawmill Creek were inserted into the “drainage area only” regression equations to estimate flood discharge values for use during initial site assessments (Table 27). The values presented include a Bias Correction Factor, as well as high and low estimates generated from standard errors of prediction. Although typically the return interval for bankfull is related to the 1.5-year recurrent event, these equations are limited to a two-year (Q2) equation for frequency. Final values were generated for Sawmill Creek using the full regression equations (Table 28), which incorporate drainage area (A), basin storage ST), mean annual precipitation (P), and basin shape index (SH). Basin storage includes the percentage of the total drainage area shown as lakes, ponds, and swamps as determined from 7.5- or 15-minute topographic maps by grid sampling, planimetering, or digitizing. Mean annual precipitation equals the average value of mean annual precipitation over the basin of interest, determined from Plate 1 in the USGS Water-Resources Investigations Report #90-4197. Basin shape index is the calculated ratio of the square of the main-channel stream length, in miles, to drainage area, in square miles (ratio of basin length to average basin width).

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Bankfull Regional Curve Data

Dunne and Leopold (1978) presented bankfull hydraulic geometry relationships, otherwise known as regional curves. Since, numerous research efforts have led to the development of regional curves for many areas of the United States. Regional cures have been developed in the eastern states including: Maryland, Pennsylvania, North Carolina, Vermont, and the Catskill region of New York. Currently the USGS is working on completing regional curves for the remaining hydrologic regions in New York. The development of regional curves has been initiated in Regions 4, 4a, 5, 6 and 7, although published data have only been presented for Region 4, 4a, 5 and 6.

The Sawmill Creek watershed lies completely within Hydrologic Region 3, which is not proposed for assessment by the USGS until the year 2007. As a guide during preliminary field investigations along the Sawmill Creek, existing regional curves from three published regions in New York as well as curves developed by David Rosgen for Southeast Pennsylvania and Eastern United States was referenced for comparison to field indicators found along Sawmill Creek. A summary of existing regional curve data used for the assessment is provided in Table 29. Average discharge values calculated above for 1.3 km2 (0.5mi2), 2.6 km2 (1.0mi2), and 3.9 km2 (1.5mi2) drainage areas were compared to corresponding values calculated using the regional regression equations. Discharge values for bankfull were slightly below the 2-year return interval flows, ranging on average between 63% and 71% of the 2-year flow. Regional USGS Gage Stations Analysis

Two primary assessments were conducted at regional USGS gauging stations to assist in the verification of bankfull flow and examine the timing and magnitude of major flood events. The first includes the validation of bankfull flow and involves calibrating field identified bankfull features with the stage-discharge relationships at a stream gauging station and generating comparisons to their equivalent recurrence interval. The second assessment utilized recorded peak flow data to identify the magnitude and timing of major flood events for association to Sawmill Creek.

Bankfull Calibration The recurrence interval method for determining bankfull discharge assumes that

bankfull is the discharge that is responsible for forming and maintaining the channel over time, which generally corresponds to the 1 to 2 year recurrence interval. As mentioned previously, stream flow gauging stations are not located within the Sawmill Creek watershed. As a surrogate for this validation, an attempt was initiated to develop relationships using stream gages located out of the Sawmill Creek watershed, but contained within Hydrologic Region 3.

Station information was obtained for over 100 USGS peak flow gauging stations located within Hydrologic Region 3, Appendix 5.3. The total number of stations was reduced to include only the stations that contain at least ten years of recorded data, have a current period of record, and which are not flow regulated. Three gauge stations were

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selected for analysis based upon drainage area, period of record, and the availability of peak discharge measurement data (Table 30).

The flood frequencies for each site were estimated using the log Pearson Type III - flood frequency analysis, and return periods for each station were determined, Appendix 5.4. The flood frequency analysis for each gauge station was used to determine recurrence intervals corresponding to the 2-year, 5-year, 10-year, 25-year, 50-year, and 100-year floods (Table 31).

Stream gauges that represent a wide range of drainage areas and bound the study reach are preferred for the development of regional curves. The three existing gauges only bound the upper limit of the study area, reducing the ability for direct calibration. Additionally, a field inspection was made at each gauging station and its contributing watershed. Notable differences between land-use, valley slope, vegetative cover and/or runoff characteristics were observed in comparison to the Sawmill Creek watershed, eliminating the ability for establishing abridged regional curves and hydraulic geometry.

Timing and Magnitude of Major Flood Events

Although bankfull discharge has been found to be highly correlated to morphology and sediment transport over long periods of time, large flood events can have significant impact of shape and stability the channel. The timing and magnitude of past flood events are important indicators in the validation of the current channel condition and potential future impacts.

As mentioned previously, stream flow gauging stations are not located within the Sawmill Creek watershed. As a surrogate for this validation, peak flow data for similar sized watersheds located within the hydrologic region were used to relate the timing and magnitude of recent and historical flood events to the Sawmill Creek basin.

Two regional USGS gage stations, located in neighboring watersheds, were selected to review recent flood magnitude and history. Selected gauges include Angel Fly Brook in White Corners, NY and Hunter Brook south of Yorktown, NY. Annual peak flood discharges were obtained from the USGS Water Resources website and plotted for both stream gauges (Appendix 5.5).

The drainage areas to the selected gauges were 4.9 km2 (3.10mi2) and 11.8 km2 (7.42mi2) respectively. Annual peak discharges were recorded on identical dates throughout their periods of record, with large peak flow events occurring on September 16, 1999, June 17, 2001, and September 18, 2004 (Table 32). Discharges measured at the Angel Fly Brook and Hunter Brook on September 16, 1999 are currently the largest peak flows on record with flows equaling 26.0 cms (918 cfs) and 148.7 cms (5,250 cfs) respectively. Field Evaluations of Bankfull Indicators

It is generally accepted that bankfull stage corresponds with the discharge that fills a channel to the elevation of the active floodplain. Numerous definitions exist of bankfull stage and methods for its identification in the field (Wolman and Leopold, 1957; Nixon, 1959; Schumm, 1960; Kilpatrick and Barnes, 1964; and Williams 1978). Field indicators include the back of point bars, significant breaks in slope, changes in

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vegetation, the highest scour line, or the top of the bank (Leopold, 1994). Hydraulic geometry relationships are empirically derived and can be developed for a specific river or watershed in the same physiographic region with similar rainfall/runoff relationships (FISRWG, 1998).

Multiple cross sections were sampled along Sawmill Creek in order to collect hydraulic geometry related to the field identified bankfull stage (Task 5). Field indicators of bankfull stage were recorded during steam reconnaissance and cross sectional surveys where available. Indicators used for the assessment included the top of bank, top of depositional point bars, significant breaks in slope and highest scour line. In areas where indicators were difficult to interpret, or deficient due to past stabilization efforts or obstructions, sections were calibrated and interpreted against hydraulic modeling (see Task 3), regional regression, and regional curve data , as well as compared to upstream and downstream areas.

Hydraulic Modeling

Multiple watershed parameters were used to generate peak discharges and runoff volumes for the Sawmill Creek watershed as part of Task 3. TR-55 methodology was utilized to compute sub-area runoff curve numbers and their corresponding times of concentration. The HEC-RAS computer program was utilized to develop hydraulic rating curves for selected stream and valley cross-sections in Sawmill Creek. HEC-RAS output data were compared at numerous cross sections to assist in calibrating bankfull stage. Stream Assessment and Morphological Description Corridor Walkover Inventories

Three corridor walkover inventories were performed in November 2003, May 2004 and May 2005, which encompassed the stream corridor from Yorktown Heights to the confluence of the New Croton Reservoir. As discussed in the description of Task 2, the objective of the initial collection effort was to locate sites of environmental degradation; subsequent inventories were performed to provide data for spatial and temporal trend analysis. The inventories included the location and information of the following stream corridor features:

• Bed Scour and Bank Erosion, • Rip Rap and Revetment, • Bedrock, • Grade Controls (natural and

manmade), • Bridge and Culvert Crossings, • Debris Blockages, • Potential Stream Classification, • Tributary Confluences, • Monitoring Stations, • Potential Reference Reaches,

• Clay Exposures, and • Notable Channel and Floodplain

Conditions.

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Additionally, potential reference and study reaches were documented for use in

further comparative analyses and Rosgen Level I classification was confirmed. The inventories provided information on channel process. Quantitative data collected and processed included length and height of individual bank failures, bank failure mechanisms, mass wasting, clay exposures, and past stabilization practices. The GIS provided spatial relationships between current and historic impacted areas and current natural and anthropogenic constraints within the corridor. The following sections describe the findings of the inventories.

Bed Scour and Bank Erosion The 2003 corridor walkover inventory identified 22 sections of streambank

exhibiting signs of instability and erosion. The inventory focused on the location of significant erosion to include large banks and banks with evidence of mass wasting. The streambank inventory revealed that the largest magnitude of erosion was limited to a small portion of the study area between station 63+40 and station 108+70.

The 2004 corridor walkover inventory identified 28 sections of eroding streambank. The inventory extended throughout the entire stream length. Large-scale bank erosion was consistent with the 2003 inventory, and was concentrated in the lower reaches of the watershed. The total length of eroded streambank inventoried was 709 m (2,325ft) of the 3529 m (11,578ft) long Sawmill Creek, which equates to 20% of the total stream channel experiencing some form of erosion or 10% total stream bank length. The total area of exposed bank was approximately 2213 m2 (23,820ft2) or 0.2 m2 (2.1ft2) per foot of stream length. The average eroded streambank was 25m (83ft) in length, and 2.9m (9.4ft) in height.

The 2005 corridor walkover inventory identified 18 sections of streambank exhibiting signs of active erosion. The inventory identified erosion concentrated in the lower reaches of the watershed from station 17+00 to the confluence with the New Croton Reservoir. The total length of eroded streambank was 504m (1,653ft) of the 3011m (9,878ft) stream channel inventoried. This equates to 17% of the stream channel experiencing active erosion. The total area of exposed bank was 1810m2 (19,478ft2), with the average eroded streambank being 28m (92ft) in length and 2.7m (9ft) in height. The inventoried stream channel had an average of 0.19m2 (2ft2) of eroded bank per foot of stream. The comparison of the 2004 and 2005 eroded banks of the lower watershed from Old Country Way to the confluence with the New Croton Reservoir revealed a small increase in eroded stream length, increasing from 13.7% to 14.2%. The height and length of the 2004 and 2005 bank erosion inventories are depicted in Figures 47 and 48. Tabular data are located in Appendix 5.6.

Riprap and Revetment

Riprap and revetment included dumped rock, stacked rock, and placed rock. A single stone walled channel section, consisting of a mortared stacked rock was also inventoried in the lower watershed. Riprap and revetment occurred 23 times and totaled 758m (2,486ft) in length through the entire stream length. The majority of the measures

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were located below Old Country Way in the lower half of watershed. The length and stream station location of riprap and revetment are shown in Figure 49. Tabular data are located in Appendix 5.7.

Spatial relationships of bank erosion and location of historic stabilization were generated to provide insight on the effectiveness of past management to control bank erosion. The comparison between the location and length of both eroding and riprap banks in Sawmill Creek are displayed in Figure 50.

This analysis revealed a relationship between the location of current bank erosion and historic attempts to control erosion. In the lower valley, combinations of placed and dumped stone have been used extensively to protect streambanks and infrastructure, primarily along the base of Route 118. Further field investigation revealed the majority of the riprap was located along the right bank of the channel, where the stream alignment impinges on the highway. Additionally, these locations were accessible for these types of treatments from the highway.

Bedrock The 2004 corridor inventory identified five outcrops of bedrock. These sections

have a significant influence on stream form and function, providing vertical and/or lateral control to the channel.

Natural and Man Made Grade Controls

In addition to the five bedrock oucrops, other instances of grade control were identified in the 2004 corridor inventory. These grade controls consisted primarily of manmade stone sills, but also included one wooden sill.

Debris Blockages

Historically, substantial effort has been made to remove woody obstructions from stream channels and floodplains; however it has been found that woody debris can assist in channel stability and biological function. The function and degree of stability provided by debris varies with quantity of debris and with the stream type in which the debris is located. The purpose of the inventory was to document the number, type and extents of debris and obstruction to flows.

The inventories along the Sawmill Creek identified 21 areas containing significant debris which consisted of downed trees and plant materials. In some locations, debris included rubbish and tires. The location and field notes describing the debris are displayed in Table 33. Large debris and potential blockages were inventoried at station 35+90, 81+30 and 105+95.

Stream Classification

Rosgen Level I Stream Classification of the Sawmill Creek was refined during the corridor inventory. The summary has been presented in later sections of the report.

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Tributary Confluences

Tributary confluences included the inventory of both mapped and unmapped drainage paths connected to the mainstem of Sawmill Creek. These confluences included culvert and significant drainage outfalls within the stream corridor. Monitoring Stations

Monitoring stations for more detailed investigation were selected during the initial walkover in 2003. These stations were established and monumented in order to meet future objectives of the study as well as to accurately replicate data for trend and spatial analysis. These sites included habitat and fisheries sampling reaches (Task 4b) and cross section stations for classification and monitoring (Task 5).

Reference Reaches

The reference reach is a stream segment that represents a stable channel within particular valley morphology. Reference reaches can be used to develop natural channel design criteria based upon measured morphological relations associated with the bankfull stage for a specific stable stream type.

Two reaches were identified during the corridor walkover as potential reference reaches. These reaches were selected due to the apparent enhanced physical condition and function to include minimal erosion and apparent general channel stability, physical habitat, and a dense supporting vegetative riparian community.

The reaches are located at Station 23+50 and Station 46+50, upstream and downstream of the impoundment structure in the upper watershed. The upstream reach is approximately 219m (720ft) long and field classified as Rosgen C-type. The 213m (700ft) long lower reach was field classified as both Rosgen C and E-type reaches.

Clay Exposures

Clay exposures were included in the inventory to identify potential sources of fine suspended sediments. A single location was identified containing clay within the channel boundary. The clay exposure was inventoried downstream of the impoundment at Station 33+75 and located in the bottom of the channel.

Notable Channel and Floodplain Conditions Notable channel and floodplain conditions included locations with evidence of channel aggradation and degradation, channel scour and head cutting, as well as beneficial and sensitive areas such as wetland areas. Locations of these features are presented within the GIS database.

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Level II Stream Classification

Level II classification uses field measurements taken along specific channel segments to produce a finer resolution of stream classification, in comparison to Level I classification. The ultimate goal of Level II data is to provide the baseline information needed to address questions of sediment supply, stream sensitivity to disturbance, potential for natural recovery, channel responses to changes in flow regime, and fish habitat potential (Rosgen, 1996). The Level II criteria are based on measurements of entrenchment, width/depth, sinuosity, channel materials, and channel slope. A summary and description of each is provided.

Entrenchment is description of the degree of vertical containment of a stream channel and is represented by the relationship between the width of the active bankfull channel, and the immediate floodplain. To evaluate a stream’s entrenchment ratio, the channels flood prone area (FPA) must first be determined. The flood prone area is identified as the width of the active channel at the elevation, which is twice the maximum bankfull depth. Bankfull stage is determined, and the maximum depth is doubled to determine the elevation of the flood prone area. In many streams, the flood prone area is represented by large, broad floodplains typical of the flatter valley floors. In other streams, the flood prone area may be narrower, or more confined due to valley morphology or in many cases manmade features. The ratio of the flood prone channel width to the bankfull channel width is expressed as the entrenchment ratio.

The width/depth ratio is the ratio of the bankfull width to the mean depth of the bankfull channel. Determining the width/depth ratio provides a rapid visual assessment of channel stability because of its ability to suggest potential sediment transport capacity. High width/depth ratios (i.e., shallow and wide channels) place stress on the near bank region. As ratios increase even further, the hydraulic stress against the banks also increases and erosion is accelerated.

Sinuosity is the ratio of stream length to valley length. Typically sinuosity is determined through the interpretation of recent aerial photography, coupled with measurements of the stream length divided by that occurring within the specific valley length.

Channel materials determine the extent of sediment transport and provide the resistance to hydraulic stress. This parameter is measured using a method presented in Rosgen (1996) referred to as the modified Wolman method (Wolman, 1954). A segment of the stream equating to 20 to 30 bankfull widths is surveyed for the frequency of riffles, pools, steps, runs, etc. Then the bed material sampling locations are adjusted so that the bed features are sampled on a proportional basis along this segment. Proportional sampling is essential for determining channel material size since bed features exhibit specific particle sizes.

Channel materials are typically determined using particle size as a determinant. Numeric indicators are used to classify the particle sizes of the channel materials. These indicators are 1 (bedrock), 2 (boulder), 3 (cobble), 4 (gravel), 5 (sand), and 6 (silt-clay).

The slope of the water surface of a stream plays a major role in determining its channel morphology and associated sediment, hydraulic, and biological functions. A longitudinal profile along a stream segment is the preferred method for determining

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slope. This approach is accomplished by measuring the difference in water surface elevation per unit stream length, usually a minimum of 20 channel widths in length.

Method

A dichotomous key approach is used to delineate streams from the Level I classifications to the Level II classifications. The methods described are those presented by Rosgen (1996) and subsequently used for this assessment.

Twenty-eight sections were established for classification along the main stem of Sawmill Creek during the initial stream reconnaissance. Cross sectional data were collected by surveying the channel geometry using total station. The dense vegetative over story combined with the relatively small size of the stream channel, did not allow for adequate use of aerial imagery for determining sinuosity. Alternatively, measurements were generated from a 2-foot resolution contour map of the watershed, provided by the Town of Yorktown Heights. Average channel slope at each cross section was also sampled from the contour map using the stream alignment for distance and elevations sampled at both 76m (250ft) upstream and downstream.

Summary and Discussion

Fourteen distinct Level II stream types were observed along Sawmill Creek at twenty-eight cross sections (Table 34). Stream types inventoried included B, C, E, F, and G-type which are all commonly found within the valley type and watershed setting.

Entrenchment ratios measured in Sawmill Creek varied from 1.1 to 11.4, indicating a significant range in flood prone channel width to the bankfull channel width. In general, the majority of stream channel above Croton Heights Road was inventoried as slightly entrenched with a stronger floodplain connection in both wetland areas. The reach connecting the wetlands between Old Country Way and the crossing under Route 118 was entrenched. Below Croton Heights stream channel was inventoried as moderately entrenched with a range of 1.4 to 2.2.

The width/depth ratio varied throughout its length with an average ratio of 14. This value indicates a predominately moderate to high relationship to the classified stream types.

Measurements of sinuosity of Sawmill Creek were consistent with both valley and anthropogenic constraints identified within the Level I characterization as well as changes in average valley slope. In general, the stream from Yorktown Heights to Croton Heights Road is less constrained and had lower valley slopes and higher sinuosity measurements. Below the Croton Heights Road Bridge, the valley slope increases and the valley floor narrows considerably, constricting the streams floodplain and belt width. Sinuosity measurements were generally low.

Channel sediment materials were predominately cobble and gravel correlating strongly with measurements of stream channel slope. Sand and silt were measured in flatter areas including the wetland complexes in the upper watershed.

Channel slope through the Sawmill Creek generally increases along its length. The upper watershed contained lower slopes in both wetland areas. The reach connecting

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the wetlands between Old Country Way and the crossing under Route 118 was substantially higher slope. Below the Croton Heights Road Bridge, there is a considerably increase in slope with a flattening at the confluence with the New Croton Reservoir.

The following sections describe the Level II classification inventory. Summaries of the stream type assessment are included in the Summary of Stream Assessment and Morphological Description section of this report. The sections and individual measurements used in the classification of the Sawmill Creek are displayed in Table 34.

“B” stream type reaches measured 620m (2035ft) in Sawmill Creek length, equivalent to 17.6% of its length. These conditions occurred in eight individual reaches located predominately in the lower watershed, below Croton Heights Road. The reaches were generally short in length with sediment composition dominated by cobble size materials.

“C” stream type reaches made up the greatest percentage of Sawmill Creek length. These reaches measured 1495m (4,906ft) in length and 42.4% of the entire length. Eleven reaches were delineated occurring throughout the watershed length. The stream type was more prevalent in the upper watershed particularly above and below the impoundment area. Sediment composition of the reaches was predominately gravel and sand with larger sized cobble materials present in steeper reaches.

“E” stream type reaches made up 571m (1874ft) or 16.2% of the Sawmill Creek length. These occurred in five individual reaches located predominately in the upper watershed in wetland complexes, above Croton Heights Road. Sediment composition was predominately finer sand materials. Two reaches were unexpectedly classified as “E” type in the lower watershed due to instability of channel bed and banks. The reaches were generally short in length, are distinguished by “E4b” classification. Sediment composition was predominately gravel materials with a steeper average stream slope.

“F” stream type reaches made up the smallest percentage of Sawmill Creek length. These reaches measured 187m (614ft) comprising 5.3% of channel length. Four reaches were inventoried entirely in the lower watershed. Sediment composition of the reaches was predominately gravel and cobble materials present in steeper reaches.

“G” stream type reaches measured 654m (2,147ft) in length, 18.5% of the Sawmill Creek length. These reaches occurred four times throughout the watershed. The longest reach measuring 355m (1165ft) in length was inventoried in the upper watershed between Old Country Way and where Sawmill Creek first crosses under Route 118. Sediment composition of the reach was gravel. Larger sized cobble materials were present in steeper reaches in the lower watershed with a single bedrock section inventoried near the confluence of the New Croton Reservoir. Level III Stream Classification

As developed by Rosgen (1996), Level III assessments are conducted at the reach scale, and include a more detailed assessment of stream condition. The application of several inventories assists in the description of specific reaches, as well as assesses the potential for instability potential at the reach and station scale.

The reach level assessment of site conditions incorporates riparian vegetation, flow regime, stream size/stream order, depositional features, meander patterns, debris/channel blockages, and stream bank erosion potential. Additionally, a reach

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specific characterization of sediment size by sampling channel substrate and point bars is added to characterize the sediment composition and condition of the reach.

