3.0 BASELINE ASSESSMENT RESULTS Study website/CityCreek... · 2010. 11. 2. · SALT LAKE CITY...

RIPARIAN CORRIDOR STUDYFINAL CITY CREEK MANAGEMENT PLAN

SALT LAKE CITY OCTOBER 2010

DEPARTMENT OF PUBLIC UTILITIES BIO-WEST, INC.3-1

3.0 BASELINE ASSESSMENT RESULTS

Watershed Conditions

Size and Land Use

City Creek is the northernmostsignificant mountain streamdrainage in Salt Lake City(Figure 3.1). The uppersubwatershed, which includes theRCS study area above

Bonneville Boulevard, drains11,189 acres of woodedmountainous terrain ranging inelevation from 9,400 to 4,300feet. Upper City Creek flows fora total length of 10.3 miles(SLCO 2009). The majority ofthe upper subwatershed isforested and undeveloped, withan estimated overall impervious

cover of 9.2% (SLCO 2009). Much of this impervious cover isassociated with the paved CityCreek Canyon Road whichextends 5.75 miles up thecanyon to Rotary Park. Morethan 99% of the uppersubwatershed is public landowned by Salt Lake City andmanaged for drinking water

Figure 3.1. City Creek watershed (map from SLCO 2009).




source protection and openspace/recreation. The City CreekWater Treatment Plant (CCWTP)is located about 3 miles up thecanyon; access to the area abovethe CCWTP is subject torestrictions designed to limitpotential water supplycontamination. Recreationalopportunities in the uppersubwatershed include biking,jogging, hiking, fishing,picnicking at designated picnicsites, skiing, and snowshoeing.

The lower City Creeksub-watershed is much smaller,draining 4,621 acres of mixeduse land below BonnevilleBoulevard. Lower City Creekflows approximately 1.5 milesthrough Memory Grove, an areaowned and managed by theCity’s Parks Division. Thecorridor is bordered by theAvenues residential area on the

east and the State Capitolcomplex on the west. Anadditional 2.3 miles of lower CityCreek south and west of the RCSstudy area is conveyed inengineered conduits to theJordan River (Figure 1.1). Estimated existing imperviouscover is 19.7% within the lowersubwatershed (SLCO 2009).

Hydrology

The hydrology of City Creek isaltered by flow diversions at theCCWTP, which commonlyremoves all available flows fromthe creek. Typical withdrawalrates average about 4.6 cfs, butrates as great as 23 cfs aresometimes diverted during thespring runoff period (F. Reynolds2010, pers. comm.). In thevicinity of the CCWTP, thestream is a “gaining reach”where surface flows are

supported by groundwaterinputs, and flows in the channelrecover rapidly below theCCWTP diversion point.

Average annual precipitationranges from 32–42 inches in thehighest portion of the uppersubwatershed, from 22–32inches in the mid-elevationportions of the uppersubwatershed, and from 11 to 22inches in the valley/lowersubwatershed (SLCO 2009). Above the CCWTP City Creek isclassified as having perennialflow; below the CCWTP thecreek is classified as“perennial-reduced,” indicatingthat flows are artificially reducedby the stream diversions formunicipal water supply (SLCO2009).

No significant water storagereservoirs are currently presenton City Creek, althoughhistorically reservoirs wereconstructed in the canyon tostore water for municipal supply. Two debris basins locatedimmediately above and belowthe Bonneville Boulevardcrossing provide some degree offlow regulation and capture asignificant portion of the creek’scoarse sediment loads. Thesedebris basins are maintained bySalt Lake County and requiredredging approximately everytwo to three years. Additionally,the formal portion of MemoryGrove Park adjacent to studyreach LCC_R02B is designed tofunction as a flow regulation

Below: City Creek debris basin above Bonneville Boulevard shortlyafter dredging.




Figure 3.2. Monthly flows at Salt Lake County’s gage at MemoryGrove.

Figure 3.3. Relationship between impervious cover and surfacerunoff. Impervious cover in a watershed results in increased surface runoff. Diagram and caption textfrom FISRWG 1998.

basin that would control floodflows entering the downstreamengineered conduits. Thiscurrent system for flow andsediment regulation wasconstructed to prevent damagessimilar to those sustained duringthe major flood event in 1983.

City Creek’s hydrology ischaracterized by a distinctspringtime peak typical ofsnowmelt-driven systems. Basedon analysis of flow data recordedat the County gage in MemoryGrove from 1980–2005, meanannual flow is 8.3 cfs, averagemonthly flow is highest in May(Figure 3.2 ), and base flowsaverage about 3 cfs. Peak dailyflow occurs on May 21st onaverage (SLCO 2009), andaverage annual high flow is 67cfs based on analysis of the1980–2005 time period.

Urbanization and development inthe lower subwatershed haveaffected surface water-groundwater patterns. Theseinfluences affect the studyreaches from LCC_R01Bdownstream, which receive waterfrom urbanized areas via stormdrain outfalls. As contributingwatershed areas becomeconverted to impervious surfaces,a greater proportion of stormwater runs off as surface flowrather than infiltrating into theground, leaving less groundwateravailable to supply baseflow tothe creek during the summer dryperiod (Figure 3.3). Therelatively high proportion of




surface flow during storm eventscan also lead to a “flashy”hydrologic pattern with rapid,brief rises in flow during storms(Figure 3.4). On City Creek,however, these influences arelimited only to the downstream-most study reaches because themajority of the contributingwatershed area has beenprotected as undeveloped openspace.

Water Quality

Above the CCWTP, City Creek’sbeneficial use classifications asdesignated by the Utah Divisionof Water Quality (DWQ) include1C (high quality drinking water),2B (secondary contactrecreation), and 3A (cold water

fishery). Below the CCWTP, thedesignated beneficial usesinclude 2B and 3A. City Creek’swater quality is considered goodand fully supports all of itsbeneficial uses (DWQ 2006).