Riparian Vegetation

The composition, vigor, density, rooting depth, and rooting density of riparian vegetation influences water quality, aesthetics, water temperature, bank protection, wildlife habitat, and detritus contribution to the stream. Likewise, stream morphology also influences the type and form of riparian vegetation.

Rosgen (1996) suggests assessing riparian vegetation either through the evaluation of existing vegetation mapping or through field observation. This study elected to conduct field observations to ensure the collection of site-specific and ground-truthed data for each stream segment. An alphanumeric descriptor (e.g., 9b) is used to indicate the type of vegetation (e.g., perennial grasses, deciduous over story, etc.) and its density (e.g., low, medium, high).

The majority of the stream corridor is dominated with large caliper deciduous species at medium to high density with limited under story. Two wetland reaches were inventoried in the upper consisting of shrub composition, and a forested wetland in the middle watershed below the low head impoundment. The upper wetland was dominated with intermittent deciduous tree species inventoried within the flood prone area with grasses and sedge species prevalent near the channel boundary. The middle forested wetland is dominated by wet tolerant deciduous tree species including maple, dogwood, willow and alder at high density.

Flow Regime

Streamflow influences channel morphology, aquatic habitat, and riparian vegetation. General streamflow categories include ephemeral (E), intermittent (I), perennial (P), and subterranean (S). Specific streamflow categories are used to describe flow dominated by snowmelt (1), storm flow (2), glacial melt (3), spring-fed (4), ice flows (5), tidal influence (6), regulated streamflow (7), and streamflow altered by development (8).

The Sawmill Creek is a perennial stream with surface water persisting yearlong. The streamflow regime is dominated by the two of the specific categories: (1) “seasonal variation in streamflow dominated primarily by stormflow runoff”, and (2) “altered due to development, and vegetation conversions that changes flow response to precipitation events.”

Stream Size and Order Stream size is based on bankfull width measured in the Level II analysis. Thirteen

categories are presented to classify generic stream size (S-1 to S-13). Knowledge of the bankfull width allows for the correct perspective in interpreting hydraulic processes, sediment transport, and biological processes.

Stream order is a measure of the relative size of streams and is generally determined using USGS topographic maps. Strahler's (1952) stream order system is a

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simple method of classifying stream segments based on the number of tributaries upstream. A perennial stream with no tributaries (headwater stream) is considered a first order stream. A segment downstream of the confluence of two first order streams is a second order stream. Thus, an nth order stream is always located downstream of the confluence of two (n - 1)th order streams. Stream sizes range from the smallest, first-order, to the largest, the twelfth-order (the Amazon River).

The Sawmill Creek main stem is classified as S-4 (1) and S-5 (2) indicating first and second order stream with a bankfull width range of 4.6-15.2m (15-50 ft).

Depositional Patterns

Depositional patterns provide insight into the effects of past land management on sediment supply and sediment storage. This in turn is related to channel form and stability. For example, delta bars indicate a contribution of coarse materials to the main stream by tributaries. Rosgen (1996) suggests the use of aerial photographs to locate depositional patterns, and then characterize them by using eight (8) types of features. These are point bars (B-1), point bars with few mid-channel bars (B-2), many mid-channel bars (B-3), side bars (B-4), diagonal bars (B-5), main branching with many mid bars and islands (B-6), mixed side bar and mid channel bars exceeding 2 - 3 times the width (B-7), and delta bars (B-8).

Sawmill Creek was inventoried for depositions patterns during field reconnaissance, due to the lack adequate visibility on recent aerial photographs. Small sidebars (B-4) dominate depositional patterns of the Sawmill. Several reaches were inventoried to contain central bar formations located near channel obstructions and blockages, as well as areas where backwater conditions are predominate. Bar formations were noted to be more frequent and larger in size in the lower watershed near eroding stream banks.

Meander Patterns

A meander indicates the way in which a stream adjusts its slope to the slope of the valley within which it occurs. Rosgen (1996) reports that meander patterns are useful for designing channel restoration, interpreting channel adjustment processes, indicating the potential onset of disequilibrium and evolutionary adjustments, and assessing the effects of changes in stream morphological variables such as width/depth ratios. The following eight (8) meander patterns are used to describe this condition: regular meander (M-1), tortuous meander (M-2), irregular meander (M-3), truncated meander (M-4), unconfined meander scrolls (M-5), confined meander scrolls (M-6), distorted meander loops (M-7), and irregular with oxbows and oxbow cutoffs (M-8). Meander patterns in this study were determined through the combined use of field observation, 2ft resolution topographic mapping, and aerial photography.

The Sawmill Creek contains predominately irregular meanders (M-3), and confined meander scrolls (M-6) in the lower basin due to constraints of the valley. The channel above and below the impoundment was inventoried containing a more natural meander pattern due to larger available floodplain.

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Debris and Channel Blockages

The presence of woody debris can profoundly affect stream channel stability, sediment storage, width/depth ratio, bank erosion, aggradation/degradation processes, and wildlife habitat. Debris is categorized by size and extent, and includes structures such as natural and man-made dams since these also obstruct streamflow. Ten (10) categories of stream channel debris/blockages are presented in the Level III analysis.

In summary, the debris and channel blockages are as follows:

D1 = None - minor amounts of small, floatable material;

D2 = Infrequent - debris consists of small, easily moved, floatable material; (e.s., leaves, needles, small limbs, twigs);

D3 = Moderate - Increasing frequency of small to medium sized material, such as large limbs, branches, small logs that when accumulated effect 10% or less of the active channel cross-sectional area;

D4 = Numerous - significant build-up of medium to large sized materials, (e.s., large limbs, branches, small logs or portions of trees that may occupy 10 to30% of the active channel cross-section area.); and

D10 = Human Influences - structures, facilities, or materials related to land use or development located within the flood-prone area, such as diversion or low-head dams, controlled by-pass channels, velocity control structures, and various transportation encroachments that have an influence on the existing flow regime, such that significant channel adjustments occur.

The majority of the stream length contains infrequent and easily moved floatable

debris (D2). Numerous areas were inventoried historically influenced by development within the stream corridor and flood prone area. These include the transportation encroachment and the low-head dams (D10). Several isolated debris jams were inventoried and are discussed in other sections.

Streambank Erosion Potential

Streambanks were scored using the Bank Erosion Hazard Index (BEHI) developed by Rosgen (1996) for the Level III analysis. This index examines bank height, bankfull height, rooting depth, bank angle, and percent surface protection to formulate a numeric score indicating bank erosion hazard categories of very low, low, moderate, high, very high, and extreme.

BEHI scoring was performed on eighteen streambanks located within the lower watershed, in May of 2005. The samples were taken at sites that contained significant bank exposures or evidence of past failures. A summary of BEHI Scores and potential for bank erosion is displayed in Table 35. A summary of each assessment is located in Appendix 5.8.

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Channel Stability Evaluation – Pfankuch

The Pfankuch (1975) system of rating channel stability, modified to incorporate the Rosgen-based classification method, was used to provide an additional evaluation of stream stability. The conventional Pfankuch system assigns a numeric rating for various stability categories, separately for the upper bank, lower bank, and channel bottom. There are 15 stability categories for these three areas. Within each category, a numeric rating is associated with a score of excellent, good, fair, and poor.

Seven stream reaches were inventoried assessed using Pfankuch scoring (Table 36). The majority of the stream length scored a good rating. A poor rating were inventoried in the lower watershed between station 86+50 and 10+850. A summary of each assessment is located in Appendix 5.9.

Altered Stream Channels

Altered stream channels include those that have been straightened, deepened, widened, lined, reshaped, relocated, routed through pipes, tunnels, turbines, and trans-basin diversions (Rosgen, 1996).

Although not directly confirmed as part of this assessment, it is apparent that several areas have been historically altered as evidenced by their relatively unnatural appearance. It is suggested that the stream channel and floodplain were altered at some point in the distant past. It is believed that a combination of channel dredging, straightening, and berming were conducted, presumably in an attempt to provide protection to the once existing railroad, as well as alleviate the rail bed from flooding. In one location, the remnants of possible dam structure or historic bridge crossing remain partially within in the stream channel and span the active floodplain. The locations and further discussion of these areas are location in further summaries.

Channel Sediment

Stream channel sediment data are commonly used for the evaluation of system wide channel process, as well as at specific locations within corridor. The composition of the streambed and banks is an important facet of stream character that influences channel form and hydraulics, erosion rates, and sediment supply. The composition of the substrate influences how streams behave. Steep mountain streams with beds of boulders and cobbles act differently than low-gradient streams with beds of sand or silt. This difference may be documented by a quantitative description of the bed material called a pebble count.

Sediment data were collected within the bankfull channel extents at 34 locations, surrounding the established cross sections (Table 37). Pebble counts were conducted, following methods developed by Wolman (1954) and modified by Rosgen (1996) to include sampling a total of 100 or more pebbles from cross sections throughout the longitudinal reach of the stream. A summary of each assessment is located in Appendix 5.10.

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Erosion Assessment and Monitoring Stream bank failure mechanisms

During the 2004 and 2005 walkover assessment, the stream bank failure mechanisms were identified. Bank failures can be categorized into several groups: shallow slip failure, planner failure, rotational failure, cantilever failure, and piping (Thorn 1982; FISRWG 1998; Fischenich and Allen 2000).

(1) A shallow slip failure occurs when a layer of bank material slides along a failure plane parallel to the bank surface. This failure mechanism is common in cohesionless banks and is mostly triggered when the bank angle exceeds the angle of repose of the bank material;

(2) The planar failure usually occurs in cohesive riverbanks with a very steep slope. Often deep tension cracks separate the failure block from the rest of the bank. This type of failure is primarily triggered by toe scour and tension cracking;

(3) A rotational failure is a deep-seated movement of all or part of the bank profile as it slips downward along a curved failure surface. This type of failure often occurs in highly cohesive banks with a relatively gentle slope. Failure is mainly triggered by toe scour;

(4) A cantilever failure is caused when an overhanging block collapses into the channel. Overhang can be found in layered banks where a resistant, cohesive layer overlies an erodible, non-cohesive layer. The overhang remains in place until a state of limiting equilibrium is reached due to increased width by further undercutting, or weakening by wetting or cracking. The overhang fails by shear, beam, or tensile failure depending on its geometry; and

(5) Piping occurs when the soil particles underneath the bank surface are eroded by flowing ground water. The process of piping depends on the seepage pressure, the chemistry of the pore water and the mineralogy of the soil (Fischenich and Allen 2000). Piping mostly occurs in the bank profile where a sandy layer is in between less permeable clay layers.

Whether bank failure occurs by shallow slip, planar, rotational, cantilever

collapse, or piping depends largely on the properties of bank materials and bank geometries. Noncohesive materials usually fail by the removal of individual particles or by shear failure along shallow, slip surfaces. Deep-seated failures occur in cohesive materials with a block of bank materials sliding into the channel along a curved failure surface. In banks with gentle slope angles, the failure surface is curved and the block tends to rotate back toward the bank as it slides. Steep banks characteristically fail along almost planar surfaces, with the detached block of soil sliding downward and outward into the channel (FISRWG 1998). Cantilevered banks are generated when erosion of a weak layer in a stratified or composite bank leads to undermining of overlying, erosion-resistant layers.

The primary failure mechanisms on the Sawmill Creek are planar failures. Other failure mechanisms identified on the Sawmill Creek are piping, cantilever and rotational failures. The bank materials of those failed banks consist of considerable amount of clays

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and silts. The failure mechanisms on the Sawmill Creek are summarized in Table 38 and the sketch and picture of each failure mechanism are shown in Figure 51. Although most failed banks are local and small in scale, for example, the heights of 90% failed banks are lower than 5m, there are several significant bank failures in the reach stationing from 83+50 to 98+00, where the failed bank is about 10 meters in height and 50m in length.

Cross Section Monitoring

Monitored cross sections were used to observe changes in the stream reach morphology, verify stream conditions, and develop stream bank erosion rates. The data assisted in determining stream bank failure mechanisms, and changes in channel maximum depth, channel cross sectional area, width, width to depth ratio, and entrenchment. Evaluating the rate of change provided information on stream processes impacting stream stability and the spatial extents of river processes. These associations assisted in developing the reach summaries and management and restoration recommendations.

Method

Cross sections locations were selected to represent various stream types, floodplain and channel conditions, and channel features (e.g., pool, rifles, etc.). Sections were established perpendicular to the active channel flow and in most cases spanned the entire 100-year floodplain. Additionally, sections were placed in both visually stable and unstable reaches.

Thirty-five cross sections were established using rebar monuments during an initial site reconnaissance. The rebar monuments were geo-referenced using GPS equipment in November of 2003 (Task 2) and are included in the GIS database and presented in Appendix 2.2.

Total station survey equipment was used to survey station and elevation across each section. Stationing began at 0+00 at the cross section monument located on the left floodplain and progressed in distance to the monument on the right floodplain. An arbitrary elevation of 30.5m (100ft) was set at each left rebar pin, and used as a temporary benchmark. The initial survey was conducted in December of 2003 and January of 2004. Cross sections were resurveyed in May of 2005. Cross sections were drafted using CAD software for common formatting, channel dimensioning and data interpretation. A field summary of each established cross sections, monitored cross sections plots, and summary data tables are provided in Appendix 5.11, 5.12, 5.13. The following provides a summary of the bankfull geometry data.

Bankfull Area

A general pattern of increasing area was present along the channel length, except for areas with significant changes in channel slope and those affected by stream crossings or impoundments. The average bankfull cross sectional area for Sawmill Creek in 2004 was 3.46m2 (37.2ft2) with a slight increase to 3.47m2 (37.4ft2) in 2005. Slight increases in

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both the average riffle and pool areas were observed. The average riffle cross sectional area increased by 0.04m2 (0.4ft2) and pool cross sectional area increased by 0.06m2 (0.6ft2).

The largest single increase in bankfull areas occurred at cross section #14, increasing in area by 25.4% from 5.1m2 (54.3ft2) to 6.3m2 (68.1ft2). Neighboring cross sections, #13.1, #13.2 and #15 also revealed a common trend of increasing area. Further evaluation of the reach determined several debris blockages were inventoried in this location, as well as significant erosion in the area.

The greatest reduction in area occurred at cross section #12, from 3.7m2 (39.7ft2) to 2.7m2 (29.4ft2). Evaluation of the section determined a large bank failure exists at the section, in which bank material slumped into the channel, reducing bankfull channel capacity nearly 26%.

Bankfull Width

Rosgen (1996) states stream width is a function of streamflow occurrence and magnitude, size and type of transported sediment and bed and bank materials of the channel. Channel width can be modified by direct channel disturbance such as channelization, changes in riparian vegetation, changes in streamflow regime due to watershed changes, and changes in sediment regime.

The average bankfull cross section width for the Sawmill Creek increased 0.9% from 6.77m (22.2ft) in 2004 to 6.83m (22.4ft) in 2005. Riffle features indicated an average increase of 1.8% from 6.61m (21.7ft) to 3.74m (22.1ft), however average pool width decreased 1.2%, from 7.44m (24.4ft) to 7.35m (24.1ft). The largest increase in width (14.6%) occurred at cross section #14, increasing from 11.92m (39.1ft) to 13.96m (45.8ft).

The greatest reduction in width (23%) occurred at downstream cross section 15, where width decreased from 8.78m (28.8ft) to 6.77m (22.2ft). An increase in bankfull area was revealed in cross section 15, indicating either a shift in channel feature type or possible streambed erosion. Evaluation of the section determined minor erosion of the channel bed, left bank migration of 0.18m (0.6ft), and increased sediment deposition along the floodplain fringe along the inside of the meander.

Bankfull Depth

Rosgen (1996) states that the mean depth of rivers varies greatly by individual reaches experiencing similar discharges due to the sequence of riffle and pool bed features.

The average bankfull cross-section mean depth for the Sawmill Creek remained consistent for both inventories measuring at 0.52m (1.7ft). Averages for both riffle and pools remained consistent measuring 0.52m (1.7ft) and 0.55m (1.8ft), respectively. The largest increase in mean depth occurred at cross section 15, increasing from 0.24m (0.8ft) to 0.34m (1.1ft). Evaluation of the section determined minor erosion of the channel bed, and a 0.18m (0.6ft) migration of the left bank. The greatest reduction in mean depth occurred at cross section #1, where a 0.15m (0.5ft) reduction occurred resulting from a 0.11m (0.36ft) increase in bed elevation.

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The average of bankfull maximum depths for the Sawmill Creek decreased slightly over the monitoring period from 0.81m (2.65ft) to 0.80m (2.63ft), 0.8%. Both inventories displayed consistent riffle and pool feature max depth. The largest increase in depth (24.9%) occurred at cross section #10.2, from 0.82m (2.69ft) to 1.09m (3.58ft). Evaluation of the section determined a decrease in channel bottom of 0.43m (1.4ft). The greatest reduction in max depth occurred cross section 3, displaying 0.24m (0.78ft) due to an increase in bed elevation.

Width to Depth Ratio

Rosgen (1996) states the width/depth ratio is the most sensitive and positive indicator of trends in channel instability. The average width/depth ratio for Sawmill Creek in 2004 was 14.5, increasing by 1.4% to 14.7 in 2005. Both years represent a moderate to high width to depth ratio. The average width/depth for riffle features increased 2.1%, from 14.3 to 14.6 and decreased in pool features 7.4%, from 16.2 to 15 confirming that generally riffle features are becoming wider and shallower and pool features are becoming narrower and deeper. This pattern was supported by consistencies in trends of both average width and average depth.

The largest decrease in width depth ratio occurred at cross section #15 indicating decrease from 33.7 to 18.7. Significant increases in width/depth occurred at cross sections #1, #22, and #25. Evaluations of cross sections #1 and #25 revealed increases of bed elevation decreasing mean depth, and cross section #22 experienced lateral erosion increasing bankfull width.

Entrenchment Ratio

The average entrenchment ratio for Sawmill Creek decreased from 3.2 to 3.1, due to trends in increasing bankfull widths in 2005. Spatial patterns reveal that the average channel is slightly to moderately entrenched. Decreases in entrenchment ratio occurred at cross section #7, #12, #21, and #25, with increase at cross section #22, this remains consistent with erosion leading to an increase in bankfull width at these sections.

Stream Classification

Level II stream classifications for the 2004 and 2005 monitoring sections were consistent in typing with the exception of one cross section. Cross section #30 changed from an F4 to a G4c due to erosion creating an increase in width/depth from low to moderate.

Channel Deposition

The mean channel deposition for the Sawmill Creek measured 0.53m2 (5.66ft2) between 2004 and 2005, smaller in comparison to average of 0.85m2 (9.15ft2) of erosion. The largest amount of deposition occurred at Section 12 in which bank material slumped into the channel measuring 11.43m (37.49ft). Significant sediment deposition was measured at Section 1 measuring 0.75m2 (8.02ft2). The section is located at the bottom of

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the watershed near the confluence with the New Croton Reservoir. Evaluation of the section displayed the section is located in the backwater area of Sawmill Creek created by the piped crossing under Route 129, Croton Lake Road. Apparent spatial patterns in channel deposition were not observed.

An increase in bed elevation (aggradation) was inventoried at section 3, measuring 0.12m (0.41ft). Evaluation of the section determined both streambanks eroded generating an increase of channel width. The bank erosion and widening depict a reduction of sediment transport capacity confirmed by the increase in bed elevation. Channel Erosion

Stream channel erosion along Sawmill Creek averaged 0.85m2 (9.15ft2) between 2004 and 2005. Patterns in erosion displayed the largest amount occurring between Station 80+00 and 99+00. The largest single measurement was at Cross Section #12 (Station 92+20). The erosion consisted of a high bank failure measuring 7.12m2 (76.6ft2). Erosion and scour along the bank toe was presumed to be a catalyst for the failure. Significant erosion also occurred at adjacent sections (#8, #9, #10.1 and #10.2). Cross sections #8 and #9 displayed evidence of older failure planes running along the same valley terrace feature.

Significant stream channel bed erosion occurred at cross sections #10.2 (Station 94+65), measuring 0.43m (1.4ft). This event was the largest single vertical change in bed elevation throughout the Sawmill. Evaluation of the area displayed the section is located immediate upstream of a hydraulically controlled section of channel. Mortared stone wall confine Sawmill Creek along both streambanks for approximately 46m (150ft). The stream channel inventory documented severely undercut wall footings and scour. Evidence of a head-cut was inventoried just upstream of the cross section.

Stream channel lateral erosion along Sawmill Creek averaged 0.21m (0.7ft) between 2004 and 2005. Lateral erosion along the western banks averaged 0.18m (0.6ft), while the migration along the eastern banks measured 0.24m (0.8ft). The decreased erosion of the western banks correlates with the increased amount of revetment on the western banks.

The largest lateral erosion occurred at cross section #14, measuring 1.10m (3.6ft) and 0.79m (2.6ft) into both right and left stream bank and general channel widening. Significant streambank movement occurred at cross sections #10 and #12 correlating with high erosion rates at the sections, and higher lateral rates from Station 80+00 to 99+00.

This reach contains sections showing significant bank failures and active bank erosion. The evidence of active bank erosion can be seen from the following pictures in Figure 52.

The eroding bank shown in Figure 52 is about 10 m (32.8 ft) in height. The tree marked in Figure 52 (a) was on the top of the bank when the picture was taken in 2004; however, in 2005, it was found that the tree fell into the stream channel due to the active bank erosion (Figure 52b).

There are 11 monitoring cross-sections on the reach stationing from 83+50 to 98+00. The average erosion rate on these cross-sections was 1.72 m2 (18.5 ft2) during the 2004-2005 monitoring season. Among these 11 monitoring cross-sections, #10.2 and #12

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exhibited significant erosion. The erosion amount at these two cross-sections during the 2004-2005 monitoring season is shown in Figure 53. At cross section #10.2, the channel bed was lowered by 0.5 m (1.64 ft) on average from 2004 to 2005 due to active erosion. The erosion occurred on channel bed increases the bank height. When bank height exceeds the critical bank height, mass failure might occur. At cross section #12, slumping occurred along the left bank. The maximum erosion rate measured on the left bank was 2 m (6.56 ft) per monitoring season. The eroded bank material deposits on the lower bank and blocks the channel bed. If the erosion at left bank continues at this cross-section, significant lateral migration might result along the opposite bank. Stream Bank Erosion Modeling

Stream bank erosion can cause severe problems to stream management, environmental protection and agricultural activities through land loss, danger to riparian and floodplain structures, and increased downstream sedimentation. Moreover, stream bank erosion and associated sediment yield may have deleterious impacts on water quality. Studies have shown, for instance, that the bank-derived sediments account for the majority of sediment loads in some urban watersheds (Trimble 1997; Bull 1997; Rosgen 2001). Sediment loads increase turbidity and water temperatures, alter aquatic habitats, and introduce pollutants such as trace metals in surface water (NYSDEC 2001). Therefore, it is important to predict the stream bank erosion on the Sawmill Creek. There are two major approaches to predict stream bank erosion. The first approach, which will be termed here the “statistical approach,” uses statistical models to establish the relationship between the bank erosion rate and various independent variables, such as channel geometry, flow properties, bank material composition, climate, and human influences. The second approach, which will be termed here the “physical approach,’’ employs bank stability analysis and excess shear strength concept to estimate the failure geometry of cohesive banks that become unstable after near bank bed degradation and/or direct fluvial shear erosion at the toe of banks. In this study, two different models were used to predict the stream bank erosion on Sawmill Creek. The first model is the Stepwise Multivariate Regression Model (SMRM) developed by Chen (2005) using data collected from the Batavia Kill watershed, New York. The second model is the Bank Stability and Toe Erosion Model (BSTEM) developed by the United States Department of Agriculture, Agriculture Research Service (USDA-ARS 2003).

The Stepwise Multivariate Regression Model

The SMRM relates the stream bank erosion to a set of influential factors. These influential factors can be categorized into several groups: (1) cross-sectional and longitudinal characteristics; (2) parameters of flow conditions; (3) rainfall conditions; (4) temperature conditions, primarily the influence of frost; (5) vegetation and soil erodibility; and (6) sediment characteristics. Each group of influencing factors contains variables that may affect stream bank erosion rates. In total, 20 explanatory variables are considered in the SMRM (Table 39).

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The variables needed in the SMRM are collected for the Turkey Mountain watershed. The cross-sectional and longitudinal characteristics are directly measured from the cross-section survey and aerial photographs. Since the Turkey Mountain watershed is un-gauged, drainage-area ratio method is used to estimate the discharge for Sawmill Creek. Angle Fly Brook watershed (41°16' N, 73°43' W) is used as the index watershed. Discharge data for Angle Fly Brook are obtained from USGS real time discharge values (01374976). The estimated mean daily discharge for the Sawmill creek from 1/1/2004 to 5/30/2005is shown in Figure 54.

The highest discharge during the 2004-2005 monitoring season was 2.82cms (99.71cfs) occurred on 9/18/2004. This discharge approximately corresponds to a storm event with a 2-year return interval (Q2, Refer to Table 17, Table 18 of interim report). Other variables needed in the SMRM are also collected from various resources.

The Bank Stability and Toe Erosion Model The BSTEM simulates two physical processes. One process is the failure by shearing of a wedge-shaped block, and the other process is the erosion on bank and bank toe caused by stream flows. The BSTEM therefore consists of two sub-models to account for these two processes: the Channel Bank Stability model (CBS) and the Bank Toe Erosion model (BTE) (USDA-ARS 2003). The CBS model is a wedge-based limit equilibrium model that calculates the Factor of Safety (Fs) for heterogeneous banks with up to five different soil layers. The BTE model computes stream bank and bank toe erosion resulting from stream flows. The BSTEM considers the effects of positive and negative pore-water pressures, soil stratification, vegetative coverage, as well as the bank and bank toe protection. The CBS model requires the user to input the bank profile, soil types, vegetation cover, water table, and other relative information, and then employs the following equation to calculate Fs.

∑ −−

−+−+∑ +=

]sin[sin

'tan)]cos(cos[]tan['

βαβ

φβαβφ

iPiWiiPiUiWb

iiSiLicFs

Where ci' = effective cohesion of ith layer (kPa); Li = length of the failure plane incorporated within the ith layer (m); Si = force produced by matric suction on the unsaturated part of the failure surface

(kN/m); φi

b = a parameter accounts for the increasing soil strength with increasing matric suction;

Wi = weight of the ith layer (kN); Ui = the hydrostatic-uplift force on the saturated portion of the failure surface

(kN/m); Pi = the hydrostatic confining force due to external water level (kN/m); α = failure-plane angle (degrees from horizontal); β = bank angle (degrees from horizontal) and φi' = the effective friction angle of ith layer (°).

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The bank is said to be “stable” if Fs is greater than 1.3, thereby providing a safety margin for uncertainties or variable data. Banks with an Fs value between 1.0 and 1.3 are said to be “conditionally stable”, while banks with an Fs value less than 1.0 are unstable (USDA-ARS 2003). The bank failure plane is computed for banks with an Fs value less than 1.3.

The BTE model allows the user to input the bank profiles, flow parameters, channel slopes, bed, bank and toe material types and erosion protection, if any. The model calculates boundary shear stresses from the channel geometry, and considers the critical shear stress and erodibility of materials on channel bed, bank toe, and bank. The model determines how much erosion may occur given the prescribed hydraulic conditions.

The bank profiles are derived from field survey data. All banks are divided into five layers to improve the computation accuracy, which is recommended by the model because it calculates pore water pressure and unsaturated soil weight as an average for the middle point of each layer. Engineering properties like friction angle, cohesion, critical shear stress and erodibility of bank, bank toe and bed materials are chosen from the representative values given in the model based on the field survey notes recorded for each bank. BSTEM assumes a fixed bed when calculating the bank toe erosion. Values of these engineering properties used in the model as listed in Tables 40 and 41.

Annual mean flow estimated for the Sawmill Creek near the bank of interest is used as the input stream flow corresponding to a particular survey season, and the flow duration is set as one year (8760 hours) so as to estimate the annual erosion amount. The vegetation and pore water pressure information are also input into the model, whenever appropriate. The eroded area on the bank and bank toe can then be calculated using BSTEM. The total annual soil erosion from a bank is the sum of failed soil profile estimated from the bank stability model and soil erosion calculated from the toe model. Stream Bank Erosion Prediction

Since most active eroding banks are located on the reach stationing from 83+50 to 98+00, the SMRM and BSTEM were applied on this reach to predict the annual erosion amount produced from this reach given certain conditions. The BSTEM was applied on 11 banks on the reach stationing from 83+50 to 98+00 on the Sawmill Creek. Bank erosion showed in Figure 52a and Figure 52b are both located in this section. The erosion amount measured during 2004-2005 monitoring season at this section averaged 1.72 m2/m along the stream length. The annual bank erosion estimated using Stepwise Multivariate Regression (SMRM) model and USDA-ARS model is given in Table 42. The estimated amount of erosion using SMRM and USDA-ARS model is very close to the measured average erosion. The predicted erosion for 2005-2006 monitoring season using SMRM model and USDA-ARS model are 1.55 m2 and 1.61 m2 respectively, which means that more than 700m3 eroded material will be produced from this section.

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In general, the Turkey Mountain Watershed is in a stable condition, but has

several mass wasting failures on the stream bank. Among the 35 monitored cross-sections, there are 3 cross-sections (10.2, 12, and14) showed excessive erosion over the 2004-2005 monitoring period. Appendix 5.1 – Aerial Series Appendix 5.2 – Aerial Assessment Appendix 5.3– Hydrologic Region 3 - Gage Site Summary Appendix 5.4a - Peekskill Hollow Creek - FFA Appendix 5.4b -Stony Brook - FFA Appendix 5.4c -Torne Brook - FFA Appendix 5.5 – Annual Peaks Appendix 5.6 – Erosion Data (GPS) Appendix 5.7 – Riprap Data (GPS) Appendix 5.8 – BEHI Data Appendix 5.9 – Pfankuch Data Appendix 5.10 – Pebble Count Data Appendix 5.11 – Cross Section Notes Appendix 5.12 – Cross Sections Plots (24x36) Appendix 5.13 – Cross Section Data

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TASK 6: Mitigation and Restoration Plan

Sawmill Creek, like all streams, is a complex natural system that will respond to every change within its watershed, no matter how large or small. Development within the Sawmill Creek watershed and consequent changes to the runoff characteristics have and will have a direct influence on water quality, water quantity, channel stability, and habitat function. The creation of impervious areas and the resultant increase in storm water runoff rates, which has occurred throughout the watershed, has certainly contributed to such conditions. These instabilities have been further aggravated by unnatural activities including channel modifications, flood plain encroachments, and redirection of drainage channels. Approximately 20% of the stream length in Sawmill Creek currently contains exposed and/or actively eroding streambanks. These unstable stream reaches are located throughout the watershed, but are more concentrated within specific segments of the stream. In contrast to many other typical urban watersheds, several factors have helped to minimize the extent of channel erosion and related stream impacts in Sawmill Creek. These factors include the relatively large wetland areas located within the upper watershed, significant undeveloped areas and a wooded riparian corridor. These areas are generally characterized as having deeply rooted woody vegetation, functional floodplains and have the ability to reduce sediment load. They also provide buffer zones to isolate roadways, buildings and other structures from erosion and flood damage. Active floodplains are also serving to protect downstream areas from increased sediment loading by providing opportunities to redistribute the sediment loads being generated during large storm events. Notable contrasts were observed throughout Sawmill Creek where the loss of either rooting stability or floodplains has resulted in considerably degraded conditions. Traditional stream management practices in Sawmill Creek have tended to focus on bank stabilization as its single objective. These traditional practices (e.g., riprap, dumped stone, stacked rock) may achieve the goal of stabilizing local streambanks; however these methods can have potential impacts outside the immediate project area, or on other stream functions such as stream and floodplain ecology, sediment transport, or water quality. In many cases, ongoing evolutionary changes in stream form are interrupted by local stabilization techniques, and these interruptions may cause stream instabilities to shift upstream or downstream. With these factors in consideration, combined with the numerous assessments completed as part of this study, general recommendations for Sawmill Creek and its watershed were developed based on seven distinct stream units. Stream Stability and Erosion The Sawmill Creek corridor was delineated into seven units (Figure 58). The delineation provides the artificial boundaries necessary for detailed discussion, data analysis, and the development of management recommendations. Numerous physical criteria used to delineate the units and based upon the previous assessments include:

• Level I and II classification, • Infrastructure location,

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• Valley morphology, • Channel confinement, • Depositional characteristics, • Channel alignment trends, • Landmark features, • General stability, • Stream bank erosion inventory, • Vertical and lateral controls, • Geological influences, and • Riparian vegetation character.

Units were labeled sequentially upstream to downstream and associated with the stream alignment stationing. Specific information about the units is given the table 43. Unit 1

Description Unit one is a relatively small section of Sawmill Creek which begins in Yorktown Heights at the bottom of the wetland, and extends approximately 142.4m (500ft) ending at the culvert under Old Country Way. Sawmill Creek initiates as a first order, perennial stream with surface water persisting throughout the year as a result of the wetland. Historically the unit appears to have been contiguous with the upstream wetland, and was dissected by the railroad bed alignment, which is now the existing bike path. The meander pattern is constrained along its length by the existing bike path. Sediment depositional patterns were not evident, most likely a result of decreased bedload from the wetland. Debris was limited to minimal amounts of small floatable material and no channel blockages were observed. The Level II stream classification determined the entire unit to be E5 stream channel. Rosgen (1996) characterizes the channel type as inherently stable bed and banks, and hydraulically efficient, maintaining a high sediment transport capacity. Typically the E5 stream type maintains stable condition unless streambanks are disturbed and/or vegetation is removed or altered. Riparian vegetation within unit consists primarily of a moderately dense combination of grass/brush with a low density deciduous overstory, Appendix 2.3, photo #101. This is consistent with the channels strong connection to its floodplain. The immediate channel boundary is dominated by overhanging deep-rooted varieties of sedges. The bankfull floodplain consists of shrub species with intermittent deciduous trees. These trees provide limited canopy cover. The area was observed to contain several invasive species including multi-flora rose, Japanese knotweed and various invasive vine species. The existing bike path limits vegetative growth along its bank length however the channel remains stable, due in part to a combination of low channel slope, velocity, and boundary shear stress. Stream bank conditions were stable with no bank erosion inventoried throughout the study period. A stream channel stability evaluation, Pfankuch (1975) and Rosgen

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(1996), was performed for the unit which indicated an overall good condition in relation to its stream type. A single monitoring cross section (cross section #31) in the unit exhibited minimal evidence of bed and bank erosion, confirming the findings of the inventory assessment. Infrastructure within the unit consisted of the single culvert conveying stream flow through a 60” corrugated metal culvert under Old Country Way, Appendix 2.3, Photo #103. No evidence of historic riprap or stream channel revetment was inventoried within the unit.

Recommendations 1. Preserve existing wetland condition

• The wetland acts to reduce the impacts of pollutant runoff to the stream channel by creating a buffer between the upper, more urban portion of the watershed and Sawmill Creek.

• The wetland assists is reducing excess sediment load to Sawmill Creek through settling of the sediment.

• The wetland allows for limited flow detention during periods of high flow Unit 2

Description Unit two is a 511m (1,675ft) long reach that begins at Old Country Way and ends at the first Sawmill Creek crossing under Route 118. Seasonal variation in streamflow within the reach is dominated by stormflow runoff and directly modified by the culvert under Old country Way. Stream size in Unit 2 indicated a bankfull width of 4.57m (15ft) -9.14m (30ft). The channel remains a first order stream. The majority of the Sawmill Creek through Unit 2 remains deeply incised within its floodplain. Existing channel morphology is indicative of historic channel and floodplain modification. Several long-standing, discontinuous berms are located adjacent to the channel adding to the reaches entrenchment. Although these features were not directly studied as part of the assessment, the relative unnatural appearance suggests that the stream channel and floodplain were altered at some point in the distant past. It is believed that a combination of channel dredging, straightening, and berming were conducted, presumably in an attempt to provide protection to the once existing railroad, as well as alleviate the rail bed from flooding. The meander pattern can be described as relatively straight and remains constrained along its length, due to the past alterations. Large caliper deciduous hardwoods including white and red oak, sugar and silver maple and white ash dominate vegetation composition within the unit. The mature vegetation provides a dense canopy cover over the channel, limiting understory vegetation. Vegetation within the bankfull channel was absent through the majority of the reach, leaving bank soils exposed to storm flow. Although the rooting density of the

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vegetation was moderate to high, the ratio of rooting depth to streambank height was low, Appendix 2.3, photo #111 and #114. This pattern indicates that riparian vegetation provides minimal bank stability, since primary erosive forces occur at a lower elevation than the average rooting depth. Sediment deposition was characterized as small point bars consisting of fine gravels. Deposition terminated at a low elevation within the channel, well below bankfull stage. This condition results in part from decreased bedload from the upstream wetland and relatively high transport capacity of the entrenched reach. The depositional features inventoried result from low to moderate stormflow, which are then conveyed through reach during higher flows. Debris was limited to minimal amounts of small floatable material and one channel blockage. The debris blockage included several large trees entangled with large floatable woody debris that completely crossed the active channel, Appendix 2.3, Photo #115. Recent sediment deposition was present immediately upstream of the jam. The blockage did not appear to be increasing boundary stress, but rather providing grade control within the incised reach. The Level II stream classification determined the entire unit as G4c stream type. Rosgen (1996) describes the G4 stream type as a very unstable stream type with high bed and bank erosion rates due to combined effects of low width depth ratios, moderate channel gradients and high sediment supply. Further the stream type is very sensitive to disturbance and tends to make significant adverse channel adjustments to changes in flow regime and sediment supply from the watershed. Pfankuch (1975) and Rosgen (1996) stream channel stability evaluation was performed for the unit, indicating fair condition in relation to stream type. The unstable characteristic of the stream type was confirmed with eroded bank lengths measuring 296m (970ft) along its 366m (1200ft) length, or 40% of total bank length. Banks were inventoried as steep, undercut to nearly vertical in areas averaging 2.13m (7ft) in height and 32.9m (108ft) in length. Bank materials were not clearly stratified, but were dominated by fine gravel, sands and silts. Failure mechanisms were primarily planer in nature with evidence of piping and sapping. Three monitoring cross sections (#28, #29, and #30) were placed at the top middle and bottom of the Unit. The upstream most section, near the outlet under Old Country Way, displayed lateral erosion of both banks averaging 0.37m/yr (1.2ft/yr) in distance, and 0.40m2 (4.3ft2) square feet of area. The middle section (#29), installed through a pool feature, displayed 0.58m2 (6.2ft2) of deposition and filling of the pool stream feature. Section #28 is located immediately upstream the culvert under Route118, at the transition of the riprap bed and bank section providing inlet protection for the culvert. The cross section displayed erosion of the left bank of 0.20m (0.65ft), which most likely correlates with adjustments in stream alignment relative to the culvert opening. The average of total erosion and deposition for the unit measured 0.15m2 (1.6ft2) of deposition and represents the lower of the two depositional units in the watershed. Measurements of bed elevations at the three sections show minor increases in elevation with an average of 0.06m (0.2ft) with the largest occurring at cross section #28. Lateral bank migration in the unit averaged 0.19m (0.61ft) with the largest migration occurring at cross section #30 along the left bank measuring0.55m (1.82ft).

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Channel slope significantly increases though the Unit, transitioning from 0.1% in the upper reach, to 0.8% in the middle reach, to 1.6% in the lower reach. A sharp increase in channel slope was observed from cross section #28 to the culvert under Route 118. Evidence of an incised headcut within the stream bed was noted in this area, and indicates the potential for a sudden lowering of streambed level upstream. This erosive feature is most obvious in coherent or semi-consolidated stream bed material, usually as a small waterfall. In a gravel-bottomed stream, a nick point may be as subtle as a small incised channel within a riffle zone. As the streambed erodes and lowers at this active headcut, the nick point migrates upstream, thereby creating progressive bed lowering of the entire reach. Where there is active erosion within the bed of a stream or river channel, the bed may be steadily lowered, creating relatively higher banks up onto the adjoining floodplain or terrace. The banks become increasingly steepened and unstable as this erosion is active at the toe of the slope. Increased streambed collapse and erosion can occur, and the channel commonly widens in conjunction with bed lowering. Additional evidence of stream bed erosion can be the exhumation of roots of trees within the channel, as observed throughout the upstream reach. Although cross section #28 did not display incision during the monitoring period, the potential for upstream incision exists. Several large rocks, appearing in the foreground of Photo #118, Appendix 2.3, are acting to halt the migration. The large rocks appear to be remnant of an old stonewall, which transversely bisects the channel in its present location. Infrastructure within the unit consisted of the two culverts discussed above, the upstream culvert under Old Country Way and the downstream culvert under Route 118 (Appendix 2.3 photo #119) at the bottom of the Unit. Approximately 48.8m (160ft) of riprap was inventoried in the Unit representing 7% of the stream length. The riprap was located primarily at the culvert inlet and outlets.

Recommendations 1. Streambank stabilization and/or channel restoration is not suggested at the present

time

• The initiation of a large scale project within the existing condition could lead to significant disturbance to the surrounding floodplain vegetation, and prove to have a rather high cost/benefit ratio.

• Route 118 and the existing bike path limit the ability for extensive channel relocation and/or realignment.

• Identification of the factors that influence the stream erosion and stream channel evolution, that have occurred at any particular site, should be a critical component of any future restoration. It is recommended that if stabilization and/or restoration is required in the future, opportunities should be investigated to include:

a. reducing the channel entrenchment by the removal of berms; b. creating additional floodplain capacity to assist with reducing floodstage and

flood water velocity; and

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c. providing grade control in combination with slope reduction techniques (increasing channel meandering) to reduce stream power and prevent further incisement.

2. Debris removal should be restricted to areas where it poses an immanent threat to

infrastructure or general safety, and/or results in undesirable flooding.

• Evidence of a headcut was recorded in the lower reach of the Unit, indicating the potential for a sudden lowering of streambed level upstream.

• Fallen trees and trapped debris can provide natural grade control, can assist in providing temporary stability, as well as aid in long term recovery of incising reaches. A single debris blockage upstream of the headcut was observed, this blockage may act to reduce the impacts of the headcut.

3. Conduct additional monitoring and assessment, focused on existing headcut and

subsequent channel incisement in the lower section of the Unit.

• Although the headcut appeared stalled by the presence of several large rocks, the size and quantity of the existing material may not provide long term protection.

• Additional monitoring and assessment should be conducted to determine the rate of migration and predict the potential impacts on upstream areas.

• Consideration should be given to stabilizing the current headcut in-place as an interim measure to minimize further upstream incision, while a more permanent strategy for stabilization is developed.