Potential nonpoint sources ofcontamination in the City Creekwatershed include fertilizers andherbicides from managed parks,runoff from roads/urbanizedareas (e.g., automotive fluids),and waste from wildlife (e.g.,deer, ducks) and pets. Bacteriological contamination isof particular concern because theJordan River, the receiving bodyfor City Creek water, is currentlylisted as impaired for E. coli(DWQ 2006). To help informongoing TMDL studies on the

Jordan River, several new E.colimonitoring stations have beenrecently established on CityCreek by DWQ and Salt LakeCounty. Samples are beingcollected monthly at these sites,but data have not yet gonethrough quality assurance/controlprocedures and have not beenreleased to the public orcomprehensively analyzed. Asadditional data are collected andanalyzed and as DWQ’s new E.coli work group becomes moreestablished, it is anticipated that abetter understanding ofbacteriological contaminationsources within City Creek willdevelop.

Geology and Soils

Within the upper subwatershed,City Creek flows primarilythrough Pennsylvanian andMississippian age sandstones andlimestones. Within the RCSstudy area, the channel flowsthrough Miocene conglomerates,Oligocene and Eocene tuffs andother volcanics, and quaternarylandslide deposits. Below reachUCC_R10C, these formations aresurrounded by Pleistocene LakeBonneville deposits ranging fromfiner-grained silt and claydeposits to coarser sand andgravel deposits. Within reachLCC_R01C and downstream, thechannel flows directly throughthese Lake Bonneville depositsand other quaternary alluvialdeposits (Bryant 1990).

Figure 3.4. A comparison of hydrographs before and afterurbanization. The discharge curve is higher and steeper for urban streams than for natural streams. Diagram and caption text from FISRWG 1998.




In the upper City Creeksubwatershed, approximately20–35 % of the soils have severeto very severe soil erosionpotential. In the lowersubwatershed, 35–50% of thesoils have severe to very severeerosion potential (SLCO 2009). Several significant hillslopeslumps are evident adjacent tothe south/east side of the channelwithin the study area. Slumpshave also occurred recently onthe steep hillside west of thechannel within areas adjacent toreach LCC_R01B anddownstream. In some areasthese instabilities appear to beinfluenced by steepened cutslopes and runoff associated withthe Bonneville Boulevardroadway; other areas are part oflarger natural landslide features.

After Lake Bonneville recededapproximately 16,000 years ago,it left a series of old shorelinedeposits that now formprominent “benches” along theedges of Salt Lake Valley. Toreach its modern base level at theJordan River, City Creek had tocarve through these deposits. Inpart because of this naturalgeologic history, stream gradientis steep, and the channel isentrenched between tall slopesthat extend up to the Bonnevillebench levels on the east and westsides of the corridor within thelower subwatershed. The degreeof channel confinement has alsobeen artificially increased byplacement of fill material forroadways.

Fish, Birds, and Wildlife

Above the CCWTP City Creeksupports a confirmedconservation population of nativeBonneville cutthroat trout, thestate fish of Utah and a state-designated special status speciesthat evolved as the top predatorwithin ancient Lake Bonneville(Lentsch et al. 2000). City Creekalso supports rainbow trout,brown trout, and rainbow-cutthroat hybrid crosses (SLCO2009). During the RCS fieldassessments, trout (speciesunknown) were observedthroughout the upper portion ofthe study area between reachesUCC_R09 to UCC_R11B. TheUtah Division of WildlifeResources (DWR) conductedfisheries sampling in a 200-foot-long portion of the stream in thisarea in 2007 and observed 19brown trout and one cutthroattrout (D. Wiley 2010, pers.comm.).

Quantitative data on fishpopulations within the lowerreaches of City Creek belowBonneville Boulevard are limited. Fish were not observed duringRCS assessments in the lowerstudy reaches, suggesting thatpopulation numbers may belower. However, fisheryinformation was not a specificfocus of the RCS assessment andobservations were merely notedwhen staff happened to see fishwhile collecting stream channelcondition information. The DWRhas not conducted fisheries

surveys in this portion of thecreek (D. Wiley 2010, pers.comm.).

During the Audubon Society’s2005 Christmas bird count,which included City CreekCanyon and Memory Grove, 19different bird species wereobserved (Carr 2009). Scientificdata describing detailedinformation about mammalpopulations are limited. However, general observationssuggest that wildlife populationsin the canyon include deer,bobcats, moose, mountain lions,coyotes, raccoons, rattlesnakes,rabbits, mice, and large numbersof elk (Hooton 2000). Evidenceof beaver was noted withinseveral of the upper RCS studyreaches.

Historical Conditionsand Current Trends

City Creek History

City Creek holds a prominentplace in Utah’s history, as itbecame an important municipaland irrigation water sourceshortly after Mormon pioneersentered Salt Lake Valley in 1847(Figure 3.5). At that time thecreek was described as beingcovered with willows and shrubs;specific types of vegetationmentioned include box elder,cottonwood, rose briar, andsquawbush. City Creek originallyexhibited a braided (multi-channel) plan form where itexited the canyon. The main




Figure 3.5. City Creek historical timeline.




branch of the channel isdescribed as following a sinuouspath to marshy swamp landsnear the Jordan River (LDSCHO 1997) and flowing througha grove of cottonwoods west ofthe Temple Block (LDS CHO1990). Pioneer accounts alsoremark on the abundant, cold,clear water in the creek.

Alterations to the RiparianCorridor

Over the last 160 years, thevarious activities associated withdevelopment and populationgrowth in Salt Lake Valley haveresulted in significant alterationsto the stream channel andriparian conditions of City Creek. Among other factors, systematicprograms to clear debris fromchannels likely contributed toreduced riparian vegetationdensity relative to pre-settlementconditions. Early citizens of SaltLake were encouraged to gatherwood in City Creek canyon byclearing the stream of deadfall(Prince and Shea 1995). Such“cleaning” programs involvedremoving debris dams andbeaver dams that increasedinundated streamside habitatarea, elevated the water table,reduced flood velocities anderosion, and trapped sedimentand nutrients (Gardner et al.1999). The removal of suchstructures decreased the natural“checks” on sediment and water,resulting in greater flow velocitiesand streambed down-cutting(Wohl 2000). Today City Creek

is maintained in a relativelynatural condition, and beaverpopulations are present in theupper subwatershed. Thepresence of beaver is uniquerelative to the other streamsassessed during the RCS;however, current populationlevels in City Creek are mostlikely significantly smaller thanduring the pre-settlement era.

Many of the direct alterations toCity Creek occurred to resolveflooding problems andaccommodate water supply formunicipal and agriculturalpurposes. Within the lowersubwatershed, the mostsignificant direct changes to thecreek involved combining themultiple branches of City Creekinto an artificial channel that randown North Temple Street(Figure 3.5). By the mid-1920sthis channel had been enclosedin underground pipes, resultingin the loss of most of the originalstreamside and riparian areaswithin the valley portion of the

Historical activities that have alteredriparian corridorconditions:

• mining for iron andother metals

• logging and timberharvest

• beaver trapping andremoval

• channel clearing anddebris removal

• construction of watersupply storage reservoirs

• flow diversion forirrigation and drinkingwater

• road and stream crossingconstruction

• residential andcommercialdevelopment

• introduction of invasive,nonnative plants

• piping of the creek inunderground conduits

• channel straighteningand ditch construction

• bank armoring

• placement of fill withinfloodplain areas

• debris basin construction




Figure 3.6. Plot of temporal trends in annual stream flow at CityCreek gage. Plot provided by the Salt Lake CityDepartment of Public Utilities.

watershed. Within both thelower and upper subwatersheds,placement of fill material for roadconstruction has directly affectedthe corridor by reducing thewidth of the active floodplain. Channel realignments have alsohistorically occurred inconjunction with construction ofmills, water supply canals, andstorage reservoirs (Figure 3.5). Following the 1983 floods,extensive channel armoring(primarily with gabion baskets)was installed to protect CityCreek Canyon Road. In mostcases the present-day streambedremains flush with the lowest tierof these gabions, suggesting thatbed elevations have remainedstable since the 1980s.

The longitudinal connectivity ofthe City Creek riparian corridorremains relatively intactcompared to the other streamsassessed as part of the RCS. Bonneville Boulevard is the onlymajor road crossing within thestudy area. However, various in-channel structures have beeninstalled to protect utilitycrossings including water, sewer,and petroleum pipelines. Someof these structures influencetransport of sediment and debrisand may impede fish passage. Channel dewatering and facilitiesassociated with the CCWTP alsointerrupt the corridor’slongitudinal connectivity. Insum, existing channel conditionswithin the City Creek corridorreflect a complex response to avariety of historical and ongoing

alterations throughout thewatershed.

Recent and AnticipatedFuture Trends

Within both the upper and lowerCity Creek subwatersheds, landuse predictions for 2030 do notindicate significant changes inimpervious cover (SLCO 2009). This is a unique situation relativeto the other lower subwatershedswithin Salt Lake City, whereincreases in imperviousness of12% or more are anticipatedover this time period.

Climate change can beanticipated to affect the CityCreek riparian corridor. Climateprojections for the southwestregion of the United States showincreased temperatures, reducedmountain snowpack, a 10–20%decrease in annual runoff

volume, reduced springtimeprecipitation amounts, andanticipated water supplyshortages (Karl et al. 2009). Therisk of drought, as well as the riskof flooding, are expected toincrease. The changes intemperature will likely result in ashift in vegetation communities,and altered precipitation patternswill influence stream hydrologyand channel conditions. Thetiming of snowmelt runoff isexpected to occur earlier in thespring, with an anticipatedreduction in summertime baseflows (Karl et al. 2009). It isdifficult to predict specificchanges to City Creek withcertainty, but recorded annualstream flow volumes on CityCreek already show a decliningtrend between 1970 and 2006(L. Alserda 2009, pers. comm.,Figure 3.6). This trend is




anticipated to continue into thefuture (Karl et al. 2009).

Stream and VegetationConditions

Stream ChannelCharacteristics

Salt Lake County has classifiedCity Creek within the RCS studyarea as entrenched, meaning thechannel is vertically confined.Composite stream stabilityratings for the area were goodduring County streamassessments completed in 2007.Also during these assessments,the County classified City Creekas Rosgen (1996) stream type B4in reach UCC_R9, and as streamtype B3 in the remaining RCSstudy reaches (K. Collins 2009,pers. comm.). County bankfullwidth estimates for the streamreaches in the RCS study arearanged from 10 feet to 23 feet,with an average value of 18 feet(K. Collins 2009, pers. comm.).

Results of RCS field surveys andGIS analyses further illustrate thefact that the City Creek channelis commonly entrenched andinset between tall, steep slopes(Figure 3.7). However, theextent of vertical confinementvaries, and in some locations thechannel shape is wider andincludes broader, flatter bankand floodplain surfaces (Figure3.7). These areas are importantbecause they allow water tospread out horizontally during

flood events, dissipating velocityand reducing erosion potential.

Surveyed channel width valuesare quite variable, ranging fromabout 4 to 20 feet at low flow,with an average value of 12 feet(Table 3.1). During the springrunoff period, field surveys wereconducted at a streamflow of 78cfs, which is close to the1980–2005 average annual highflow value of 67 cfs. Width atthis high flow value varies fromabout 12 to 35 feet, with anaverage of 19 feet. WithinLCC_R02B, channel width hasbeen directly affected by installedbank hardening measures.