Unit 3

Description Unit three is a 427m (1400ft) long reach that begins at the upstream most Sawmill Creek culvert under Route 118 and ends at the impoundment structure (online pond) outlet. Streamflow within the reach is dominated by stormflow runoff from upstream drainage and the Unit remains a first order stream channel. Stream size in the unit was classified as S-4, indicating a bankfull width of 4.57-9.14m (15-30ft). A strong mixed species composition was inventoried within the riparian buffer of the unit. The riparian vegetation was classified as predominately deciduous trees with grass understory. The composition and structure was consistent with the overall good physical condition of the channel. Numerous types of sedges and grasses were documented growing within and along the channel boundary. These deep-rooted species provide enhanced channel and low bank protection, as well as habitat. The bankfull floodplain was dominated by mixture of deciduous trees at moderate to high density. Depressed wet areas contained varieties of willow and wet tolerant shrubs species with larger deciduous trees providing good canopy cover. The plan form pattern can be described as meandering. Constraints to channel meanders and belt width were only present in the first 30m (100ft) of the reach where

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Sawmill Creek runs parallel with Route 118. Sediment deposition was characterized as small point bars consisting of fine gravels. Debris was limited to minimal amounts of small floatable material and one minor debris jam in 2003. Stream classification determined the unit to contain B4 and C4 stream reaches. The upstream B4 reach was approximately 152m (500ft) in length and represented 39% of the unit. Rosgen (1996) describes the B4 stream type as a moderately entrenched channel typical of structurally controlled drainage ways. The stream type is a relatively stable and is not a high sediment supply channel. The downstream C4 section measured 266m (872ft) long comprising 61% of the unit. Rosgen (1996) describes the C4 stream type as a slightly entrenched, meandering, gravel-dominated, riffle/pool channel with a well-developed floodplain. Rates of lateral adjustments are influenced by the presence and condition of riparian vegetation. Sediment supply is generally moderate to high, unless streambanks are in stable condition. The stream type is susceptible to shifts in both lateral and vertical stability caused by direct channel disturbance and changes in flow and sediment regimes from the contributing watershed. Pfankuch (1975) and Rosgen (1996) stream channel stability evaluation was performed for the unit, indicating good condition in relation to stream type. The majority of the Unit is considered a reference area/reach due to the physical characteristics and overall general stability (Appendix 2.3, Photo #122 & #123). A smaller reach within the unit was inventoried as part of the Habitat Assessment (Task 4b). The 2004 bank erosion inventory found no bank erosion in the unit, while the 2005 inventory measured three eroded banks totaling approximately 76.2m (250ft) of erosion, 9% of the reach length. The three eroded banks exhibited toe erosion and scour, two of which were located immediately downstream of the culvert under Route 118. Bank Erosion Hazard Index (BEHI) ratings were taken at the three eroded banks identified in the 2005 corridor inventory. The banks scored 41.6, 37.1 and 26.4, indicating very high, high and moderate erosion potential for the three banks. The area located along Route 118 was characterized as having high bank height to bankfull height, relatively low rooting depth in relation to bank height, and low rooting density. Additionally, a high bank angle, greater than 60o, and minimal surface protection suggest that the bank has a high potential for further erosion. Two monitoring cross sections established in the Unit (#26, #27) indicated the second lowest amount of erosion of the four units dominated by channel erosion. Cross section #27 contained minimal channel erosion and deposition, changes in bed elevation, and channel migration. Erosion at downstream section #26 measured 0.24m2 (2.59ft2), and 0.23m (0.76ft) of lateral erosion. Infrastructure includes the Route 118 culvert as well as the concrete weir at the outlet of pond. The general condition of the weir is poor and impoundment is essentially filled with sediment. The impoundment provides limited stormflow attenuation due to the outlet structure and moderate sediment storage relative to its current capacity. The structure provides grade control for lower section of the Unit.

Recommendations 1. Streambank stabilization and/or channel restoration is suggested at one location

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• The upper reach of the Unit running parallel along Route 118, has shown an

increase in streambank erosion and bank scour during the monitoring period. This specific area running along Route 118, should involve stabilization and/or restoration.

• The erosion in this area is currently located only a short distance from Route 118. Consideration should be given to stabilizing the bank in-place as an interim measure to minimize further impacts on the roadway, while a more permanent strategy for stabilization is developed.

• Interim stabilization should not result in the decreasing the channel capacity, nor redirect stress to the opposite side of the channel.

• Although Route 118 limits the ability for extensive channel relocation and/or realignment the relative low sinuosity of the reach suggests that the channel will continue to increase channel meandering in an attempt to reduce slope and resultant energy.

• Final stabilization should include investigating opportunities to create additional floodplain capacity on the western side of the channel, incorporate grade control, and reduce channel boundary stress along both banks.

• Although more formal revetment may be required along Route 118, increasing floodplain capacity along the opposite bank will result in reduced boundary stress. This approach can decrease the size and amount of structural protection required along the roadway, and also allow for use of less structural measures along the western bank. Additionally, the increased floodplain capacity will allow the installation of grade control structure, as needed, without elevating the risk of flooding along Route 118.

2. Maintain/Preserve the existing vegetative buffer along the stream corridor

• The stream types present through the reach (primarily C and E) have a very high sensitivity to disturbance and a high potential for erosion.

• Vegetation in these areas have a direct impact on the width/depth ratio stability. • Stability of the reach, in its current condition, is particularly dependant upon and

maintained by the existing vegetation. • Disturbance of the vegetation (cutting, removal, etc.) will most likely lead to

increased lateral channel migration, channel widening and a reduction in mean channel depth through siltation.

3. Improve sediment storage capacity of the online pond

• The existing pond appears relatively shallow and nearly full of sediment. • Possibilities may exist to increase/maintain the pond capacity for sediment storage

as well as improve the removal of pollutants and excess organics. • Increasing the pond depth may reduce current thermal impacts to downstream

reaches created by the shallow water, provide a buffer from upstream thermal impacts, as well as provide refugia for fish during periods of increased water temperature.

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4. Improve/repair the outlet structure of the pond to facilitate fish passage during base

flow. 5. Modification on of the pond outlet for flow detention is not recommended.

• The adjacent elevation of Route 118 relative to the existing pond water surface does not facilitate using the pond as a flow retention facility.

6. Maintain/Preserve the existing vegetation surrounding the pond.

• Provides shading for pond to reduce thermal impacts. • Provides buffer from sediment and pollutant runoff from Route 118.

Unit 4

Description Unit four is a 975m (3,200ft) long reach that begins at the impoundment (pond) structure outlet and ends at the culvert under Croton Heights Road. Streamflow within the reach dominated by stormflow runoff from upstream drainage and is directly modified by the weir outlet of the impoundment. The Sawmill Creek changes to a second order stream channel near the top of the Unit, with the confluence of a small 1st order tributary. The meander pattern is classified as meandering containing no lateral constraints through a majority of its length. Sediment deposition and formations were infrequent and contained small point bars consisting of fine gravel and sand. Sand and gravel substrate dominate the unit. The riparian vegetation of Unit 4 is characterized as a forested wetland, dominated by wet tolerant deciduous tree species including maple, ash, and alder. Shrub species of willows and dogwood occupy the area where the utility transmission lines cross the corridor. The relatively low, wide floodplain and naturally wet conditions provide for excellent wildlife habitat and corridor diversity. The mixed composition, excellent health and density of the diverse vegetative community in the unit provide numerous benefits to water quality, habitat and channel stability. Numerous debris and channel blockages were observed within the Unit. Significant build-up of medium to large sized woody material including large limbs, branches, small logs or portions of trees was observed, often occupying 10 to 30% of the active channel cross section. Additionally, substantial amounts of litter and refuse including tires and metal were intermixed with the debris. One large debris blockage, Appendix 2.3, Photo 131, was observed occupying 90% of the channel cross section. Streams affected by urbanization and development often lack a sufficient quantity of the large woody debris necessary to maintain an ecologically healthy and stable ecosystem. Streams with adequate large woody debris tend to have greater habitat diversity, a natural meandering shape, and greater resistance against high water events. Debris jams may be positive or negative depending on the perspective and the specific

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site characteristics. The debris inventoried in Unit 4 did not appear to impact or affect the stability of the channel. Five separate reaches alternating between Rosgen C and E type channels were inventoried. The unit contains C4, C5, and E5 stream reaches. The upstream C4 reach was approximately 226m (740ft) in length and 23% of the unit, and the downstream C5 section measured 266m (872ft) long comprising 22% of the unit. E5 reaches make up the 362m (1,188ft) or 37% of the unit. Rosgen (1996) describes the C4 stream type as a slightly entrenched, meandering, gravel dominated riffle/pool channel with a well-developed floodplain. Sediment supply is moderate to high, unless streambanks are in a very low erodibility condition. The stream type is susceptible to shifts in both lateral and vertical stability in response to channel disturbance. Rosgen (1996) describes the C5 stream type having similar in physical characteristics to the C4 describe above, however, bed material is dominated by sand. The C5 typically has a higher width/depth ratio than C4 and C3 and has greater tendency for lateral migration. Sediment supply is high to very high unless stream banks have a low erodibility condition. Rosgen (1996) characterizes the E5 channel type as inherently stable bed and banks, and hydraulically efficient, maintaining a high sediment transport capacity.

The 2004 and 2005 bank erosion inventory found minimal bank exposure in the unit. Four monitoring cross sections were initially established in the Unit (#22, #23, #24 and #25). Cross section #23 was removed from the monitoring set due to the wetland condition providing insufficient physical channel characteristics. Cross sections #22, #24, and #25 indicated the lowest rates of erosion of the four units dominated by erosion. Evaluation of the sections revealed an average of 0.06m2 (0.63ft2) square feet of erosion through the Unit. Cross section #25, located just downstream of the impoundment (Station 31+92), revealed signs of deposition 0.28m2 (3.0ft2) and a bed elevation increase of 0.09m (0.28ft). Stream channel erosion occurred on both banks, measuring 0.15m (0.5ft) and 0.12m (0.4ft). Cross section #24 revealed 0.32m (1.06ft) and 0.09m (0.28ft) of lateral erosion, with no change in bed elevation. Cross section #22, upstream of the culvert under Croton Heights Road, demonstrated the highest rate of erosion in the Unit measuring 0.46m2 (4.9ft2) of erosion and 0.50m (1.63ft) of lateral erosion. A Pfankuch (1975, modified by Rosgen 1996) stream channel stability evaluation was performed for the unit, resulting in an overall score of 67, indicating a good condition with respect to the inventoried stream types. The majority of the Unit is considered stable. One of the two reference reaches identified in the 2004 walkover was located between Station 43+00 and 50+00 in Unit 4. The reference area contained a C5 stream type and appeared to be in excellent condition due to the physical characteristics and general stability (Appendix 2.3, Photo #202). No evidence of aggradation, degradation or significant bank erosion was observed. This reach may be suitable for physical reference parameters including cross sectional channel dimensions, pattern and profile in the event of future channel modification projects.

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Infrastructure inventoried within the reach included the concrete weir outlet at the impoundment, the culvert crossing under Route 118 and the Croton Heights Road crossing. In summary, the reaches within the unit maintain a relatively stable condition unless direct channel disturbance occur, or changes in sediment or flow regime. Stability of the stream types identified within the unit is highly dependent on the presence and condition of riparian vegetation. Monitoring cross sections confirmed the types and rates of erosion, depositional characteristics, and general channel processes are relatively stable in relation to the other Units along Sawmill Creek.

Recommendations 1. Streambank stabilization and/or channel restoration is not suggested at the present

time.

• The initiation of a large scale project within the existing condition could lead to significant disturbance to the surrounding floodplain vegetation, and prove to have a rather high cost to low benefit ratio.

2. Enhance the riparian vegetation near the tributary confluence downstream of the pond

area.

• Field observations and aerial image assessments documented minimal shading and overhead cover near the confluence of a small tributary entering Sawmill Creek from the east along Revere Drive.

• Temperature is a critical influence in aquatic ecosystems, affecting both the physical and biological characteristics of the stream. Higher stream temperatures can reduce the streams oxygen carrying capacity, increase rates of organic decomposition, and influence the rate at which nutrients are released from suspended sediments. Overhead cover and shading is critical to maintaining lower water temperatures, especially in the warmer summer months.

• Increasing the size and density of the riparian vegetation near the confluence may assist in reducing and/or maintaining water temperatures from the tributary at the confluence.

• It is recommended that native types of shrub size species (willow, alder, dogwood, etc.) be used along the boundary of the tributary, to provide rapid growth, establishment, and cover. Larger tree species should be planted in the surrounding area to provide long-term overstory and shade.

3. Maintain/Preserve/ the existing vegetative buffer along the stream corridor.

• The stream types present through the reach (primarily C and E) have a very high sensitivity to disturbance and a high potential for erosion.

• Vegetation in these areas have a direct impact on the width/depth ratio stability. • Stability of the reach, in its current condition, is particularly dependant upon and

maintained by the existing vegetation.

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• Disturbance of the vegetation (cutting, removal, etc.) will most likely lead to increased lateral channel migration, channel widening and a reduction in mean channel depth through siltation.

4. Preserve wetland

• A large section of Unit 4 contains federal and state regulated wetlands the majority of which are forested wetlands.

• Maximize the buffer areas between the wetland and Route 118.

• Maximize the buffer areas between the wetland and the utility crossing.

a. Sawmill creek runs parallel with the utility lines for approximately 488m (1,600ft).

b. Recent clearing of large trees and vegetation was observed along the boundary of the utility crossing extending to Sawmill Creek and into the forested wetland.

c. Areas requiring the maintenance or clearing of vegetation should be limited to maximize and/or create additional buffer width.

Unit 5

Description Unit five is a 715m (2,345ft) long reach that begins at Croton Heights Road (Station 63+00), and ends at a culvert conveying the stream under a driveway to a private residence (Station 86+45). The valley and channel morphology differ from that of the adjacent Units. The Sawmill Creek transitions from a channel with a wide well connected floodplain and lower channel slopes, dominated by sand and gravel bed materials, to an entrenched channel, with higher channel slopes, dominated by gravel and cobble bed materials. Riparian vegetation converts from a generally wide buffer with wetland vegetation to a narrower buffer dominated by large deciduous trees with limited under story. Entrenchment and channel slope in the unit increase from the top of the unit to the bottom. The downstream segment of Unit 5 contains several bank failures, debris jams and the remnants of an earthen dam or bridge crossing that once completely or partially impounded the creek. Large deciduous trees including oak, maple and ash were inventoried in Unit 5. The mature trees provide a dense canopy cover, limiting under story vegetation. The riparian corridor in Unit 5 is much narrower than Unit 4 upstream, restricted by the narrower valley floor and confined by Route 118 and the bike path. The Sawmill Creek is a second order stream channel in Unit 5, with stream size classification of S-5, indicating a bankfull width of 9.1-15.2m (30-50ft). The meander pattern is classified as meandering containing irregular meanders, with increasing entrenchment through the unit’s length. Sediment deposition and formations are

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infrequent and generally in the form of point bars and side channel bars consisting of coarse gravel and cobble. The majority of the debris and channel blockage in the unit consisted of small easily floatable material. However, some larger debris jams were inventoried in the lower portions of the unit between stations 78+05 and 81+90. Several of the larger jams were found in the area of a remnant dam/bridge structure. Ten reaches of various Rosgen stream types were inventoried in the unit, and included B, C, F and G channels. Table 44 summarizes the Level II stream types and corresponding lengths found in Unit 5. The majority of the unit consists of C type channel (54%) with substrate dominated by coarse gravel and small cobble. Comprising the C-type channel was C3b (11%), C4b (10%), and C4 (33%). The C3b stream type represent slightly entrenched, meandering, cobble dominated, riffle/pool channels with a well-developed floodplains. The “b” indicates a steeper channel slope than typical C-type channel. Sediment supply is generally low, however the streambanks are generally composed of unconsolidated, heterogeneous, non-cohesive, alluvial materials that are finer than the cobble dominated bed material, which can be lead to accelerated bank erosion (Rosgen, 1996). Rates of lateral adjustment are influenced by the presence and condition of riparian vegetation. C4 and C4b stream types are slightly entrenched, meandering, gravel dominated, riffle/pool channels with well-developed floodplains. The “b” indicates a steeper channel slope than typical C-type channel. Sediment supply is generally high and streambank erosion potential is very high. This stream type is susceptible to shifts in both lateral and vertical stability, and has a high sensitivity to disturbance. Riparian vegetation influences width/depth ratio stability, affecting both stability and recovery. F stream types compose 25% of the unit. Rosgen (1996) interprets the entrenched F stream types, with gravel to cobble bed materials, to be moderately to extremely sensitive to disturbance. Furthermore, Rosgen (1996) asserts that F3 and F4 stream types have poor recovery potential, very high sediment supply, very high stream bank erosion potential, and states that vegetation has a moderate controlling influence on width/depth ratio-stability. The G3 stream type represents 12% of the stream length in Unit 5. G-type channels are entrenched and to have a low width/depth ratio. G3 stream types are highly unstable due to the very high sediment supply available from both upslope and channel derived sources (Rosgen, 1996). G3 stream types also have a very high sensitivity to disturbance, poor recovery potential, and very high streambank erosion potential. The B3 stream type represents 9% of the channel length in Unit 5. Rosgen (1996) describes the B3 stream type as a moderately entrenched channel; typically having stable bed and banks, contributing only small quantities of sediment during runoff events. To facilitate further discussion of Unit 5, it has been divided into two segments based upon distinct differences in channel erosion. A more detailed summery of each segment is provided below.

Unit 5 - Upper Segment The upper segment (Station 63+00 to 75+00) includes several stream types. Initially a C3b stream type, the channel transitions through C4b, F4, C4, B3, and G3

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stream types heading downstream. Field assessments recorded only limited amounts of stream bank erosion in the upper segment. One isolated area of stream bank erosion was inventoried in 2005 downstream of Croton Heights Road. The erosion was located along the right bank immediately downstream of the outfall under the road and totaled 13.4m (44ft) in length. Four occurrences of bank stabilization were documented in the upper segment of the unit, totaling 145m (475ft). Of the existing stabilized banks, approximately 91.4m (300ft) were installed along the bike path and 52.2m (168ft) along Route 118. Additionally, bedrock was inventoried at Station 72+05, near the middle of the unit along the channel bottom. A Bank Erodibility Hazard Index (BEHI) rating was conducted on the single eroding bank. Shallow rooting depth, low rooting density, and minimal surface protection (5%) indicated a high potential for erosion. Although the potential for erosion is high, the bank is isolated from any nearby infrastructure and has a comparably low bank height (0.61m-0.91m (2ft - 3ft)), which reduces concern. Four monumented cross sections (#21, #20, #19, and #18) were located in the upper segment of Unit 5. The cross section monitoring data did not reveal any significant channel erosion, deposition or incision.

Unit 5 – Lower Segment The lower segment, between Stations 75+00 and 86+45, transitions between C and F stream types. This segment contains numerous eroded banks and significant debris. Eight BEHI evaluations and five monitoring cross sections were established in this segment. The first reach in the lower segment is a 91m (296ft) long section of C4 stream type. This reach is of special concern from a geomorphic perspective due to the high percentage of eroded banks observed during the field inventories. Eroded banks in this reach totaled 75.2m (247ft) in 2005, nearly 60% of the total bank length of the reach. Three BEHI evaluations were conducted in the first reach. Two of the evaluations indicated a high potential for erosion due to the combination of low rooting density, high bank angles (75° to 90°), moderate surface protection, and low rooting depth relative to bank height. The third evaluation indicated a moderate potential for erosion having slightly higher rooting density, lower bank angle (45°) and lower bank height. A remnant earthen dam or bridge type structure was observed in near the middle of the segment, near Station 80 +50 (Appendix 2.3, Photo #220 & #221). The structure lies at the downstream end of a 90.8m (298ft) long F4b reach. The structure creates an abrupt change in channel confinement, and interrupts a significant portion of the west floodplain. The 45.7m (150ft) of channel immediately upstream of, and through, the structure exhibited an extremely high percentage of eroded banks (63%). Additionally the length of eroded bank increased between 2004 and 2005. A change in bed material size from gravel, upstream of the dam, to cobble, downstream of the dam, was also observed. Field observations suggested that the dam may be promoting upstream aggradation, downstream incision and exacerbating lateral erosion in the reach. BEHI evaluations conducted on two exposed banks, one just upstream of the structure and one

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at the structure both indicated an extreme potential for continued erosion. Both banks were characterized as having low rooting depth and density, high bank angles (85° and 55°), moderate surface protection, and high bank heights relative to bankfull stage. Additionally, stratification of bank material was noted at the exposed bank near the structure, increasing the potential for erosion. However, the monitored cross sections located upstream and downstream of the structure, #17 and #16, did not document any significant erosion or deposition during the monitoring period. A notable stormwater outfall, draining runoff from Route 118 (Station 80+15) was located approximately 15ft above the channel, allowing flow to cascade down the bank. There was minimal to no outfall protection noted and the outfall flow had created excessive scour and bank erosion. A significant frequency of medium to large size material, such as large limbs, branches and small logs were located in the reach. A single debris jam, occupying nearly 85% of the channel was inventoried near Station 82+50. While much of the debris appeared to originate from upstream areas, a substantial section of floodplain was filled with wood chips and landscaping waste. The fill extended several feet above the floodplain elevation and could impact floodplain hydraulics and increase channel energy. A single monitoring cross section (#13.2) was located near the bottom of the Unit (Station 85+87). This area is an over-wide section of channel upstream of a culvert crossing to a private residence. No significant erosion was observed. Although, minor deposition on the right gravel bar was documented, the channel area and width remains significantly larger than those in adjacent reaches. A small manmade grade control structure (Appendix 2.3, Photo #227) located immediately downstream of the cross section may have induced the localized deposition and created the over-wide condition. The final C4 reach ends at the culvert crossing to a private residence. A large diameter, steel squash pipe coveys the Sawmill Creek under a private driveway, at the downstream limit of Unit 5. The culvert was noted to be in poor condition. The culvert bottom is rusted through and significant flow escapes from the bottom of the culvert, eroding the material under the invert.

Revetment Summary Seven occurrences of revetment (riprap, dumped stone, etc.), totaling 221m (726ft), were inventoried in Unit 5. The revetment accounts for 15.5% of the total streambank length, or 31% of the channel length in the Unit. Ninety-four meters (307ft) of the revetments were found stabilizing the bike path, which runs adjacent to the channel on the east. Route 118, which is located adjacent to the channel on the west, is stabilized along 218 feet. The remaining revetments were found just upstream of the private driveway crossing that marks the downstream extent of Unit 5. The revetment identified in Unit 5 appeared to be in good condition.