Channel slope, as determined foreach stream reach from digitalelevation data, varies from 2.9%to 4.5% within the RCS studyarea, with an average value of3% (Figure 3.8, Table 3.1). CityCreek does not show anyobvious consistent spatial trendsin gradient through the studyarea. Historically, channelgradient would have shifted to aflatter, less confined, moresinuous channel type in theflatter valley area west of StateStreet; however, this portion ofthe creek is now pipedunderground in the NorthTemple conduit.

Median (D50) streambed particlesize at the measured crosssections ranges from 26 to 74millimeters (mm), indicating thatmedium- and large-sized gravelare the dominant substrate sizes

in riffle areas of City Creek(Table 3.1). Medium gravelcomprises the D16 particle size atall cross sections, and cobble-sized material typically comprisesthe D84 particle size (Table 3.1). Embeddedness values are highlyvariable. In flatter-gradientportions of the channel such asrun and pool areas, particle sizesare smaller, with fine gravel andsand often dominant. Noconsistent upstream-to-downstream trends are evident inthe pebble count results; rather,bed material size andembeddedness appear to belargely a function of local factorssuch as sediment inputs fromerosion areas, composition ofbank material, and the presenceor proximity of bedrock.

Vegetation Characteristics

Table 3.2 lists all plant speciesnoted on the data forms duringthe study area mapping effort. Species are identified by theircommon and scientific names,mapping code, wetland indicatorstatus (USFWS 1988), andwhether the species is native toUtah or introduced (NRCS2009). A total of 82 differentspecies were noted during theCity Creek mapping work, with50 of these species being nativeto Utah. As seen in Table 3.3,most of the nonnative specieswithin the corridor occur in thecanopy and understoryvegetation layers, while the shrublayer is dominated almostentirely by native species.




Figure 3.7. Cross-section plots extrapolated from digital elevation data. Plots on left (in red) exhibit lessvertical confinement and include relatively well-developed active floodplain surfaces. Plotson right (in blue) exhibit a greater degree of vertical confinement between tall, steep side slopes.




Table 3.1. Summary of streambed material, channel geometry, and slope data.

REACHNUMBER

MEASURED VALUES AT RIFFLE CROSS SECTIONREACH DATA

STREAMBED MATERIAL SIZE DATA CHANNEL GEOMETRY

D16(mm) a

D50(mm) a

D84(mm) a

PercentEmbedded

Low FlowWetted Width

(ft) b

WettedWidth (ft) b

at 78 cfs c

Local Slope(ft/ft) d

Reach Slope(ft/ft) d

Reach Length(ft) b

UCC_R09 8 26 99 12 7.0 21.0 0.008 0.029 1565

UCC_R10A 8 29 56 3 4.9 35.6 0.020 0.031 1427

UCC_R10B 17 39 102 22 4.4 13.4 0.014 0.036 1905

UCC_R10C 15 46 121 20 14.8 16.9 0.033 0.032 1612

UCC_R11A 17 43 148 29 13.6 16.6 0.024 0.033 1836

UCC_R11B 16 58 171 0 7.6 14.1 0.034 0.032 1207

UCC_R11C 14 40 79 9 13.4 19.7 0.015 0.038 1357

LCC_R01A 14 34 108 8 19.9 21.0 0.041 0.036 1686

LCC_R01B 32 74 187 7 10.9 13.7 0.031 0.045 836

LCC_R01C 15 38 128 13 19.0 23.2 0.028 0.035 1303

LCC_R01D02A 13 37 89 11 13.4 19.0 0.019 0.032 681

LCC_R02B - - - - 10.0 12.0 0.016 0.034 748

averages 15 42 117 12 11.7 19.5 0.024 0.034 1401

a The 16th, 50th, and 84th percentile values of the particle size distribution, in millimeters.b Feet.c Cubic feet per second.d Feet per feet.

Bigtooth maple (Acergrandidentatum), box elder (Acernegundo), cottonwood (Populussp.), and water birch (Betulaoccidentalis) are the mostcommon trees along the stream-side areas of City Creek, withGambel oak (Quercus gambelii)common on upper slope areas. Siberian elm (Ulmus pumila), anintroduced invasive tree, is alsocommon in the lower reaches ofthe study area and comprises thedominant canopy species insome reaches (Table 3.3). Common shrub species include

chokecherry (Prunus virginiana)and redosier dogwood (Cornussericea), with Wood’s rose (Rosawoodsii) and skunkbush sumac(Rhus trilobata) common onupper portions of slopes. Theunderstory vegetation layershows the greatest speciesvariety. Creeping barberry(Mahonia repens) and fieldhorsetail (Equisetum arvense) arethe most common nativeunderstory species in stream-sideareas, with bluebunchwheatgrass and ragweed(Ambrosia sp.) common on

upper slopes. Nonnative turfgrass dominates the understorycover within the formal MemoryGrove lawn area in reachLCC_R02B. Common invasiveunderstory species include lesserburdock (Arctium minus),cheatgrass (Bromus tectorum),and Dalmatian toadflax (Linariadalmatica) (Table 3.3).

Near-stream canopy (tree) coveris generally high throughout thestudy area, averaging 67%(Table 3.4.). Tree cover islimited by the proximity of the




Figure 3.8. Longitudinal profile plot of City Creek streambed based on 2006 digital elevation data. Blackcross marks indicate culvert inlets and outlets; red and blue lines indicate open channel streamsections.

canyon road in some areas, andis lacking in the formal MemoryGrove lawn area (reachLCC_R02B). Because of thegenerally high-quality tree coverwithin the City Creek ripariancorridor, the riparian functions ofshading and water-temperaturecontrol are met to a high degree. Thirty-seven percent of theoverall mapped area has greaterthan 25% cover in all threevegetation layers (canopy, shrub,understory); these areas areparticularly effective at providingthe riparian functions of nutrientfiltration and sediment trapping.

Invasive weeds are a problem inall study reaches, and 52% of theoverall mapped area has a weedclassification of moderate, high,or majority (Table 3.4). Understory cover is minimal inthe reaches downstream fromreach LCC_R01A.

Issues AffectingRiparian Functions

During the baseline assessmentwork, several common issueswere observed to be affectingand limiting riparian functions inthe City Creek corridor. These

issues are discussed by functionbelow.

Aesthetics

Although many visuallyappealing portions of City Creekexist, the presence of trash anddebris degrades corridoraesthetics in a number oflocations. Common types oftrash include miscellaneous smallitems such as bottles, cans, foodwrappers, ropes, tarps, etc. These items are common in themore accessible stream reachesthat are used for recreation.




Table 3.2. Plant species noted during City Creek mapping work.

SCIENTIFIC NAME COMMON NAME WETLAND INDICATOR STATUS NATIVE TO UTAHOR INTRODUCED

Aibes concolor white fir not listed nativeAcer glabrum Rocky Mountain maple facultative nativeAcer grandidentatum bigtooth maple not listed nativeAcer negundo box elder facultative wetland nativeAegilops cylindrica jointed goatgrass not listed introducedAesculus hippocastanum horse chestnut not listed introducedAgrostis gigantea redtop no indicator introducedAilanthus altissima tree of heaven no indicator introducedAlnus incana gray alder facultative wetland nativeAmbrosia artemisiifolia annual ragweed facultative upland nativeArtemisia cana silver sagebrush facultative nativeArctium minus lesser burdock not listed introducedArtemisia tridentata big sagebrush not listed nativeBetula occidentalis water birch facultative wetland nativeBromus tectorum cheatgrass not listed introducedCatabrosa aquatica water whorlgrass obligate wetland nativeCardaria draba whitetop no indicator introducedCentaurea maculosa spotted knapweed not listed introducedCentaurea solstitialis yellow star-thistle not listed introducedCeanothus velutinus snowbrush ceanothus not listed nativeChenopodium album lambsquarters facultative upland nativeChrysothamnus viscidiflorus yellow rabbitbrush not listed nativeCirsium arvense Canada thistle facultative upland introducedCichorium intybus chicory not listed introducedCirsium vulgare bull thistle facultative introducedCornus sericea redosier dogwood facultative wetland nativeCrataegus rivularis river hawthorn not listed nativeCynoglossum officinale gypsyflower (houndstongue) not designated introducedDactylis glomerata orchardgrass facultative upland introducedElaeagnus angustifolia Russian olive facultative introducedEquisetum arvense field horsetail facultative nativeEuphorbia myrsinites myrtle spurge not listed introducedFraxinus pennsylvanica green ash facultative wetland nativeGrindelia squarrosa curlycup gumweed facultative upland nativeGutierrezia sarothrae broom snakeweed not listed nativeHelianthus annuus common sunflower facultative upland nativeHedera helix English ivy not listed introducedHordeum jubatum foxtail barley facultative nativeIsatis tinctoria Dyer's woad not listed introducedJuniperus osteosperma Utah juniper not listed nativeLactuca serriola prickly lettuce facultative upland introduced




Table 3.2. Plant species noted during City Creek mapping work (cont.).

SCIENTIFIC NAME COMMON NAME WETLAND INDICATOR STATUS NATIVE TO UTAHOR INTRODUCED

Linaria dalmatica dalmatian toadflax not listed introducedLonicera involucrata twinberry honeysuckle facultative nativeMahonia repens creeping barberry not listed nativeMaianthemum stellatum starry false lily of the valley facultative nativeMelilotus officinalis yellow sweetclover facultative upland introducedMedicago sativa alfalfa not listed introducedOnopordum acanthium scotch cottonthistle not listed introducedParthenocissus quinquefolia Virginia creeper no occurrence nativePhalaris arundinacea reed canarygrass obligate wetland nativePhragmites australis common reed facultative wetland nativePicea engelmannii Engelmann spruce facultative upland nativePinus nigra Austrian pine not listed introducedPinus ponderosa ponderosa pine facultative upland nativePicea pungens blue spruce facultative nativePopulus angustifolia narrowleaf cottonwood facultative nativePopulus deltoides eastern cottonwood facultative wetland nativePoa pratensis Kentucky bluegrass facultative upland introducedPrunus virginiana chokecherry facultative upland nativePseudoroegneria spicata bluebunch wheatgrass obligate upland nativeQuercus gambelii Gambel oak not listed nativeRhus glabra smooth sumac not listed nativeRhus trilobata skunkbush sumac no indicator nativeRibes montigenum gooseberry currant not listed nativeRobinia pseudoacacia black locust facultative upland nativeRosa woodsii Woods' rose facultative nativeRumex crispis curly dock facultative wetland introducedSalix bebbiana bebb willow facultative wetland nativeSalix exigua narrowleaf willow obligate wetland nativeSalix fragilis crack willow facultative introducedSchoenoplectus acutus hardstem bulrush obligate wetland nativeScirpus microcarpus panicled bulrush obligate wetland nativeSecale cereale cereal rye not listed introducedSolanum dulcamara climbing nightshade facultative introducedSolidago missouriensis Missouri goldenrod not listed nativeSorbus scopulina greene's mountain ash no indicator nativeSymphoricarpos albus common snowberry facultative upland nativeSymphyotrichum eatonii Eaton's aster facultative nativeThinopyrum intermedium intermediate wheatgrass not listed introducedToxicodendron rydbergii western poison ivy facultative upland nativeTragopogon dubius yellow salsify not listed introducedUlmus pumila Siberian elm not listed introduced




Table 3.3. List of mapped canopy, shrub, and understory plant species found in each assessed streamreach.