Erosion Summary The 2004 erosion inventory found 174m (570ft) of exposed streambank in the unit, approximately 12% of the total bank length of Unit 5. The length of exposed bank

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documented during the 2004 inventory in Unit 5 represented 25% of all the erosion found throughout the watershed. Erosion in 2005 measured 203m (665ft), 14% of the total bank length in Unit 5. The bank erosion documented in Unit 5 during the 2005 inventory represented 40% of the bank erosion documented watershed wide. These figures indicate not only a trend toward increasing bank erosion in the unit during the study period, but also reveal that Unit 5 had the third highest percentage of eroding bank length of the seven Units in 2004, and the second highest in 2005. Further analysis of the location of erosion documented in Unit 5 reveals that erosion is limited, almost exclusively, to the downstream-most 349m (1145ft) of the unit, between stations 75+00 and 86+45. That stream length represents less than half of the entire unit length. An analysis of erosion limited to the lower portion of the unit reveals that 25% of the bank length in the lower portion of the unit was experiencing erosion in 2004, increasing to 27% in 2005. During the same monitoring period, the same analysis of the upper unit (Station 63+00 to Station 75+00) reveals that 0% of the banks were experiencing bank erosion in 2004, increasing to just 4% in 2005. A comparison of the existing revetment and the currently exposed banks shows that nearly 30% of the streambank length, 59% of the channel length, in the Unit has been previously stabilized or is actively eroding. It is evident that past attempts of channel stabilization have provided localized protection to Route 118 and the bike path. The increasing trend in bank exposures, suggests that the revetment has not resulted in an overall stabilization of the reach and could potentially be exacerbating erosion, by redirecting associative channel stress.

Upper Segment Recommendations 1. Streambank stabilization and/or channel restoration is not suggested at the present

time.

• The initiation of a large scale project within the existing condition could lead to significant disturbance to the surrounding floodplain vegetation, and prove to have a rather high cost to low benefit ratio.

• It is recommended that if stabilization and/or restoration is required in the future, opportunities should be investigated to include:

a. creating additional floodplain capacity to assist with reducing flood stage and

flood water velocity. b. providing grade control in combination with slope reduction techniques

(increasing channel meandering) to reduce stream power and prevent further incisement.

2. It is recommended debris removal in the upper be restricted to areas that pose an

imminent threat to infrastructure or general safety, and/or results in undesirable flooding.

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• Fallen trees and trapped debris can provide natural grade control, can assist in providing temporary stability, as well as aid in long term recovery of incising reaches.

Lower Segment Recommendations 1. Stream bank stabilization and/or channel restoration is recommended in the lower

segment.

• This area represents an opportunity to address a large percentage of the exposed banks inventoried, watershed wide, through treatment of less than 10% of the overall stream length.

• Restoration opportunities should include the use of natural channel design principals to promote the stability of channel form and function.

• A combination of “assisted recovery” and “full restoration” could be used to facilitate channel stability.

a. Assisted recovery involves direct management intervention on a small scale

and may be as simple as planting riparian vegetation to maintain or enhance bank stability, or providing stabilization in key areas to reduce overall impacts. Assisted recovery must be done carefully and with a good understanding of the stream type and setting to avoid further instability.

b. Full Restoration requires the most intensive management, and is generally reserved for the most severe locations of stream instability with the greatest impact to management goals. Full restoration requires complete assessment, design and reconstruction of the stream channel, banks and in some cases adjacent hillsides, to return the channel to a stable, functioning condition that satisfies multiple management goals. This level of management requires much greater time and financial resources, as well as technical expertise, to ensure stability restoration is consistent both with management goals, as well as the stream type and setting that will ensure project success and longevity.

2. Any method of stabilization or restoration designed for the lower segment of Unit 5

should be done with the entire segment in consideration. Stabilization of small areas should only be conducted after determining the impacts to upstream and downstream reaches, and as part of a comprehensive plan for the entire segment.

3. Stabilization and/or restoration should integrate and build upon the existing

monitoring data to verify the channel process prior to the initiation of a large scale restoration project.

4. Conduct further assessment of the large remnant dam/bridge structure and its impact

on channel stability and effects on upstream/downstream floodplain transition.

• The structure creates a significant reduction in floodplain width and may exacerbating erosion and channel incision downstream, as well as inducing

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sediment deposition and channel widening upstream. Active erosion was noted through the structure.

5. Conduct further assessment of the area containing significant floodplain fill to

determine its impact on channel stability and effects on upstream/downstream floodplain transition.

• A substantial section of floodplain was filled with wood chips and landscaping

waste. The fill extended several feet above the floodplain elevation and could impact floodplain hydraulics and increase channel energy.

6. Detailed hydraulic modeling should be conducted for the lower Unit and incorporated

into any stabilization and/or restoration design.

• Modeling should include the effects of the dam/bridge structure, include the effects of modifications (filling) of floodplain areas, determine energy and shear stress along exposed banks, and used to determine proper sediment conveyance.

7. Stabilization and/or restoration should include methods to reduce channel

entrenchment and create additional floodplain capacity as well as provide grade control in combination with slope reduction techniques (increasing channel meandering) to reduce stream power and prevent further incisement.

8. Provide outfall protection and/or employ alternative methods to reduce the impacts of

the drainage outfall from Route 118.

• Installation of additional outfall protection is recommended for this culvert to prevent scour at the outfall and ensure stability of the Route 118 embankment. Outfall protection should allow for energy dissipation before discharge into Sawmill Creek.

Unit 6

Description Unit six is a 678m (2,225ft) long reach that begins at the culvert that conveys the Sawmill Creek under a private driveway (Station 86+45), and ends at the downstream most crossing under Route 118 (Station 108+70). The stream channel through the unit is highly entrenched within its valley, with limited floodplain due to natural and anthropogenic confinement. The entrenched condition results in high potential for bank erosion and mass wasting, resulting from relatively small adjustments in channel plan form. The Sawmill Creek impinges on Route 118 in several locations with historic and contemporary applications of riprap, and dumped to control stream erosion. Areas of mass wasting and bank erosion were inventoried at several areas within the unit.

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The Sawmill Creek is a second order stream channel in Unit 6 with a stream size classification of S-5, indicating a bankfull width of 9.1-15.2m (30-50ft). The meander pattern is classified as meandering containing irregular meanders, strictly constrained by valley confinement. Sediment deposition and formations were infrequent, consisting of small point bars composed of coarse gravel and cobble. Large deciduous trees including oak, maple and ash were inventoried in Unit Six. The mature trees provide a dense canopy cover, limiting under-story and protective vegetation within the channel boundary. Although riparian vegetation plays a significant role in providing stability and promoting channel recovery, the majority of the stream types inventoried in the unit are less dependent on vegetation for physical stability than other areas. The majority of the debris and channel blockage in the unit consisted of small easily floatable material. However, three larger debris jams were inventoried at stations 97+50, 99+30, and 105+95. Eleven individual stream types were inventoried in Unit 6, consisting of B, C, E, F and G-type stream channels, Table 45. Channel sediment was dominated by coarse gravel transitioning into small cobble substrate in slightly steeper reaches, resulting in B3, B4, C3, C4, F3, E4, and G3 stream types. The majority of the unit is B type channel, comprising 58% of the unit. Rosgen (1996) describes the B3 stream type as a moderately entrenched channel; typically having stable bed and banks, contributing small quantities of sediment during runoff events. Rosgen (1996) describes the B4 stream type as a moderately entrenched channel typical of structurally controlled drainage ways. The stream type is generally relatively stable and is not a high sediment supply channel. Additionally, large woody debris is important component to fisheries habitat when available in both B3 and B4 reaches. The G stream type represents 20% of the stream length in Unit 6. Rosgen (1996) describes G stream types to be entrenched and to have a low width/depth ratio. Furthermore, Rosgen (1996) interprets G3 streams to have a very high sensitivity to disturbance, poor recovery potential, very high sediment supply and very high streambank erosion potential. The C stream type accounts for 13% of the channel length in Unit 6. Rosgen (1996) describes the C3 stream type as a slightly entrenched, meandering, cobble-dominated riffle/pool channel with a well-developed floodplain. Sediment supply is generally low, however streambanks are typically finer than the bed material and are susceptible to accelerated bank erosion when riparian vegetation is modified. The 2004 and 2005 bank erosion inventories found similar lengths of erosion measuring 239m (785ft) and 224m (735ft) of erosion respectively. Unit 6 had the second highest percentage of eroded bank length of the seven Units in 2004, and the highest percentage of eroded bank length in 2005. Cross section monitoring revealed that Unit 6 exhibited the highest rates of erosion in the entire stream corridor, measured over the study period. Cross section monitoring in the Unit determined the average rate of erosion to be 1.43m2 (15.4 ft2) over the study period. To facilitate further discussion of Unit 6, it has been divided into three segments based upon distinct differences in channel erosion and morphology. A more detailed summery of each segment is provided below.

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Unit 6 - Upper Segment The upper segment begins at the culvert outfall at the private driveway, Station 86+45, and extends to Station 90+80, where the valley bottom and floodplain narrow, impinging Route 118 and Sawmill Creek. This 133m (435ft) long G3 reach is highly modified, with 43% (114m (374ft)) of its bank length containing revetment in the form of riprap and dumped stone. Significant bank erosion at Station 90+50 was inventoried in the lower section of the segment measuring 34m (112ft) long and ranging from 1.2m (4ft) to 6.7m (12ft) in height. The erosion is located on the east bank, opposite the Route 118 embankment which has been stabilized with a combination of stacked riprap and dumped stone, Appendix 2.3, Photo #304. Although the revetment in this reach provides continued stabilization along Route 118, the entrenched channel condition combined with the steep adjacent bank allows for continued erosion of the eastern bank. Two monitoring cross sections were located in the upper segment. Cross section 13.1, located in at Station 86+63, showed no evidence of erosion during the monitoring period. A small protrusion of the east bank monitored was attributed to dumped yard waste, which did not appear restrict the channel or destabilize the channel. Monitoring cross section #13 (Station 90+15) documented no significant erosion, however the corridor inventory indicated an increase in eroded bank length from 19m (63ft) in 2004 to 34m (112ft) in 2005. Evidence of increased bank erosion was located just downstream of the cross section. A BEHI evaluation was conducted on the bank in 2005. A moderate bank angle (45°) and low surface protection, combined with low rooting depth and density indicated a high potential for erosion. Bank erosion initiated from toe scour is likely to continue on the eastern streambank as a result of the entrenched channel, steep bank condition, and limited ability of vegetation to increase stability.

Unit 6 - Middle Segment The middle segment of Unit 6, located between station 90+80 and 98+45, exhibited the most significant evidence of erosion and mass wasting in the Sawmill Creek study area. The 2004 corridor inventory determined that 38% (178m (583 ft)) of the banks in the unit were experiencing erosion, increasing to 41% (190m (623 ft)) in 2005. Seven cross sections and five BEHI evaluations conducted in the segment, indicated moderate to high erosion potential. The average exposed bank height inventoried in this portion of the unit was over 6.7m (22ft). A large active bank failure is present along the eastern slope between Stations 92+00 and 92+50. The bank measured 42.7m (140ft) long and the exposure ranged from 9.1m (30ft) to 13.7m (45ft) in height. The failure contains numerous large standing and fallen trees and originates more than 15.2m (50ft) above the stream channel, Appendix 2.3, Photo #305. The failure appears to be a combination of shallow rotational and planar failures, exacerbated by saturated soil conditions. Slumping of soil and vegetation down the slope has resulted in a partial channel blockage. The reduced channel width created by the failure results in a stream type

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change from C4b to E4b, which is atypical of the valley setting. Water was observed piping from the bank face and the bank soil remained saturated during numerous site visits and throughout the monitoring period. A single monitoring cross section (cross section #12, Station 92+10) is bisects the failure. The cross section documented extreme slumping along the eastern bank, resulting in the displacement of more than 1.67m2 (18ft2) of material into the channel above the adjacent floodplain elevation. Nearly 1.30m2 (14ft2) of material was added to the bankfull channel, resulting in total reduction in bankfull area by more than 1.11m2 (12ft2; 31%) and bankfull width by 0.94m (3.1ft; 24%). Although a BEHI evaluation of the bank (BEHI #38) indicated a high potential for erosion, reduced from an extreme potential due to a slightly lower bank angle and limited surfaced protection, continued erosion and slumping is evident. Reduced channel cross sectional area and width will certainly result in erosion of the material as the channel attempts to regain capacity. Additional erosion along the bank toe will act to increase the bank angle in turn promoting increased bank failure. Erosion extends from this bank into and through a majority of the downstream B3a reach averaging 2.13m (7ft) in height. This area contains a small floodplain area, providing protection to the high bank. Monitoring cross section #11 (Station 93+51), near the middle of the reach, verifies the B3a stream type. Although the monitoring cross section remained stable through the monitoring period, the corridor inventory documented nearly 30.5m (100ft) of exposed streambank. A BEHI evaluation (# 39) indicated a high potential for erosion due in part to high bank height to bankfull height, low rooting density, and minimal surface protection. Thirteen point seven meters (46ft) of riprap was located on the right bank immediately downstream of the erosion, protecting a terrace behind a private residence. Channel migration along the eastern valley wall, near Station 95+00, resulted in 45.1m (148 ft) of bank exposure. A BEHI evaluation (BEHI #40) conducted in 2005 indicated a moderate potential for erosion. Cross section #10.2 (Station 94+65) documented 0.43m (1.4ft) of channel incision, and 0.24m (0.8ft) of erosion of the left bank. Both of these processes are likely to further decrease the stability of the bank. The erosion extends into a highly modified channel reach behind a private residence. The modified channel reach behind the private residence contains an in-ground swimming pool and a patio area. The stone patio around pool is located along the floodplain, bounding Sawmill Creek. Two mortared stone walls, approximately 45.7m (150ft) in length and five to six foot in height, line the channel, Appendix 2.3, Photo #310, #311. The characteristics of the artificial channel are atypical of the natural physical characteristics and cause severe confinement of Sawmill Creek. Site visits inventoried erosion and undermining of the wall footings and documented severe damage along the wall to include cracking and shifting of the wall and significant settling of the patio area. A small wooden footbridge located in the area of the pool and supported by the walls, was apparently damaged and removed during the monitoring period. The channel modification does not include continuous armoring of the channel bottom. A sharp increase in channel slope was observed just upstream of cross section #10.2. Evidence of an incised headcut within the stream bed was noted in this area, and confirmed by the cross section monitoring, indicating the potential for a sudden lowering of streambed level upstream. This erosive feature is most obvious in coherent or semi-

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consolidated stream bed material, usually as a small waterfall. In a gravel-bottomed stream, a nick point may be as subtle as a small incised channel within a riffle zone. As the streambed erodes and lowers at this active headcut, the nick point migrates upstream, thereby creating progressive bed lowering of the entire reach. Where there is active erosion within the bed of the stream, the bed may be steadily lowered, creating relatively higher banks up onto the adjoining floodplain or terrace. The banks become increasingly steepened and unstable as this erosion is active at the toe of the slope. Increased streambed collapse and erosion can occur, and the channel commonly widens in conjunction with bed lowering. Downstream of the modified area, the channel runs along an exposed high bank between Stations 96+50 and 98+25, Appendix 2.3, Photos #312 and #313. The exposure is 49.4m (162ft) long and 10.7m (35ft) high, with minimal vegetation established on the bank face. A saturated bank condition with water piping from the bank was documented during the study. Four cross sections (#10.1, 10, 9, 8) were established in the area for monitoring. Cross section #10.1 is located just downstream of the modified channel and cross sections #8, #9, and #10 transect the exposed bank. Although the cross sections displayed only minimal amounts of erosion and channel migration, evidence of planar and cantilever type failures were present along the upper slope of the bank. Numerous large trees fell and remained in the channel between the 2004 and 2005 monitoring. A BEHI evaluation (BEHI #43) indicated a high erosion potential for the bank. A high bank height to bankfull height ratio and low rooting depth and density signified an extreme potential for erosion, but were reduced slightly by the presence of large fallen trees and slumped soil from the upper slope of the bank, which was acting to provide surface protection. Although the material does provide some element of stability, it is felt that it is only temporary. Personal interviews in 2003 with property owners revealed the area of the bank had been stabilized after the 1996 storm event. The property owners described the stream flows during the storm event as completely inundating the western floodplain behind the house, filling the pool with stream sediment, and causing substantial damage to the pool and patio area. The owners confirmed that the destabilization of the high bank occurred during that storm event. The opposite bank also remained exposed during the monitoring period. Although the height and extent of the erosion are limited and fairly insignificant, a discoloration of the bank soil and a small volume of leachate were noted. The leachate runs down the bank, and into the active channel.

Unit 6 - Lower Segment The lower segment of Unit 6 (Station 98+45 to 108+70) exhibited minimal erosion during the monitoring period. Forty-two meters (139ft) of minor erosion was documented in the 312m (1025ft) long lower portion of the unit in 2004, and no erosion was documented in 2005. The first reach in the lower segment contains a 90.8m (298ft) long C3b stream type. The reach begins with 40.2m (132ft) of stone revetment on the right, protecting the Route 118 embankment near Station 98+75. Erosion along 8.8m (29ft) of the left bank

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was noted during the 2004 walkover, opposite the revetment. The erosion was fairly insignificant, and was not documented during the 2005 walkover. Cross section #7 (Station 99+48) located downstream, remained relatively stable. Erosion was also documented in 2004 near the bottom of the reach at Station 101+20. The erosion, on the right bank, measured 15.8m (52ft) long and just 1.22m (4ft) high in 2004, but was not evident at the time of the 2005 walkover, most likely becoming increasingly stabilized with vegetation. The stream type transitions to B3a just downstream of the eroded right bank. Stone revetment was inventoried along the left and right bank adjacent to a bedrock grade control near Station 102+20. The revetment on the right bank extends 41.5m (136ft), and protects the Route 118 embankment. Near the downstream end of the revetted right bank, 17.7m (58ft) of erosion is evident on the left bank. This is the most significant erosion noted in the lower portion of the unit at 6.1m (20ft) in height. Notes from the 2004 walkover indicate that slumping at the bank appeared old and that the bank toe was well protected. Cross section #5 (Station 103+35) showed evidence of a small slide present along the upper slope of the left bank and minor scour along the right bank. The final reach of Unit 6 is a 84.1m (276ft) long B4 stream type beginning near Station 105+95. A debris blockage was noted in the reach near Station 106+25, occupying approximately 40% of the channel capacity in 2005, increasing from 20% in 2004, Appendix 2.3, Photo #401. Numerous large trees span the channel, and may accelerate the accumulation of additional debris. The blockage is located approximately 75.2m (250ft) upstream of a culvert crossing under Route 118, and has potential to block streamflow through the culvert. Cross section #3 (Station 106+76) located in the area of the blockage showed evidence of erosion along both banks, including the right bank which contains riprap along Route 118. Approximately 0.31m (1ft) of channel incision was also measured at the cross section.

Revetment Summary Eleven occurrences of revetment (riprap, dumped stone, etc.), totaling 221m (726ft), were inventoried in Unit 6. The revetment accounts for 29% of the total streambank length, or 58% of the channel length in the Unit. Route 118, which is located adjacent to the channel on the west, is stabilized along 233m (763ft). Of the remaining revetment, over 21% (273 feet) of the revetment inventoried in Unit 6 was accounted for by the mortared stone walls found behind the private residence. With the exception of the mortared stonewalls, the revetment identified in Unit 6 appeared to be in good condition.

Erosion Summary The 2004 inventory documented 239m (785ft) exposed bank in Unit 6, approximately 18% of the total bank length. The length of exposed bank documented during the 2004 inventory in Unit 6 represented 34% of all the erosion found throughout the watershed. Bank exposures in 2005 measured 224m (735ft), 17% of the total bank length in Unit 6. The exposures documented in Unit 6 during the 2005 inventory represented 44% of the bank erosion documented watershed wide.

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Further analysis of the location of erosion documented in Unit 6 reveals that erosion is confined largely to the middle 233m (765ft) of the unit, between stations 90+80 and 98+45. That stream length represents only 34% of the entire unit length. An analysis of erosion limited to the middle reaches of the unit reveals that 38% of the bank length in the middle portion of the unit was experiencing erosion in 2004, increasing to 41% in 2005. Analysis of the upper reach during the same monitoring period (Station 86+45 to Station 90+80) reveals that only 7% of the banks were experiencing bank erosion in 2004, increasing to 13% in 2005. Analysis of the lower unit (Station 98+45 to Station 108+70) reveals that 5% of the banks were experiencing bank erosion in 2004, decreasing to 0% in 2005. The primary slope failure mechanisms in units 5 and 6 are planar failures. The bank materials of the failed banks consist of considerable amount of clays and silts. Although most failed banks are local and small in scale, for example, the heights of 90% failed banks are lower than 5m, there are several significant bank failures in the reach stationing from 83+50 to 98+00, where the failed bank is about 10m in height and 50m in length. Several slopes in Unit 5 and 6 are show planar failure. To keep these slopes stable, the toe of these slopes must be protected against further erosion. The mass stability of such slopes is governed by topographic, geologic and climate variable, which in turn, controls shear stress and shear resistance in a slope. If no measures are taken to stabilize some of these slopes, erosion, sloughing and planar failure will continue to occur, delivering sediments to the downstream reached. Further, the bank loss will also result in loss of mature trees into stream, thus blocking the stream. The area of primarily concern for the slope stability is between cross-section 92+00 and 97+00. In this region, several planar failure of different degree has been observed. This type of failure is most likely to be influenced by vegetation and biotechnical treatment. Therefore, it is suggested that the Vegetation Reinforcement Soil System (VRSS) be considered to stabilize these slopes. This system usually consists of a rock toe to prevent further scour and undercutting of the bank and soil lifts encapsulated by geotextiles and geogrids to hold and strengthen the soil. Additionally, living braches are placed between the lifts, which are strengthened by interwoven network of roots and soil particles. The branches overhang beyond the face of the slope, which provides cover, and will intercept water and soil flowing over the slope. The living plants will provide a more aesthetic landscape compared other alternative (concrete crib walls, Gabbian walls and rip-rap blanket) which are often considered in stabilizing natural slopes. There have been many successful implementations of the VRSS in stabilization of natural slopes and Gray and Sotir (1996) and USDA(1992) provide an excellent basis for the use of VRSS in units 6.