PLANT SPECIES

UCC_

R09

UCC_

R10

A

UCC_

R10

B

UCC_

R10

C

UCC_

R11A

UCC_

R11B

UCC_

R11C

LCC_

R01A

LCC_

R01B

LCC_

R01C

LCC_

R01D

_R0

2A

LCC_

R02B

Common Name Scientific Name

CANO

PY

White fir Aibes concolor X X

Rocky mountain maple Acer glabrum X

Bigtooth maple Acer grandidentatum X X X X X X X X X X X X

Box elder Acer negundo X X X X X X X X X X X X

Water birch Betula occidentalis X X X X X X X X X

Green ash Fraxinus pennsylvanica X X X X X X

Utah juniper Juniperus osteosperma X X

Engelmann spruce Picea engelmannii X X

Ponderosa pine Pinus ponderosa X

Blue spruce Picea pungens X X

Narrowleaf cottonwood Populus angustifolia X X X X X X X X X X X X

Eastern cottonwood Populus deltoides X X X X X

Gambel oak Quercus gambelii X X X X X X X X X X X X

Smooth sumac Rhus glabra X

Austrian pine a Pinus nigra a X

Horse chestnut a Aesculus hippocastanum a X X X X

Russian olive b Elaeagnus angustifolia b X X X X

Black locust c Robinia pseudoacacia c X

Tree of heaven c Ailanthus altissima c X X

Siberian elm b Ulmus pumila b X X X X X X

SH

RUB

Chokecherry Prunus virgniana X X X X X X X X X X

Gray alder Alnus incana X

Common snowberry Symphoricarpos albus X

Snowbrush ceanothus Ceanothus velutinus X X

Greene's mountain ash Sorbus scopulina X

Redosier dogwood Cornus sericea X X X X X X X X X X

River hawthorn Crataegus rivularis X X

Twinberry honeysuckle Lonicera involucrata X

Skunkbush sumac Rhus trilobata X X X X X X X X X

Gooseberry currant Ribes montigenum X X

Woods' rose Rosa woodsii X X X X X X X X X X X

Bebb willow Salix bebbiana X

Narrowleaf willow Salix exigua X X X X X X




Table 3.3. List of mapped canopy, shrub, and understory plant species found in each assessed stream reach(cont.).

PLANT SPECIES

UCC_

R09

UCC_

R10

A

UCC_

R10

B

UCC_

R10

C

UCC_

R11A

UCC_

R11B

UCC_

R11C

LCC_

R01A

LCC_

R01B

LCC_

R01C

LCC_

R01D

_R0

2A

LCC_

R02B


SH

RUB

Yellow rabbitbrush Chrysothamnusviscidiflorus

X X X

Common reed Phragmites australis X

Silver sagebrush Artemisia cana X X X

Big sagebrush Artemisia tridentata X

Crack willow a Salix fragilis a X X X X X

UNDE

RSTO

RY

Annual ragweed Ambrosia artemisiifolia X X X X X X X X X X X

Bluebunch wheatgrass Pseudoroegneria spicata X X X X X X X X X X

Broom snakeweed Gutierrezia sarothrae X X X

Common reed Phragmites australis X X X

Common sunflower Helianthus annuus X X X X X X X X

Creeping barberry Mahonia repens X X X X X X X X X X X X

Curlycup gumweed Grindelia squarrosa X X X X X X X X

Eaton's aster Symphyotrichum eatonii X X X X X X X X

Field horsetail Equisetum arvense X X X X X X X X X X

Foxtail barley Hordeum jubatum X X X X

Hardstem bulrush Schoenoplectus acutus X

Missouri goldenrod Solidago missouriensis X X X X X X X

Panicled bulrush Scirpus microcarpus X

Reed canarygrass Phalaris arundinacea X X

Starry false lily of the valley Maianthemum stellatum X X X X X X X X

Virginia creeper Parthenocissusquinquefolia

X X X

Water whorlgrass Catabrosa aquatica X X X X X

Western poison ivy Toxicodendron rydbergii X X X X X X

Redtop a Agrostis gigantea a X X X X X X X X

Yellow sweetclover a Melilotus officinalis a X X X X X X X X X

Alfalfa a Medicago sativa a X X X

Lambsquarters a Chenopodium album a X

Orchardgrass a Dactylis glomerata a X X X X X X X X X X

Kentucky bluegrass a Poa pratensis a X X X X X

Intermediate wheatgrass a Thinopyrum intermedium a X X X

Yellow salsify a Tragopogon dubius a X

Chicory a Cichorium intybus a X X X




Table 3.3. List of mapped canopy, shrub, and understory plant species found in each assessed stream reach(cont.).

PLANT SPECIES

UCC_

R09

UCC_

R10

A

UCC_

R10

B

UCC_

R10

C

UCC_

R11A

UCC_

R11B

UCC_

R11C

LCC_

R01A

LCC_

R01B

LCC_

R01C

LCC_

R01D

_R0

2A

LCC_

R02B


UNDE

RSTO

RY

Climbing nightshade a Solanum dulcamara a X X X X X X X X

Prickly lettuce a Lactuca serriola a X X X X X X X X X

Curly dock a Rumex crispis a X

Cereal rye a Secale cereale a X X

Lesser burdock b Arctium minus b X X X X X X X X X X

Cheatgrass c Bromus tectorum c X X X X X X X X

Spotted knapweed b Centaurea maculosa b X X

Yellow star-thistle b Centaurea solstitialis b X X

Myrtle spurge b Euphorbia myrsinites b X X X

English ivy c Hedera helix c X X X

Dyer's woad b Isatis tinctoria b X X X X X

Dalmatian toadflax b Linaria dalmatica b X X X X X X X X X

Scotch cottonthistle b Onopordum acanthium b X X X X X

Whitetop b Cardaria draba b X

Gypsyflower(houndstongue) b Cynoglossum officinale b X X X X

Jointed goatgrass b Aegylops cylindrica b X X

Bull thistle b Cirsium vulgare b X X

Canada thistle b Cirsium arvense b X Xa Species not native to Utah.b State- or City-listed, nonnative, noxious weed species.c Nonnative, invasive species.

Another common category oftrash is remnant/obsolete piecesof infrastructure such as pieces ofconcrete and asphalt or old pipesand barrels. In some instancesthe concrete and asphalt piecesare associated with poorlydesigned bank stabilizationefforts. A total of 43 individuallitter areas were mapped in thestudy area during the RCSbaseline assessment work. Otheritems impacting aesthetics

include utility pipe crossings andgabion basket bank protection.

Wildlife Habitatand Connectivity

A wide range of native bird andmammal species rely on nativeinsects as a key food source(Tallamy 2009). These insectsmust share an evolutionaryhistory with plants in order torecognize them and use them as

a food source. Therefore,healthy native plant communitiesare necessary for a ripariancorridor to function to itsmaximum potential in terms ofwildlife habitat. As discussedabove, invasive nonnative plantspecies are a concern in all of thestudy reaches within the CityCreek corridor, and significantlyaffect the composition of theunderstory and canopyvegetation layers. In some areas




Table 3.4. Percent cover and invasive species class for mapped vegetation polygons.

REACH POLYGONNUMBER

PERCENT CANOPYCOVER

PERCENTSHRUB COVER

PERCENT UNDERSTORYCOVER

INVASIVE SPECIESCLASS

UCC_R09 1 0 0 51—75 high

UCC_R09 2 51—75 26—50 1—5 none

UCC_R09 3 51—75 51—75 76—100+ low

UCC_R09 4 0 6—25 76—100+ none

UCC_R09 5 25—50 6—25 76—100+ none

UCC_R09/10A 6 51—75 6—25 51—75 none

UCC_R09 7 51—75 26—50 76—100+ high

UCC_R09 8 76—100+ 6—25 76—100+ none

UCC_R09 9 51—75 26—50 76—100+ moderate

UCC_R09 10 76—100+ 26—50 26—50 low

UCC_R09 11 51—75 26—50 51—75 moderate

UCC_R09 12 6—25 1—5 51—75 majority

UCC_R09 13 51—75 6—25 51—75 moderate

UCC_R09 14 6—25 0 26—50 none

UCC_R10A 15 51—75 6—25 51—75 moderate

UCC_R10A 16 76—100+ 6—25 76—100+ low

UCC_R10A 17 76—100+ 51—75 26—50 none

UCC_R10A 18 51—75 6—25 26—50 low

UCC_R10A 19 76—100+ 76—100+ 26—50 moderate

LCC_R02B 20 26—50 6—25 51—75 majority

LCC_R02B 21 26—50 26—50 76—100+ majority

LCC_R01D/02A/02B 22 51—75 26—50 6—25 moderate

LCC_R01D/02A/02B 23 26—50 26—50 51—75 moderate

LCC_R02B 24 26—50 0 76—100+ moderate

LCC_R01D/02A/02B 25 0—25 0—5 51—100+ low

UCC_R10A 26 76—100+ 26—50 51—75 low

UCC_R10A 27 76—100+ 51—75 26—50 low

UCC_R10B 28 76—100+ 76—100+ 76—100+ none

UCC_R10A/10B 29 76—100+ 51—75 51—75 moderate

UCC_R10A/10B 30 51—75 76—100+ 6—25 none

UCC_R10B 31 76—100+ 6—25 76—100+ low

UCC_R10B 32 76—100+ 51—75 76—100+ none

UCC_R10B 33 76—100+ 51—75 51—75 low

UCC_R10B 34 76—100+ 26—50 76—100+ none

UCC_R10B 35 76—100+ 26—50 76—100+ low

UCC_R10B/10C 36 76—100+ 26—50 51—75 low

UCC_R10B 37 76—100+ 26—50 76—100+ none

UCC_R10C 38 76—100+ 26—50 26—50 low

UCC_R10C 39 76—100+ 6—25 51—75 low

UCC_R10C 40 76—100+ 26—50 76—100+ none




Table 3.4. Percent cover and invasive species class for mapped vegetation polygons (cont.).