Upper Segment Recommendations (Station 86+45 to Station 90+80) 1. Streambank stabilization and/or channel restoration is not suggested at the present

time.

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2. Debris removal should be restricted to areas where it poses an immanent threat to infrastructure or general safety, and/or results in undesirable flooding.

• Fallen trees and trapped debris can provide natural grade control, can assist in

providing temporary stability, as well as aid in long term recovery of incising reaches. A single debris blockage upstream of the headcut was observed, this blockage may act to reduce the impacts of the headcut.

Middle Segment Recommendations (Station 90+80 to 98+45) 1. Full channel restoration and bank stabilization is recommended in the lower segment.

• This area represents an opportunity to address a large percentage of the exposed banks inventoried, watershed wide, through treatment of less than 7% of the overall stream length.

• Restoration opportunities should include the use of natural channel design principals to promote the stability of channel form and function as well as incorporate further geotechnical investigations and possible drainage issues.

• Conduct further assessment through the area to determine the impact of the channel confinement on the stability of the reach and impacts and risk to the residence.

• Restoration efforts should include investigating option for relieving and/or reducing the channel confinement, increasing floodplain width where applicable, and reducing bed and boundary shear stress.

• Grade control should be provided through the Unit to prevent further channel incision.

2. Detailed hydraulic modeling should be conducted for the middle Unit and

incorporated into any stabilization and/or restoration design. Modeling should include the effects of the stone walls and floodplain modifications (fill, pool and patio area) on the downstream eroding banks.

3. Stabilization and/or restoration should integrate and build upon the existing

monitoring data to verify the channel process prior to the initiation of a large scale restoration project.

4. Consideration should be given to stabilizing the current headcut in-place as an interim

measure to minimize further upstream incision, while a more permanent strategy for stabilization is developed.

5. Conduct a more detailed investigation of the uphill drainage and saturated condition

of the failing banks.

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Lower Segment Recommendations (Station 98+45 to 108+70) 1. Streambank stabilization and/or channel restoration is not suggested at the present

time.


2. Debris removal should be conducted upstream of the culvert under Route 118 (Station

106+76).

• The debris currently poses a threat to the downstream culvert and has the potential to completely clog or block the structure. Additionally it is felt that the debris provides little or no benefit to grade control or instream habitat.

Unit 7

Description Unit 7 is a 216m (708ft) long reach that begins at the downstream most crossing with Sawmill Creek and Route 118 and ends at the Sawmill Creek culvert crossing under Route 129, where it discharges to the New Croton Reservoir. Infrastructure includes a culvert crossing under Route 118, marking the upstream extents of the unit, as well as crossing under Route 129, marking the downstream extent of Unit 7 and the study area. The type of crossing under Route 129 was not determined due to deep water ponded over the structure during site inspections. The Sawmill Creek remains a second order stream channel throughout Unit 7, exhibiting a stream size classification, indicating a bankfull width of 4.6m (15ft) – 9.1m (30ft). The meander pattern can be described as confined meanders. Sediment deposition patterns were characterized as small point bars, consisting predominately of medium grain gravels. No debris or channel blockages were identified in the active channel, however wood debris and litter were observed in the backwater pond at the bottom of the unit. A strong mixed species composition was inventoried within the riparian buffer to the west of the channel in the unit. Route 118 runs adjacent to the channel on the east, leaving only nominal riparian vegetation on the left bank. The riparian vegetation was classified as predominately deciduous overstory. The composition and structure was consistent with the heavily modified physical condition of the channel. Stream classification determined the unit to contain G1 and C4 stream reaches. The upstream G1 reach was approximately 82.3m (270ft) in length and represented 38% of the overall unit length. Rosgen (1996) describes G stream types to be entrenched and to have a low width/depth ratio. Furthermore, Rosgen (1996) interprets G1 streams to have a low sensitivity to disturbance, good recovery potential, low sediment supply and

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low streambank erosion potential. A mixture of old and new riprap was inventoried on the left bank, protecting 57.6m (189ft) of the Route 118 embankment. The downstream C4 reach measured 137m (449ft) long comprising 62% of the unit. Rosgen (1996) describes the C4 stream type as a slightly entrenched, meandering, gravel dominated, riffle/pool channel with a well-developed floodplain. Rates of lateral adjustments are influenced by the presence and condition of riparian vegetation. Sediment supply is a moderate to high influence unless streambanks are in a very low erodibility condition. The stream type is susceptible to shifts in both lateral and vertical stability caused by direct channel disturbance and changes in flow and sediment regimes from the contributing watershed. The 2004 and 2005 bank erosion inventory found no bank erosion in the unit. Monitoring sections confirmed those findings, documenting minimal evidence of erosion in the unit. Cross Section #2 documented minor toe erosion at the left bank despite bedrock grade control in the reach. Monitoring cross section #1 documented 0.74m2 (8ft2) of deposition. The deposition is likely the effect of frequent backwater inundation of the area, resulting from a potential blockage of the outfall structure under Route 129. Pfankuch (1975) and Rosgen (1996) stream channel stability evaluation was performed for the unit, indicating good condition in relation to stream type.

Recommendation

1. Streambank stabilization and/or channel restoration is not suggested at the present

time. 2. Conduct further investigations of the structure outlet to the Croton Reservoir. Stream Bank Erosion Modeling and Prediction

Since stream bank erosion can cause severe problems to stream management, environmental protection and agricultural activities through land loss, danger to riparian and floodplain structures, and increased downstream sedimentation, the prediction of stream bank erosion is a critical part in the assessment of the physical condition of a watershed. In this study, the stream bank erosion prediction model developed by Chen et al. (2005) and Chen et al. (2005) is applied to the Turkey Mountain watershed to estimate the potential stream bank erosion rate. Chen et al.’s model uses statistical method to establish the relationship between the bank erosion rate and various independent variables, such as channel geometry, flow properties, bank material composition, climate, and human influences. Since the majority of eroding banks in the Turkey Mountain watershed are located on the reach stationing from 83+50 to 98+00, the model is applied on this reach to estimate the reach average erosion. The independent variables needed for the prediction are collected from various sources such as field surveys, walkover, and other public database. The results show that in general, this problem reach (stationing 83+50 to 98+00) will produce about 800 m3 to 900 m3 eroded materials from stream banks a year.

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The stream bank erosion prediction will help stream mangers and environmental agents to analyze the physical conditions of a watershed, and thus prioritize the mitigation methods in order to effectively manage the watershed. Water Quality

The Metcalf & Eddy study of 1975 pointed out alternatives to the Town of Yorktown for stormwater management in the Sawmill Creek Watershed. At that time their options included maintaining the existing wetlands and provide on-site retention of excess stormwater for the future industrial and residential development. They had also noted the potential of a control structure at the outlet of the North wetland to take advantage of the storage available during high flows. This would now be in the location of Old Country Way. Since 1975 the watershed has practically been built out. There have been detention structures incorporated with most of the new residential and commercial sites, but not all. Based on this study recommendations for mitigation for water quantity include: 1. Preservation of existing land use.

As a minimum, this maintains the current volume of runoff in the watershed. The major land unit that could experience changes is the golf course west of NYS-Route 118 located in stream segment units 5 and 6.

2. Investigate the potential for outlet control of the North wetland. Land use changes in unit 1 (drainage sub-areas 1 and 2) include residential and commercial expansion. Some of these sites have incorporated stormwater management. The North wetland does have a substantial potential for runoff storage.

3. Evaluate the opportunities for the design and construction of additional stormwater detention ponds.

A number of stormwater outfalls along Pine Brook Road on the east side of the watershed from discharge unchecked above the wetland in Unit 1 and three outfalls to the terrain in Unit 3 below Chestnut Court and Pine Brook Road.

4. Evaluate the existing stormwater ponds for water quantity performance. A number of detention structures were installed over the past 30 years. During that time methods, procedures and hydrologic criteria have changed. These structures should be documented for their actual field performance.

The pollutant load analysis produced a snapshot of the impact various land uses have regarding the wash off of total phosphorous and total suspended solids in the Sawmill Creek watershed. The pollutant load in some stormwater discharges that flow through existing detention basins and/or sheet flow through existing vegetation or wetlands will be reduced. Due to the variability of these practices and their spatial relationships in the watershed, the actual reduction is not quantifiable. However, a number of areas were identified in the watershed where stormwater management practices or supplemental modifications could be implemented to further reduce potential pollutant wash off. Based on a field reconnaissance and identification of

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critical stormwater hotspots, the following recommendations are made by stream segment unit:

Unit 1 1. Enhance the depression area between Cardinal Court and NYS Route 118.

Runoff from high density residential development enters this depression area then exits under NYS Route 118 to the wetland behind the Town Hall. This area should be adapted to capture TSS prior to discharge.

2. Plunge pools should be oversized at the stormwater outfalls where practicable. There are 17 stormwater outfalls in Unit 1. These typically discharge directly to streams segments, many causing erosion. Plunge pools should be used where practical to reduce energy and provide stabilized velocities. The specific locations are noted on maps in the appendices.

3. Utilize appropriate SMP’s for water quality improvement at hot spot locations. Critical hot spot sources of pollutants were identified in the industrial section along the west side of Front Street. These sources included the town highway residency, a construction company storage yard, and a school bus parking garage and depot. Stormwater management practices such as oil/grit separators, deep sump catch basins, grass swales, filter areas and the use of forebays at the existing wetland should be evaluated at each site.

4. Construct island structures in the concentrated flow channels of the east side of the wetland.

The stormwater runoff from the industrial area along Front Street discharges directly into channels in the wetland which enter the main stem of Sawmill Creek. Island structures acting as channel plugs could be constructed at specific sites to force flow out of the channel into surface of the wetland which will slow it down and filter pollutants.

Unit 3 1. Design and construct a stormwater detention pond for the drainage from Chestnut

Court and Pine Brook Road. Stormwater runoff from the residential drainage areas east of Sawmill Creek is concentrated in three exposed outfalls between Chestnut Court and Pine Brook Road. Erosion at these outfalls is significant. A stormwater extended detention pond will provide water quantity and quality protection to the creek from these discharges.

Unit 4 1. Incorporate island structures in the south wetland channels to reduce velocities and

force flow to the wetland surface to filter the runoff.

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Unit 5 1. Stabilize the outlets of the culverts under NYS Route 118.

Four culverts discharge under NYS Route 118 in this unit to sawmill Creek. Their outlets are unstable. Options for these outlets include paver mats, cable crete, and rock riprap plunge pools. Deep sump catch basins can be incorporated at some locations to trap sediment.

2. The stormwater basin located on the east side of the creek above the trail should be modified for water quality and have its outlet flow path stabilized to the creek.

Unit 6 1. Provide sediment capture capability for the five culverts in this unit discharging

stormwater runoff under NYS Route 118 and stabilize their outlets.

Unit 7 1. Remove the deposited sediment from the backwater basin at NYS Route 129.

There is a significant volume of sediment and detritus accumulated at the outlet of sawmill Creek at NYS Route 129. This basin area is a backwater to the New Croton Reservoir. The existing sediment should be removed and the function of this system restored.

The locations of the sites for remediation are shown on the aerial maps in the appendix 4.3. Table 46 in the appendix summarizes the remediation location, existing appurtenance, problem, and recommended action. General Habitat Recommendations

1. Some cold water inflow, either from tributaries or ground water, may not be chemically suitable for fish due to low DO or pH.

• Perform an additional stream reconnaissance to walking the stream during periods

of warm water temperatures and lower flows to located areas which are being used by trout.

• Once refugia are documented every effort should be made to preserve the way water flows in those areas so that the cooler water is not quickly mixed with the warmer water of the main stream flow.

2. An additional consideration for the protection of trout from extreme changes in water

temperature would be the use of techniques for ground water recharge with storm

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water runoff from highways to delay hot stormwater runoff direct access to the stream.

3. Stormater outfalls should not be located so that the discharge is upstream or even

nearby downstream from thermal refugia. Central to all those considerations is the idea that habitat for trout is seasonally discontinuous and even isolated in most New York State trout streams.

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TABLES

109

Table 1. Data layers for the Turkey Mountain/Sawmill Creek Watershed.

Coverage Format Source

Bedrock Geology Vector USGS

Surface Geology Vector USGS

DEM Grid USGS

Ctour 100 Vector USGS

Slopes Vector USGS

Soils Vector SSURGO

Lakes Vector USGS

Land Use 1996 Vector Westchester County Planning Department

Wetlands Inventory Vector USGS

FEMA 100yrs Flood Vector USGS

Streams Vector USGS

Aquifer Vector USGS

Roads 1991 Vector NYDOT

Census 2000 Blocks Vector US Census Bureau

Drainage Area Vector USGS

Hazardous Waste Point Vector US EPA

Topograph Vector Yorktown Heights

Orthographic Images Rastor Westchester County

110

Table 2. Soil units and their percentage composition in the watershed.

Name Symbol % of Total Area

Carlisle muck Ce 3.94 Charlton-Chesterfield complex, rolling, very rocky CrC 18.88 Charlton-Chesterfield complex, hilly, very rocky CsD 14.88 Chatfield-Hollis Rock outcrop complex, rolling CtC 4.19 Chatfield-Hollis Rock outcrop complex, hilly CuD 5.00 Hollis- Rock outcrop complex, very steep HrF 3.76 Leister loam, 0-3% slope, stony LcA 0.40 Leister loam, 3-8% slope, stony LcB 0.40 Leister loam, 2-8% slope, very stony LeB 2.51 Paxton fine sandy loam, 2-8% slopes PnB 1.76 Paxton fine sandy loam, 8-15% slopes PnC 8.44 Paxton fine sandy loam, 15-25% slopes PnD 4.10 Paxton fine sandy loam, 8-15% slopes, very stony PoC 1.67 Paxton fine sandy loam, 215-25% slopes, very stony PoD 3.38 Ridgebury loam, 0-3% slopes RdA 2.17 Sun Loam Sh 1.76 Sutton loam, 3-8% SuB 0.35 Udorthents, smoothed Ub 0.10 Udorthents, wet substratum Uc 0.82 Urban land Uf 1.75 Urban land Charlton-Chatfield complex, rolling UlC 8.24 Urban land- Paxton- complex, 2-8% slopes UpB 4.79 Urban land- Paxton- complex, 15-25% slopes UpD 0.62 Urban- land Woodbridge complex, 2-8% slopes UwB 2.40 Woodbridge loam,8-15% WdC 0.18 Woodbridge loam, 0-3% slopes WdA 0.74 Woodbridge loam, 3-8% slopes WdB 3.75

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Table 3. Environmental degradation features and the number of occurrences along Sawmill Creek.

Feature # of Occurrences Bank Erosion 28 Bank Revetment 23 Bedrock 5 Stream Crossings (bridge, culvert) 6 Stormwater Outfalls 22 Debris Blockages 9 Stream Type Changes 32 Tributary Confluences 10 Reference Reaches 2 Clay Exposures 1 Notable Channel/Floodplain Features 7

Table 4. Summary of total length and percent of stream length where bank erosion and bank revetment occur along Sawmill Creek.

Feature # of Occurrences Total Length (ft) % Stream Length Bank Erosion 28 2325 20%

Bank Revetment 23 2486 21% Table 5. Summary of parameters for the hydrologic sub-area in Sawmill Creek Watershed.

Hydrologic Sub-Area Drainage Area (ha) RCN Tc (hrs)

1 37.8 83 0.40 2 154.3 79 1.30 3 161.0 71 1.21 4 65.7 71 0.43

Where: RCN is and Tc is time of concentration.

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Table 6. Rainfall and peak discharges for 1, 2, 5, 10, 25, 50 and 100 year storm events.

Sawmill Creek Outlet (Sta. 10+00) Event Rainfall (cm) Discharge (1,000 m3/d) 1 Year 7.1 158 2 Year 8.9 280 5 Year 11.4 480 10 Year 12.7 592 25 Year 15.2 826 50 Year 17.8 1,072 100 Year 19.0 1,199

Table 7. A comparison of estimated discharges under flood frequency for Sawmill Creek in 1000 m3/day for this study and FEMA (1993).

Study 10 Year 50 Year 100 Year This Study 593 1,072 1,200

FEMA-FIS (93) 472 779 910 Table 8. Sawmill Creek Basin - sub-area hydrologic parameter summary.

Sub-Area Drainage Area (km2)

Runoff Curve Number

Time of Concentration

(hrs.) 1 0.38 83 0.40 2 1.54 79 1.30 3 1.61 71 1.21 4 0.66 71 0.43

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Table 9. Sawmill Creek pollutant load summary - total suspended solids (in kg/yr).

Land Use Area 1 Area 2 Area 3 Area 4 Total Woods 849 3,223 1,309 5,381 Open Space 1,940 740 1,397 4,077 L.D. Residential 4,523 1,007 4,827 1,345 11,702 M.D. Residential 9,916 1,798 11,714 H.D. Residential 2,283 12,638 1,036 15,957 Apartments 4,348 4,348 Commercial 4,990 2,860 7,850 Light Industrial 8,556 8,556 Industrial 5,530 5,530 Roads/Hwys 2,633 11,191 2,155 3,292 19,271 Wetlands 966 1,424 2,390 Brush Area 251 251 Water 28 18 46 TOTALS 14,429 59,801 14,446 8,397 97,073

Table 10. Sawmill Creek pollutant load summary - total phosphorus (in kg/yr).

Land Use Area 1 Area 2 Area 3 Area 4 Total Woods 1.8 6.9 2.8 11.5 Open Space 1.0 1.6 3.0 5.6 L.D. Residential 33.6 7.5 35.9 10.0 87.0 M.D. Residential 73.7 13.3 87.0 H.D. Residential 5.7 30.4 2.6 38.7 Apartments 10.8 10.8 Commercial 21.4 12.2 33.6 Light Industrial 18.4 18.4 Industrial 11.9 11.9 Roads/Hwys 8.0 34.1 6.6 10.0 58.7 Wetlands 12.9 19.0 31.9 Brush Area 0.5 0.5 Water 0.4 0.2 0.6 TOTALS 68.7 214.7 84.2 28.6 396.2

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Table 11. Sawmill Creek pollutant load summary by major land use categories.

Major Land Use TSS Load (kg/yr)

TSS % of Total

TP Load (kg/yr)

TP % of Total

Resource Areas: woods, wetlands, open space, brush, water

12,145 12.5 50.2 12.7

Residential: L.D., M.D., H.D., and Apartments 43,392 44.8 223.4 56.4

Commercial & Industrial 21,935 22.7 63.9 16.1

Roads/Hwys 19,270 20.0 58.7 14.8

Totals 96,742 100.0 396.2 100.0

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Table 12. Summary of analytical methods.

Solute Method Reference

SO42-, NO3

-, Cl-, F- Ion chromatography EPA 300.0A US EPA 1994b

NH4+ Phenate colorimetry;

autoanalyzer; EPA 350.7 US EPA 1983

Ca2+, Mg2+, K+, Na+ Atomic absorption Spectrophotometry (AAS) Slavin 1968

PH Potentiometrically w/glass electrode EPA 150.1 EPA 9040B

APHA 2005 (Standard Methods)

ANC Strong acid titration w/ Gran plot analysis Gran 1952; Kramer 1984

Dissolved Inorganic Carbon/Dissolved Organic Carbon

UV enhanced persulfate oxidation (for DOC), detection of CO2 by IR spectrophotometry

US EPA 1983

Dissolved Silica

Heteropoly blue complex colorimetry; autoanalyzer EPA 370.1 SM 4500 Si-D

US EPA 1983

Specific Conductance Conductivity Bridge APHA 2005 (Standard Methods)

Total, suspended, volatile, sediments

filtration, drying and weighing EPA 160 US EPA 1983

Total Coliform Filtration, incubation and enumeration

APHA 2005 (Standard Methods)

Turbidity Turbidimeter HACH

116

Table 13. Mean temperatures for the wetlands sites (1, 2, 3) and freely draining sites (4, 5, 6). Winter (Dec, Jan, Feb) Summer (July, Aug, Sept)

Wetlands 1.8 °C 19.8 °C

Freely flowing reach 1.2 °C 19.6 °C Table 14. Spearman Rank Order Correlation for pH and temperature measurements with the major solutes measured. Significant at p<0.05.

pH Temp Site Turbidity -0.41 0.29 0.56 DO 0.39 -0.64 0.50 TSS -0.07 0.15 0.06 TDS -0.08 0.14 0.07 TN -0.44 -0.40 -0.61 OrgN -0.25 -0.26 -0.37 NH4 -0.28 -0.47 -0.32 NO3 -0.40 -0.15 -0.59 TP -0.04 0.62 -0.23 SRP -0.02 0.55 -0.11 DOC -0.38 0.44 -0.44 DIC -0.53 0.17 -0.77 Ca -0.16 -0.05 -0.23 Mg -0.49 -0.06 -0.70 Na -0.50 -0.31 -0.64 K -0.64 -0.09 -0.70 SO4 -0.09 -0.56 -0.06 Cl -0.24 -0.57 -0.06

* Highlighted values indicate statistically relevant correlations at p<0.05.

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Table 15. Summary statistics of concentrations of total nitrogen and nitrogen species in stream water by season. All units are in mg N /l.

Seasons TN NO3

- NH4 OrgN

Mean Min Max Mean Min Max Mean Min Max Mean Min Max

Fall 0.75 0.30 1.82 0.30 0.13 0.95 0.19 0.01 0.50 0.25 0.01 0.81 Winter 1.27 0.77 2.29 0.42 0.14 1.25 0.41 0.06 0.78 0.43 0.00 1.01 Spring 0.94 0.43 1.97 0.36 0.17 0.77 0.28 0.07 0.56 0.30 0.01 0.77 Summer 0.75 0.37 1.90 0.26 0.13 0.65 0.20 0.02 1.02 0.28 0.05 0.99

Table 16. Spearman Rank Order correlations for total nitrogen and nitrogen species. (significant at p< 0.050). TN OrgN NH4 NO3

-

Site -0.61 -0.37 -0.32 -0.59 Temp -0.40 -0.26 -0.47 -0.15 pH -0.44 -0.25 -0.28 -0.40 Turbidity 0.48 0.35 0.26 0.38 DO 0.05 0.06 0.16 -0.11 TSS -0.21 -0.08 -0.11 -0.07 TP -0.06 0.21 -0.30 0.09 SRP -0.11 0.2 0 -0.34 0.12 TDP -0.13 0 .82 -0.27 0.04 PP 0.22 0.32 -0.23 0.41 DOP -0.21 -0.18 -0.04 -0.19 DOC -0.01 -0.06 -0.19 0.08 DIC 0.54 0.33 0.29 0.50 Ca 0.16 0.07 0.01 0.23 Mg 0.59 0.43 0.35 0.46 Na 0.59 0.26 0.60 0.43 K 0.42 0.22 0.30 0.38

SO4

2- 0.16 0.14 0.27 -0.05

Cl- 0.17 -0.01 0.53 -0.09

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Table 17. Summary statistics of total phosphorus and phosphorus species in stream water by season. All units are in µg/l. Seasons TP SRP DOP PP Mean Min Max Mean Min Max Mean Min Max Mean Min Max

Fall 20.2 12.2 29.9 5.4 0.0 12.0 5.4 0.4 13.0 9.2 4.5 14.3 Winter 16.8 8.4 30.9 3.6 1.0 10.0 3.7 0.2 10.8 6.2 2.7 11.7 Spring 18.0 10.3 29.4 4.9 1.5 11.1 4.4 0.3 13.0 7.9 3.3 21.8 Summer 17.2 7.5 27.5 7.1 2.0 12.3 4.3 0.2 14.8 10.8 5.2 18.0

Table 18. Spearman’s Ranked Correlation of total phosphorus and phosphorus species to the other water quality parameters.

TP SRP TDP PP DOP Site -0.23 -0.11 -0.06 -0.29 0.03 Turbidity 0.33 0.10 0.17 0.37 0.11 DO -0.55 -0.46 -0.41 -0.47 -0.01 TSS 0.13 0.30 0.15 0.05 -0.10 TDS 0.12 0.30 0.16 0.04 -0.10 TN -0.06 -0.11 -0.13 0.01 -0.05 DON 0.04 0.07 -0.08 0.10 -0.03 NH4 -0.30 -0.34 -0.27 -0.23 -0.00 DOC 0.39 0.26 0.66 0.37 0.06 DIC 0.40 0.22 0.22 0.41 0.06 Ca -0.06 -0.04 0.09 -0.02 -0.07 Mg 0.11 0.03 -0.04 0.19 -0.06 Na -0.04 -0.11 -0.03 0.01 -0.02 K 0.25 0.00 -0.10 0.37 0.03 SO4 -0.57 -0.47 -0.53 -0.37 -0.17 NO3 0.09 0.12 0.04 0.09 -0.07 Cl -0.38 -0.30 -0.28 -0.35 -0.05 pH 0.04 -0.02 0.08 -0.08 0.11 Temp 0.62 0.55 0.52 0.49 0.09

119

Table 19. Principal factor scores.

Parameters Factor 1 Factor 2 Factor 3 Site -0.84 -0.12 0.07 Temp 0.02 0.91 0.04 pH -0.64 0.25 0.15 Turbidity 0.60 0.51 -0.27 DO -0.25 -0.73 0.05 TSS 0.23 0.23 -0.34 TDS 0.72 0.23 -0.03 TN 0.79 -0.06 0.19 OrgN 0.27 0.21 0.58 NH4 0.40 -0.45 -0.34 TP 0.11 0.87 0.17 SRP 0.13 0.51 0.63 TDP -0.20 0.69 0.05 PP 0.35 0.73 0.22 DOC 0.09 0.56 -0.22 DIC 0.78 0.37 0.09 Ca 0.83 0.13 0.05 Mg 0.82 0.15 0.07 Na 0.79 -0.29 0.01 K 0.72 0.00 -0.12 SO4 0.03 -0.47 -0.18 NO3 0.83 0.19 0.00 Cl 0.38 -0.56 0.08 Expl.Var 8.64 5.59 2.1 Prp.Totl 33.25 21.52 8.06

Table 20. Comparison of total phosphorus and total suspended solids loss estimated for different locations in Sawmill Creek with values estimated using the Simple Method. Data are shown by hydrologic sub-areas (and water quality sampling sites) and for the entire watershed.

Estimated from the Simple Method Observed values

Hydrologic Sub area

TP (kg/yr)

TSS (kg/yr)

Water Quality Sampling Site

TP (kg/yr)

TSS (kg/yr)

1 68.7 14429 1 440 86353 2 214.7 59801 2 280 40199 3 84.2 14446 4 768 63176 4 28.6 8397 6 323 20487 Total 396.2 97073 Total 1811 210215

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Table 21. Comparison of water quality for Sawmill Creek (this study), unnamed watershed (adjacent to Sawmill Creek; citation) and the New Croton Reservoir (after Effler et al. 2003). Parameter Unnamed Watershed Sawmill Creek New Croton Reservoir pH 7.4-7.7 7.7 Not measured NH4 0.006-0.007 mg/l 0.02 – 1.02 mg/l 0.005-0.150 mg/l NO3 0.5 mg/l 0.34 mg/l 0.005-0.428 mg/l OrgN 0.1-0.2 mg/l 0.01-1.01 mg/l 0.271-0.369 mg/l TN 0.08-0.2 mg/l 0.92 mg/l 0.281-0.947 mg/l TP 0.012-0.02 mg/l 0.0075 – 0.0309 mg/l 0.0002-0.045 mg/l DOC 1551-3979 ug/l 1400 – 8200 ug/l 3550 mg/l TSS 1-1.4 mg/l 0.12 – 15 mg/l 2.64 mg/l

121

Table 22. Percent oxygen saturation of water at Sawmill Creek, sample sites from October 2002 to September 2003.

Percent Oxygen Saturation Year Day Site 1 Site 2 Site 3 Site 4 Site 5 Site 6

18-Oct 66 67 72 70 86 84 22-Nov 74 42 77 76 85 85 2002 13-Dec 73 73 79 75 97 96

30-Jan 75 75 79 81 96 94 26-Feb 79 54 87 76 92 93 31-Mar 85 71 91 89 87 92 24-Apr 92 75 79 92 89 90 19-May 87 78 86 86 87 87 16-Jun 72 32 72 79 87 89 11-Jul 46 28 101 77 83 83 13-Aug 58 24 63 69 82 88 18-Sep 44 26 62 80 96 93 15-Oct 73 15 71 74 88 85 19-Nov 72 26 67 79 89 86

2003

12-Dec 74 67 87 89 93 100

26-Jan 70 29 79 87 95 95 22-Feb 63 51 96 99 110 109 25-Mar 79 78 92 108 117 112 30-Apr 99 68 97 102 102 111 18-May 74 38 87 87 98 98 16-Jun 70 28 76 88 107 105 15-Jul 65 24 82 89 99 102 19-Aug 57 59 72 76 86 85

2004

25-Sep 81 24 85 94 98 99 * Percentage DO saturation of water determined as the measured DO in water over theoretical DO in water at a given surface water temperature.

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Table 23. Physical habitat features at macroinvertebrate sampling stations.

Habitat Feature Sawmill Creek – Upper Sawmill Creek – Lower Depth (meters) 0.1 – 0.3 0.15 – 0.25 Wetted width (meters) 1.0 – 3.0 2.5 – 5.5 Current (cm/s) 40 55 Canopy (% Cover) 90 75 % Embeddedness 40 50 % Rock 10 15 % Rubble 70 70 % Gravel 10 10 % Sand 10 5 % Silt 0 0 Filamentous Algae None 3% of reach covered Diatoms None 40% of reach covered Air Temperature (C) 20.6 20.4 Water Temperature (C) 16.3 12.3

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Table 24. Water quality data (10/18/02-9/25/04) with regard to macroinvertebrate sampling.

Upstream Downstream

Site 1 2 * 3 4 * 5 6

mean 11 11 11 11 11 11 min 0 0 0 0 0 0

Temp (° C)

max 22 24 23 23 24 24

mean 6.99 5.51 3.58 2.83 2.26 2.40 min 2.20 1.77 1.44 1.64 0.71 0.71

Turbidity (NTU)

max 14.69 11.99 7.74 6.10 8.45 7.14

mean 7.39 7.32 7.64 7.73 8.11 8.15 min 7.01 7.04 7.20 7.31 7.76 7.73

pH (standard units) max 7.66 8.51 8.40 8.30 8.73 8.75

mean 7.80 5.47 8.97 9.39 10.35 10.39 min 4.05 1.59 5.29 5.77 7.00 7.27

DO mg/l

max 10.81 10.00 12.90 12.92 14.40 14.20

mean 4.3 3.0 1.6 1.5 18.6 1.3 min 0.8 0.6 0.4 0.4 0.1 0.2

TSS mg/l

max 15.7 9.0 8.5 4.7 217 7.0

mean 106.92 81.07 62.42 50.76 48.19 47.27 min 74.54 32.30 24.39 21.30 24.20 23.98

TN mg/l

max 140.33 163.73 123.71 100.45 97.86 92.78

mean 35.01 26.99 24.74 18.03 16.10 16.32 min 4.44 2.74 1.20 0.82 0.61 1.05

OrgN mg/l

max 57.71 68.40 72.28 68.57 54.60 56.29

mean 44.75 29.56 21.02 18.38 15.94 15.42 min 17.80 15.99 10.21 9.45 9.28 9.88

NO3 umol/l

max 86.96 89.05 61.68 39.26 35.10 32.45

mean 27.71 24.13 18.21 14.45 16.32 16.29 min 8.10 6.08 2.62 1.59 1.02 0.91

NH4 mg/l

max 59.50 72.90 46.10 43.57 42.83 40.57

mean 21.65 17.82 17.91 18.47 16.17 18.26 min 13.13 11.10 10.25 9.00 7.52 8.40

TP mg/l

max 29.90 25.80 29.37 32.84 36.93 37.46 * Location of macroinvertebrate sampling site

124

Table 25. Valley segments determined by valley slope for Sawmill Creek.

Valley Segment Station ∆ Elevation (ft.) % Slope 1 0+00 – 04+50 N/A N/A 2 04+50 – 22+00 83 ft 4.74 % 3 22+00 – 48+50 95 ft 3.58 % 4 48+50 – 78+00 13 ft 0.44 % 5 78+00 – 96+50 33 ft 1.78 % 6 96+50 – 137+00 9 ft 0.22 % 7 137+00 – 145+25 19 ft 2.30 %

Table 26. Percentage of undeveloped areas in Sawmill Creek watershed in 1974, 1985, and 2004.

Open Space Year acres mi2

Total Open Space Total Developed Area

1974 564 0.881 54.5% 45.5% 1985 422 0.659 40.8% 59.2% 2004 378 0.591 36.6% 63.4%

Table 27. Regression equations for hydrologic region 3 or Sawmill Creek at 1.5mi2.

Discharge (CFS) at 1.5mi2 Return Interval Regression Equation Low Average High

2-year Q2 = 75.5(A)0.700 68.3 105.9 212.1 5-year Q5 = 127(A)0.687 113.4 177.2 354.7

10-year Q10 = 167(A)0.686 146.1 233.8 467.9 25-year Q25 = 225(A)0.687 190.3 317.2 634.8 50-year Q50 = 273(A)0.689 223.9 387.3 775.1

100-year Q100 = 326(A)0.691 259.5 465.9 932.3

125

Table 28. Full regression equations for Sawmill Creek at 1.5mi2, 1.0mi2, 0.5mi2.

Discharge (CFS) at 1.5mi2 Return Interval Regression Equation Low Average High 2-year Q2 = 45.6(A)0.723(ST+1)-0.390(P-20)0.491(SH)-0.273 74.5 107.5 140.5 5-year Q5 = 33.0(A)0.718(ST+1)-0.405(P-20)0.806(SH)-0.347 132.0 195.6 259.2

10-year Q10 = 29.2(A)0.717(ST+1)-0.424(P-20)0.977(SH)-0.401 180.1 275.4 370.7 25-year Q25 = 27.4(A)0.717(ST+1)-0.452(P-20)1.155(SH)-0.470 252.6 406.8 561.0 50-year Q50 = 27.5(A)0.717(ST+1)-0.475(P-20)1.263(SH)-0.521 312.4 525.9 739.5

100-year Q100 = 28.5(A)0.718(ST+1)-0.499(P-20)1.354(SH)-0.571 374.3 663.7 953.0



100-year Q100 = 28.5(A)0.718(ST+1)-0.499(P-20)1.354(SH)-0.571 279.8 496.0 712.3



100-year Q100 = 28.5(A)0.718(ST+1)-0.499(P-20)1.354(SH)-0.571 170.1 301.6 433.0

Table 29. Regional hydraulic geometry and bankfull discharge data for selected areas of New York and Eastern U.S.

Hydrologic Region Drainage Area (mi2)

Discharge (cfs)

Area (ft2)

Width (ft)

Depth (ft)

0.5 25 6.0 9.6 0.6 1.0 46 10.6 13.2 0.8 Region 5 1.5 64 14.8 15.9 0.9 0.5 27 49.0 12.6 0.9 1.0 48 77.6 16.9 1.0 Region6 1.5 68 101.5 20.0 1.2 0.5 34 6.8 8.5 0.8 1.0 63 12.0 12.1 1.0 Region 4, 4a

(Catskill Mtns.) 1.5 90 16.7 14.9 1.1 0.5 36 14.0 12.0 1.3 1.0 62 22.0 15.0 1.5 Southeast, PA &

Eastern U.S. 1.5 85 29.0 17.0 1.7

126

Table 30. Summary of site information for selected USGS gauging stations.

Site Number Site Name Drainage

Area Period of Record

Number of Years

of Record

01374250 Peekskill Hollow Creek at Tompkins Corners, NY 14.9 mi2 9/25/75 – 3/22/03 29

01199477 Stony Brook near Dover Plains, NY 1.93 mi2 1/28/76 – 3/22/03 28 01387410 Torne Brook @ Ramapo, NY 2.6 mi2 8/19/60 – 5/14/02 43 Table 31. Log Pearson – Type III Distribution of Gauging Sites.

Discharge (CFS) – Log Pearson Type III USGS Gage Station mi2 Q2 Q5 Q10 Q25 Q50 Q100 Peekskill Hollow Creek 14.9 352 629 880 1289 1672 2132

Stony Brook 1.93 116 203 284 417 544 700 Torne Brook 2.6 315 576 808 1181 1524 1930

Table 32. Annual peak stream flow recorded at two regional USGS gauging stations.

Peak Discharge (cfs) Angel Fly Brook Hunter Brook Water Year Date

(USGS # 01374967) (USGS # 0137499350) 1996 07/15/96 356 ---- 1997 12/02/96 157 ---- 1998 05/31/98 176 ---- 1999 09/16/99 918 5,250 2000 11/02/99 64 177 2001 06/17/01 382 637 2002 06/06/02 85 205 2003 01/02/03 119 528 2004 09/18/04 618 1,100

127

Table 33. Location and type of debris inventoried along Sawmill Creek.

Station Description 12+15 Small Debris Jam Across Channel Consisting of Several Downed Trees 24+30 Small Debris Jam Across Channel Consisting Logs and Tires, Provides Grade Control 31+95 Minor Debris Blockage Minimal Obstruction of Flow 33+15 Minor Debris Blockage Minimal Obstruction of Flow 34+10 Milled Timber Debris, Obstruction at Low Flow 34+35 Debris Spans Low Flow, Obstruction at High Flow 34+95 Several Clumps of Woody Debris, Obstruction to Low Flow 35+90 Large Debris Jam, Obstruction of Low and High Flows 37+15 Small Woody Debris, Obstruction of Low and High Flows, Minor 44+85 Small Debris Located at Right Bank, Gravel Deposition Upstream 45+60 Debris with Vegetation in Jam, Channel Migration Left 76+45 Two Debris Jams, Consists of Root Ball and Sticks 78+05 Debris, Log with Sticks 80+95 Debris Consisting of Wood and Metal, Obstruction at All Flow 81+30 Large Logs And Wood, Obstruction at All Flow 81+90 Logs And Sticks, Obstruction at All Flow 84+30 Debris Consisting of Logs And Sticks 97+50 Debris, Consisting of Very Large Tree Trunk, Obstruction at High Flow 99+30 Minor Debris Blockage Minimal Obstruction of Flow

105+95 Debris Consisting of Large and Small Logs, Obstruction at High Flow

128

Table 34. Rosgen Level II classification for 28 cross sections on Sawmill Creek.

Cross Section W/d ER Slope Sinuosity D50 Stream Type1 11.3 3.5 0.0120 1.0 14 C4 2 9.5 1.5 0.0320 1.1 426 G1 3 7.2 1.9 0.0384 1.1 59 B4 4 23.2 1.6 0.0430 1.1 145 B3a 5 10.4 1.7 0.0560 1.1 202 B3a 6 12.6 2.2 0.0480 1.1 132 B3a 7 18.5 3.9 0.0466 1.2 80 C3b 8 10.8 1.9 0.0380 1.1 121 B3 9 8.9 1.7 0.0400 1.1 121 B3a

10 26.4 1.4 0.0400 1.1 121 F3b 10.1 10.9 1.4 0.0397 1.1 121 B3 10.2 10.4 3.1 0.0360 1.1 45 E4b 11 17.3 1.9 0.0436 1.1 69 B3a 12 4.3 4.8 0.0492 1.2 53 E4b 13 8.4 1.2 0.0440 1.2 72 G3 14 28.2 1.2 0.0426 1.1 80 F3b 17 21.4 1.1 0.0440 1.1 28 F4b 18 11.8 1.2 0.0330 1.2 145 G3 19 11.1 1.8 0.0360 1.0 77 B3 20 15.9 3.6 0.0247 1.1 50 C4b 21 11.4 6.4 0.0228 1.1 98 C3b 22 17.7 6.4 0.0080 1.2 0.08 C5 24 11.1 9.1 0.0040 1.0 0.20 E5 25 13.5 11.4 0.0120 1.1 3 C4 26 23.7 7.7 0.0118 1.2 5 C4 27 20.5 1.6 0.0280 1.1 42 B4 28 9.0 1.4 0.0160 1.0 43 G4c 30 11.1 1.4 0.0010 1.0 17 G4c/F4

129

Table 35. BEHI scores for 18 banks along Sawmill Creek.

Station BEHI Score Bank Erosion Potential 18+35 41.6 Very High 19+50 37.1 High 24+10 26.4 Moderate 63+95 35.3 High 75+00 33.1 High 76+35 38.6 High 77+00 26.1 Moderate 78+00 34.1 High 78+55 46.1 Extreme 80+35 47 Extreme 80+40 55 Extreme 83+25 25.4 Low 90+15 39.6 High 91+30 28.9 Moderate 92+50 38.7 High 93+50 37.8 High 95+00 27.8 Moderate 97+50 38.9 High

Table 36. Pfankuch stability ratings for seven reaches along Sawmill Creek.

Stream Channel Stability Rating

Upstream Station

Downstream Station

Upper Banks

Lower Banks

Streambed Features

Total Rating Points

Reach Rating

0 5+00 15 18 26 59 Good 5+00 17+00 35 38 45 118 Fair

17+00 31+00 16 26 25 67 Good 31+00 63+00 14 26 25 65 Good 63+00 86+50 22 24 36 82 Good 86+50 108+50 27 33 36 96 Poor

108+50 115+78 20 14 19 53 Good

130

Table 37. Particle size analysis for 34 locations along Sawmill Creek.

Size Class Indices (in mm) (From Cumulative Frequency Plots)

Cross Section D95 D84 D50 D35 D15

1 84 34 14 8 0.31 2 Bed Rock Bed Rock 426 148 35 3 386 247 59 31 12 4 490 365 145 88 36 5 Bedrock 207 202 128 37 6 Bedrock 594 132 89 46 7 336 106 80 66 41 8 484 276 121 80 33 9 484 276 121 80 33

10 484 276 121 80 33 10.1 484 276 121 80 33 10.2 346 168 45 20 0.23 11 334 173 69 44 20 12 237 125 53 34 0.33 13 678 328 72 46 25

13.1 379 126 38 27 13 13.2 286 108 41 25 11 14 482 264 80 47 16 15 482 264 80 47 16 16 479 272 35 12 0.26 17 465 261 28 14 0.37 18 907 417 145 76 1 19 759 271 77 31 2 20 349 159 50 26 0.33 21 559 369 98 40 2 22 0.23 0.18 0.08 Silt Silt 24 58 33 0.2 0.1 Silt 25 20 10 3 1 0.12 26 60 36 5 1 0.16 27 337 217 42 8 0.14 28 395 137 43 22 1 29 124 54 2 0.46 0.14 30 100 41 17 8 0.42 31 0.23 0.18 0.08 Silt Silt

131

Table 38. Summary of failure mechanisms on the Sawmill Creek. Year Planar Rotational Cantilever Piping Total 2004 14 0 8 6 28 2005 14 3 0 1 18 Table 39. Independent variables considered in the stepwise multi-variate regression model (SMRM).

Factors Variables

(1) Cross-sectional and longitudinal characteristics

Drainage Area

Cross-section area

Bankfull width

Cross-section maximum depth

Cross-section mean depth

Width/depth ratio

Bank height

Bank angle

Radius of curvature

Radius of curvature/Bankfull depth

Sinuosity

Channel slope

(2) Flow conditions Product of real time stream discharge and flow duration during a monitoring season

(3) Rainfall Condition Amount of rainfall per season

Duration of rainfall per season

(4) Temperature Froze-thaw circles per season

Frozen days per season

(5) Vegetation Vegetation coverage index

(6) Bank and Bed Materials Soil erodibility k, Bed materials size distribution D50

132

Table 40. Engineering properties for different bank materials (Source: USDA-ARS 2003),

Bank Material Description Friction angle φ' Cohesion c' kPa (lb/ft2)

Saturated unit weight kN/m3 (lb/ft3)

Matric suction φb (degrees)

Gravel 36 0 (0) 20 (127.32) 5 Angular sand 36 0 (0) 18 (114.59) 15 Rounded sand 27.0 0.0 (0.0) 18 (114.59) 15 Silt 25.0 5.0 (104.5) 18 (114.59) 15 Stiff clay 10.0 15.0 (313.4) 18 (114.59) 15 Soft clay 30 10 (208.9) 16 (101.86) 15

Table 41. Critical shear stress and erodibility for bank, bank toe, and channel bed materials (Source: USDA-ARS 2003).

Material Description (Size)

Critical Shear Stress τc Pa (lb/ft2)

Erodibility Coefficient k cm3/N-s (in3/N-s)

Boulders (256 mm) 248.83 (5.23) 0.006 (0.0003) Cobbles (64 mm) 62.21 (1.31) 0.013 (0.0008) Gravel (20 mm) 19.44 (0.41) 0.023 (0.0014) Coarse sand (1 mm) 0.71 (0.01) 0.118 (0.0072) Fine sand (0.125 mm) 0.09 (0.002) 0.335 (0.0205) Resistant cohesive 50.00 (1.05) 0.014 (0.0009) Moderate cohesive 5.00 (0.105) 0.045 (0.0027 Erodible cohesive 0.10 (0.0021) 0.316 (0.0193) Table 42. Annual stream bank erosion estimated by the stepwise multi-variate regression model (SMRM) and the USDA-ARS Model (stationing 83+50 – 98+00).

Measured m2 (ft2)

SMRM m2 (ft2)

USDA-ARS m2 (ft2)

2004-2005 1.72 (18.51) 1.73 (18.62) 1.68 (18.08) 2005-2006 Prediction - 1.55 (16.68) 1.61 (17.33)

133

Table 43. Location and length of delineated units at Sawmill Creek.

Station Unit Start End

Length (m)(ft)

1 0+00 5+00 152.4 (500) 2 5+00 17+00 365.76 (1200) 3 17+00 31+00 426.72 (1400) 4 31+00 63+00 975.36 (3200) 5 63+00 86+50 716.28 (2350) 6 86+50 108+50 670.56 (2200) 7 108+50 115+78 221.89 (728)

Table 44. Stream types inventoried in Unit 5 at Sawmill Creek.

Station Stream Type Begin End

Length m (ft)

C3b 63+00 65+60 79(259) C4b 65+60 67+95 71(233) F4 67+95 68+80 25(83) C4 68+80 70+15 41(133) B3 70+15 72+20 63(205) G3 72+20 74+95 84(275) C4 74+95 77+90 90(296) F4b 77+90 80+90 91(298) F3b 80+90 82+95 63(205) C4 82+95 86+45 107(352)

134

Table 45. Summary of stream types in Unit 6 in Sawmill Creek.

Station Stream Type Begin End

Length (ft) Percent of Unit Length

B3 94+55 96+30 175 B3 97+45 98+45 99 B3a 91+95 93+90 193 B3a 96+60 97+45 86 B3a 101+45 105+95 450 B4 105+95 108+70 276

58%

C3b 98+45 101+45 298 13% E4b 90+80 91+95 111 E4b 93+90 94+55 66

8%

F3b 96+30 96+60 28 1% G3 86+45 90+80 437 20%

135

Table 46. Summary of stormwater problems and potential mitigation for Sawmill Creek.

Location # Description Problem Action

1 Depression area Connectivity Amend for water quality 2 Town building Stormwater outfall scour Plunge pool 3 Stormwater outfall Stormwater outfall scour Plunge pool 4 Truck yard Pollutant wash off Amend for water quality 5 Construction yard Pollutant wash off Amend for water quality 6 Stormwater outfall Stormwater outfall scour 7 Stormwater outfall scour 8 Stormwater outfall scour 9 Bus garage and

storage Pollutant wash off Amend for water quality

& island structures 10 Stormwater outfall scour Plunge pool 11 Stormwater outfall Connectivity Plunge pool 12 Stormwater outfall Connectivity Plunge pool 13 Stormwater outfall Connectivity Plunge pool 14 Stormwater outfall Connectivity Plunge pool 15 Wetland channel Connectivity Utilize island structure 16 Stormwater outfall Connectivity & stormwater outfall scour 17 Stormwater outfall Connectivity & stormwater outfall scour 18 Stormwater outfall Connectivity & stormwater outfall scour 19 Stormwater outfall Connectivity & stormwater outfall scour 20 Stormwater outfall Connectivity & stormwater outfall scour 21 Stormwater outfall Connectivity & stormwater outfall scour

* Evaluate the potential to link as many of these outfalls as possible to stormwater detention basin

22 Stormwater outfall Connectivity & stormwater outfall scour 23 Stormwater outfall Stormwater outfall scour 24 Stormwater outfall Stormwater outfall scour 25 Stormwater outfall Stormwater outfall scour 26

Stabilize outfalls and run to a stormwater basin location area for stormwater basin for stormwater outfall’s 23, 24, & 25

27 Wetland channel Connectivity Utilize island structures 28 Stormwater outfall Stormwater outfall scour Armoring 29 Stormwater outfall Stormwater outfall scour Armoring 30 Stormwater outfall Stormwater outfall scour Armoring 31 Stormwater outfall Stormwater outfall scour Armoring 32 Stormwater basin Pollutant wash off Scour amend for water

quality & stablize outlet 33 Stormwater outfall Stormwater outfall scour 34 Stormwater outfall Stormwater outfall scour

Stabilize outfalls as required

35 Stormwater outfall Stormwater outfall scour 36 Stormwater outfall Stormwater outfall scour 37 Stormwater outfall No outlet found 38 Croton Reservoir

backwater basin Sediment accumulation Remove sediment

136

FIGURES

137

Figure 1. Topographic map of Turkey Mountain Watershed.

138

Figure 2. Westchester County, showing drainage divides and Turkey Mountain Watershed.

139

Figure 3. Bedrock geology of Turkey Mountain Watershed.

140

Figure 4. Surficial geology of Turkey Mountain Watershed.

141

Figure 5. The distribution of steep slopes and a 100m stream buffer in Turkey Mountain Watershed.

142

Figure 6. Hydrological features within Turkey Mountain Watershed.

143

Figure 7. Land use distribution of Turkey Mountain Watershed.

144

Figure 8. Population density within Turkey Mountain Watershed.

145

Figure 9. Roads within Turkey Mountain Watershed.

146

Figure 10. Distribution of soil units within Turkey Mountain Watershed.

147

Figure 11. Hydrography of Turkey Mountain Watershed showing sampling sites and road networks.

148

Figure 12. 30m buffer around Sawmill Creek. Shown are Route 118 and stream sampling sites.

149

Figure 13. Distribution of major land-use class within Turkey Mountain Watershed.

150

Figure 14. Proportion of land use of Turkey Mountain Watershed in and near 100m buffer around wetlands.

151

a) b)

Figure 15. Critical undeveloped and residential areas within Turkey Mountain Watershed, including: a) underdeveloped areas that are on steep slopes, and b) residential areas that are on steep slopes.

152

Period of Study

Oct-02 Feb-03 Jun-03 Oct-03 Feb-04 Jun-04 Oct-04

Dis

char

ge (

m3 /d

)

0

25000

50000

75000

100000200000

Figure 16. Estimated of mean daily discharge values for Sawmill Creek predicted from Angle Fly Brook using the proportional area discharge relationship. Sample collection dates and shown. Monthly water samples were collected under a range of flow conditions.

153

a) Temporal b) Spatial

Tem

pera

ture

(o C)

-505

1015202530

Tem

pera

ture

(o C)

0

5

10

15

20

25

30

pH

7.0

7.5

8.0

8.5

9.0

pH6.5

7.0

7.5

8.0

8.5

Ttur

bidi

ty (N

TU)

0

2

4

6

8

10

12

Turb

idity

(NTU

)

02468

1012

DO

(mg/

l)

02468

101214

DO

(mg/

l)

02468

101214

Month

OCTNOV

DECJA

NFEB

MARAPR

MAYJU

NJU

LAUG

SEP

TSS

(mg/

l)

-2

0

2

4

6

8

10

SITES1 2 3 4 5 6

TSS

(mg/

l)

-2

0

2

4

6

8

Figure 17. Variation of selected surface water characteristics of Sawmill Creek. Temporal patterns (a) are the mean value monthly of the six sampling sites. Spatial patterns (b) are illustrated by the mean of monthly observations for each of the longitudinal sites.

154

Sites1 2 3 4 5 6

DO

(mg/

l)

02468

101214

Figure 18. Spatial variation of dissolved oxygen concentrations at Sawmill Creek. Shown are mean and standard deviation based on monthly collections. The NYS water quality standard for dissolved oxygen is 5 mg/L.

155

OCTNOV

DECJA

NFEB

MARAPR

MAYJU

NJU

LAUG

SEP

Oxy

gen

Def

icit

(mg/

L)

-4-202468

1012

Figure 19. Temporal variation of dissolved oxygen deficit for Sawmill Creek. Shown are mean and standard deviation of the study sites.

156

OCTNOV

DECJA

NFEB

MARAPR

MAYJU

NJU

LAUG

SEP

DO

(mg/

l)

-5

0

5

10

15

20

25

-5

0

5

10

15

20

25DO Temp

Tem

pera

ture

(o C)

Figure 20. Seasonal dissolved oxygen and temperature variations for Sawmill Creek. Shown are mean and standard deviation of sampling sites. The NYS standard for dissolved oxygen is 5 mg/L.

157

Sites1 2 3 4 5 6

Oxy

gen

Def

icit

(mg/

L)

-4-202468

101214

Figure 21. Spatial variation of dissolved oxygen deficit for sampling sites along Sawmill Creek. Mean and standard deviation are shown.

158

Turbidity (NTU)

0 2 4 6 8 10 12 14 16

TS

S (

mg/

L)

0

2

4

6

8

10

12

14

16

18Site 1Site 2Site 3Site 4Site 5Site 6Tur vs. TSS

R2=0.10

Figure 22. Concentrations of total suspended solids (TSS) as a function of turbidity for the sampling sites along Sawmill Creek.

159

Turbidity (NTU)

0 2 4 6 8 10 12 14 16

TS

S (

mg/

L)

0

2

4

6

8

10

12

14

16

18WinterSpringSummerFall

Figure 23. Total suspended solids as a function of turbidity in stream water of Sawmill Creek with values shown by season.

160

DO

C (

mg/

L)

0

2

4

6

8

10

Sites

1 2 3 4 5 6

DO

C (

mg/

L)

0

2

4

6

8

10

A

B

Oct Nov Dec Jan

Feb Mar Apr May Jun

Jul

Aug Sep

Figure 24. Temporal and spatial patterns of dissolved organic carbon (DOC) in Sawmill Creek. Mean and standard deviation of study sites are shown.

161

DO

C (

mg/

L)

0

2

4

6

8

10

Tem

pera

ture

(o C

)

-5

0

5

10

15

20

25

30

Oct Nov Dec Jan

Feb Mar Apr May Jun

Jul

Aug Sep

Figure 25. Temperature and dissolved organic carbon variation over the study period in Sawmill Creek. Mean and standard deviation of study sites are shown.

162

Nitr

ogen

(m

g/L)

0.0

0.5

1.0

1.5

2.0

2.5

TN NH4 NO3- DON

Figure 26. Mean concentrations (and standard deviation) of total nitrogen (TN) and its species for all samples collected at Sawmill Creek.

.

163

Sites

0 1 2 3 4 5 6 7

Tot

al N

itrog

en (

mg/

L)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

NH4NO3DON

Figure 27. Proportion of nitrogen species for sampling sites of Sawmill Creek.

164

Seasons

Fall Winter Spring Summer

Nitr

ogen

(m

g/L)

0.0

0.5

1.0

1.5

2.0

TN NO3 NH4 OrgN

Figure 28. Seasonal variation of the means and standard deviations of total nitrogen and nitrogen species for samples collected at Sawmill Creek.

165

Tot

al N

itrog

en (

mg/

L)

0.0

0.5

1.0

1.5

2.0

2.5

Oct Nov Dec Jan

Feb Mar Apr May Jun

Jul

Aug Sep

Figure 29. Monthly mean and standard deviations of total nitrogen concentrations at sampling sites of Sawmill Creek.

166

Sites

1 2 3 4 5 6

Tot

al N

itrog

en (

mg/

L)

0.0

0.5

1.0

1.5

2.0

2.5

Figure 30. Spatial variation of mean and standard deviation for total nitrogen concentrations for sampling sites at Sawmill Creek.

167

Tot

al P

hosp

horu

s (u

g/L)

5

10

15

20

25

30

35

Oct Nov Dec Jan

Feb Mar Apr May Jun

Jul

Aug Sep

Figure 31. Monthly variation of mean and standard deviation for total phosphorus concentrations of stream sites in Sawmill Creek. Note that there is a New York Department of Environmental Protection advisory level of 20 µg/L.

168

Sites

1 2 3 4 5 6

Tot

al P

hosp

horu

s (u

g/L)

5

10

15

20

25

30

35

Figure 32. Mean total phosphorus and standard deviation by site. Values represent mean of monthly samples. Note that there is a New York Department of Environmental Protection advisory level of 20 µg/L.

169

Phosphorus Species

TP SRP DOP PP

Con

cent

ratio

n (u

g/L)

0

5

10

15

20

25

30

35Fall Winter SpringSummer

Figure 33. Mean and standard deviation of total phosphorus and phosphorus species seasonally. SRP is soluble reactive phosphorus. DOP is dissolved organic phosphorus. PP is particulate phosphorus.

170

Figure 34. Distribution of parameters in the plane defined by principal components 1 and 2.

171

Figure 35. Linkage diagram showing similarities in water chemistry between sampling sites.

172

Est

imat

ed

Tot

al P

Loa

ding

(g

-mo)

0

200

400

800E

stim

ated

T

otal

N L

oadi

ng (

kg-m

o)

0

5000

10000

15000

20000

25000

30000

Est

imat

ed

TS

S L

oadi

ng (

kg-m

o)

0

5

10

15

20

25

30

Time

Oct-02 Feb-03 Jun-03 Oct-03 Feb-04 Jun-04 Oct-04

Est

imat

edF

low

(m/m

o)

0

1

2

3

4

5

6

Figure 36. Estimated monthly loads of total phosphorus, total nitrogen and total suspended solids (TSS) transported from Turkey Mountain Watershed. Estimated flow is also presented as a reference.

173

Flo

w(c

fs)

010203040506070

X Data

Tur

bidi

ty(N

TU

)

05

101520253035

X Data

TS

S(m

g/l)

0

10

20

30

40

50

X Data

DO

C(m

g/L)

0

100

200

300

400

X Data

TP

(ug/

L)

0.00.20.40.60.81.01.2

Time

Fri 13 Sat 14 Sun 15 Mon 16

Nitr

ogen

(mg/

l)

0

20

40

60

80

TN NH4 NO3

Figure 37. Flow, turbidity, total suspended solids (TSS), dissolved organic carbon (DOC), total phosphorus (TP) and nitrogen species of upper Sawmill Creek site (site 3) during January storm event.

174

Flo

w(c

fs)

010203040506070

X Data

Tur

bidi

ty(N

TU

)

0

5

10

15

20

25

X Data

TS

S(m

g/l)

0

10

20

30

40

50

X Data

DO

C(m

g/L)

0

100

200

300

400

X Data

TP

(ug/

L)

0.0

0.2

0.4

0.6

0.8

Time

Fri 13 Sat 14 Sun 15 Mon 16

Nitr

ogen

(mg/

l)

0

10

20

30

40

50

60

70

TNNH4NO3

Figure 38. Flow, turbidity, total suspended solids (TSS), dissolved organic carbon (DOC), total phosphorus (TP) and nitrogen species of lower Sawmill Creek site (site 5) during January storm event.

175

TP

(ug

/L)

0.0

0.5

1.0

1.5

2.0

2.5

TS

S (

mg/

L)

0

200

400

600

800

1000

Col 2 vs TSS

Time

03:00 07:00 11:00 15:00 19:00 23:00 03:00

Nitr

ogen

(m

g/L)

0

50

100

150

200

NO3 NH4 TN

Flo

w (

cfs)

0

50

100

150

200

Tur

bidi

ty (

NT

U)

0

20

40

60

80

Figure 39. Flow, turbidity, total suspended solids (TSS), dissolved organic carbon (DOC), total phosphorus (TP) and nitrogen species of the upper site (site 3) of Sawmill Creek during October storm event.

176

Figure 40. Location of habitat analysis sampling locations in Sawmill Creek.

177

Figure 41. Water quality rating for Sawmill Creek sites.

Figure 42. Hilsenhoff Biotic Index values in Sawmill Creek

SPP

SPP

EPT

EPT

HBI

HBI

PMA

PMA

Score

Score

0 1 2 3 4 5 6 7 8 9

10

Upper Lower

Non-impaired 7.5 – 10

Moderate 5 – 7.5

Slight 2.5 – 5

Severe 0 – 2.5

0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00

10.00

Upper #1 Upper #2 Upper #3 Upper Avg. Lower #1 Lower #2 Lower #3 Lower Avg.

178

Figure 43. Taxa richness in Sawmill Creek.

Figure 44. EPT taxa richness in Sawmill Creek.

0.00

5.00

10.00

15.00

20.00

25.00

30.00

35.00

40.00

Upper #1 Upper #2 Upper #3 Upper Avg.

Lower #1 Lower #2 Lower #3 Lower

Avg.

0.00

2.00

4.00

6.00

8.00

10.00

12.00

Upper #1 Upper #2 Upper #3 Upper Avg. Lower #1 Lower #2 Lower #3 Lower Avg.

179

Figure 45. Percent model affinity in Sawmill Creek.

0.00

10.00

20.00

30.00

40.00

50.00

60.00

70.00

80.00

90.00

100.00

Upper #1 Upper #2 Upper #3 Upper Avg.

Lower #1 Lower #2 Lower #3 Lower Avg.

180

Figure 46. Mean community similarity values in Sawmill Creek.

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

1

Upper Lower Between

181

Figure 47. Bank height measurements along eroded banks in Sawmill Creek.

2004-2005 BANK EROSION HEIGHT

0

5

10

15

20

25

30

35

40

45

50

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 11000

STATION

ERO

SIO

N H

EIG

HT

(FT)

2004 EROSION 2005 EROSION

182

2004-2005 BANK EROSION LENGTH

0

50

100

150

200

250

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 11000

STATION

ERO

SIO

N L

ENG

TH (F

T)

2004 EROSION 2005 EROSION

Figure 48. Bank length measurements along eroded banks in Sawmill Creek.

183

Figure 49. Length and location of stabilized streambanks along Sawmill Creek.

RIPRAP AND REVETMENT

0

50

100

150

200

250

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 11000 12000

STATION (FT)

LEN

GTH

(FT)

Rip Rap and Revetment

184

Figure 50. Length and location of stabilized and eroding streambanks along Sawmill Creek.

2004 BANK EROSION -RIPRAP LENGTH

0

50

100

150

200

250

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 11000 12000

STATION

LEN

GTH

(FT)

2004 EROSION RIPRAP

185

Failure Mechanisms

Sketches

Field Pictures

Planar

Location: near xs8 (98+00)

Rotational

Location: near xs11 (94+00) Cantilever

Piping

Picture not available

Figure 51. The failure mechanisms of the stream bank in the Turkey Mountain Watershed.

Line of failure

Tension crack

Sandy pervious soil layer

Line of failure

Line of failure

Tension crack

186

Figure 52a. Active bank erosion near cross-section 10 (St. 97+00) Picture taken in 2004 (notice the tree was on the stream bank).

Figure 52b: Active bank erosion near cross-section 10 (St. 97+00) Picture taken in 2005 (notice the tree fell into the stream channel).

187

(a) Cross-section 10.2 (Stationing 94+50)

(b) Cross-section 12 (Stationing 92+00)

Figure 53: Erosion at cross-section 10.2 (a) and cross-section 12 (b) during 2004-

2005 monitoring season.

Erosion occurring during 2004-2005 monitoring season, total area lost: 7.6m2

Deposition occurring during 2004-2005 monitoring season

Erosion occurring during 2004-2005 monitoring season, total area lost: 1.8m2 (19.4 ft2)

188

0

10

20

30

40

50

60

70

80

90

100

1/1/2004 4/10/2004 7/19/2004 10/27/2004 2/4/2005 5/15/2005

Date

Dis

char

ge (c

fs)

Figure 54. Estimated daily mean discharge for Sawmill Creek (1/1/2004 – 5/30/2005).

189

Appendix Summary

(Attached in CD format) Task 1 Appendix 1.1 Sawmill Creek Geographic Information Database Task 2 Appendix 2.1 GPS Data Dictionary Appendix 2.2 Map of Environmental Degradation Appendix 2.3 Walkover Notes 2003

Walkover Notes 2004 2004 Photographs Walkover Notes 2005

Task 3 Appendix 3.1 Supporting Documentation for Hydrological Modeling Task 4a Appendix 4.1 Water Quality Monitoring Chemistry Data Task 4b Appendix 4.2– Macroinvertebrate Bioassessment of Sawmill Creek (Complete Report) Task 5 Appendix 5.1 – Aerial Series Appendix 5.2 – Aerial Assessment Appendix 5.3 -Hydrologic Region 3 - Gage Site Summary Appendix 5.4a - Peekskill Hollow Creek - FFA Appendix 5.4b -Stony Brook - FFA Appendix 5.4c -Torne Brook - FFA Appendix 5.5 – Annual Peaks Appendix 5.6 – Erosion Data (GPS) Appendix 5.7 – Riprap Data (GPS) Appendix 5.8 – BEHI Data Appendix 5.9 – Pfankuch Data Appendix 5.10 – Pebble Count Data Appendix 5.11 – Cross Section Notes Appendix 5.12 – Cross Sections Plots (24x36) Appendix 5.13 – Cross Section Data

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