REACH POLYGONNUMBER

PERCENT CANOPYCOVER

PERCENTSHRUB COVER



UCC_R10C 41 76—100+ 6—25 51—75 low

UCC_R10C 42 6—25 0 26—50 moderate

UCC_R10C 43 51—75 26—50 51—75 low

UCC_R10C 44 76—100+ 6—25 51—75 none

UCC_R10C 45 76—100+ 26—50 51—75 none

UCC_R10C 46 76—100+ 26—50 76—100+ none

UCC_R10C 47 76—100+ 6—25 51—75 none

UCC_R10C/11A 48 76—100+ 6—25 51—75 none

UCC_R10C 49 76—100+ 51—75 51—75 low

UCC_R10C 50 76—100+ 51—75 51—75 none

LCC_R01B 51 51—75 26—50 6—25 none

LCC_R01B 52 51—75 26—50 26—50 moderate

LCC_R01B 53 6—25 0 76—100+ moderate

LCC_R01B 54 51—75 6—25 26—50 low

LCC_R01B/01C 55 51—75 6—25 26—50 low

LCC_R01B/01C 56 76—100+ 6—25 6—25 moderate

LCC_R01B 57 76—100+ 6—25 6—25 none

LCC_R01B 58 6—25 1—5 51—75 high

LCC_R01B 59 76—100+ 6—25 26—50 none

UCC_R11A 60 51—75 51—75 76—100+ low

UCC_R11A 61 51—75 26—50 76—100+ high

UCC_R11A 62 51—75 26—50 26—50 moderate

UCC_R11A 63 51—75 76—100+ 51—75 high

UCC_R11A 64 51—75 26—50 26—50 high

UCC_R11A 65 26—50 26—50 26—50 moderate

UCC_R11A 66 76—100+ 51—75 51—75 low

UCC_R11A 67 76—100+ 51—75 26—50 high

UCC_R11A 68 51—75 26—50 51—75 low

UCC_R11A 69 26—50 6—25 6—25 moderate

UCC_R11A 70 51—75 51—75 51—75 low

UCC_R11B 71 26—50 26—50 26—50 none

UCC_R11B 72 76—100+ 26—50 26—50 none

UCC_R11B 73 76—100+ 51—75 51—75 moderate

UCC_R11B/11C 74 51—75 26—50 51—75 none

UCC_R11B 75 51—75 51—75 51—75 moderate

UCC_R11B 76 76—100+ 51—75 76—100+ none

UCC_R11B 77 26—50 6—25 26—50 moderate

UCC_R11B 78 26—50 51—75 26—50 moderate

UCC_R11A/11B 79 51—75 6—25 26—50 moderate

UCC_R11C 80 6—25 6—25 26—50 none




Table 3.4. Percent cover and invasive species class for mapped vegetation polygons (cont.).

REACH POLYGONNUMBER

PERCENT CANOPYCOVER

PERCENTSHRUB COVER



UCC_R11C 81 51—75 26—50 6—25 noneUCC_R11C 82 51—75 26—50 6—25 moderateUCC_R11C 83 26—50 6—25 26—50 moderateUCC_R11C 84 26—50 26—50 26—50 moderateUCC_R11C 85 51—75 26—50 26—50 moderateUCC_R11C 86 51—75 26—50 26—50 noneUCC_R11C 87 26—50 6—25 26—50 highUCC_R11C 88 51—75 51—75 26—50 moderateUCC_R11C 89 51—75 26—50 51—75 highUCC_R11C 90 6—25 26—50 26—50 highUCC_R11C 91 76—100+ 51—75 6—25 moderateLower Debris Basin 92 76—100+ 51—75 26—50 majorityLCC_R01A 93 26—50 6—25 26—50 moderateLCC_R01A 94 51—75 6—25 26—50 highLCC_R01A 95 51—75 26—50 26—50 highLCC_R01A 96 26—50 6—25 6—25 moderateLCC_R01A 97 51—75 26—50 26—50 moderateLCC_R01A 98 51—75 26—50 26—50 highLCC_R01A 99 26—50 6—25 26—50 moderateLCC_R01A 100 76—100+ 26—50 26—50 moderateLCC_R01A 101 76—100+ 6—25 6—25 noneLCC_R01A 102 51—75 26—50 6—25 moderateLCC_R01A/01B 103 51—75 26—50 26—50 noneLCC_R01A 104 51—75 26—50 26—50 moderateLCC_R01B 105 26—50 6—25 26—50 moderateLCC_R01A 106 26—50 26—50 51—75 highLCC_R01A 107 6—25 6—25 26—50 moderateLCC_R01C 108 6—25 1—5 26—50 highLCC_R01C 109 6—25 1—5 51—75 highLCC_R01C 110 51—75 6—25 6—25 moderateLCC_R01C 111 6—25 6—25 51—75 highLCC_R01C 112 26—50 6—25 51—75 highLCC_R01C/01D/02A 113 6—25 6—25 76—100+ moderateLCC_R01C 114 76—100+ 6—25 0 highLCC_R01C/01D/02A 115 76—100+ 6—25 51—75 highLCC_R01C 116 26—50 6—25 51—75 moderateLCC_R01C 117 76—100+ 26—50 6—25 highLCC_R01D/02A 118 26—50 0 51—75 highLCC_R01D/02A 119 51—75 51—75 51—75 highLCC_R01D/02A 120 76—100+ 51—75 6—25 moderateLCC_R01D/02A 121 51—75 26—50 51—75 highLCC_R01D/02A 122 51—75 6—25 51—75 majority




invasive species comprise themajority plant cover within avegetative layer, limiting theability of native plants to thriveand support native insects, birds,and wildlife. A lack of understoryand shrub cover in some reachesalso limits habitat quality in termsof structural diversity, which is

particularly important for birdpopulations.

As discussed previously, thelongitudinal connectivity of CityCreek within the RCS study areais relatively intact compared toother streams within Salt LakeCity. However, the debris basinsand associated culverts nearBonneville Boulevard create aninterruption in the corridor thatimpedes fish passage (Figure3.12, Table 3.5). The othercrossings within the study areaare generally footbridges that donot significantly impedeconnectivity with regards to fishor wildlife (Table 3.5). However,several concrete in-channelapron/sill structures that protectpipe crossings create significantvertical drops that may hinderfish passage (Table 3.6).

Nutrient Filtrationand Sediment Trapping

Several areas within the CityCreek corridor lack the densenear-stream understory andshrub cover that is needed tomaximize the ability of theriparian corridor to filtersediment, nutrients, andpollutants from storm runoff. Another factor limiting thefiltration function in portions ofCity Creek is the design of stormdrain outfalls. Some pipesdischarge directly into the mainflow of the stream, eliminatingthe opportunity for runoff to befiltered by riparian vegetation.

Invasive plants of concernin the study area:

• spotted knapweed

• yellow starthistle

• dalmation toadflax

• dyer’s woad

• whitetop

• Canada thistle

• gypsyflower(houndstongue)

• myrtle spurge

• Scotch thistle

• bull thistle

• common burdock

• Russian olive

• Siberian elm

• tree of heaven

• jointed goatgrass

• English ivy

• black locust

• cheatgrass

Dominant canopy species/vegetation types in thestudy area:

• bigtooth maple

• bigtooth maple-Gambeloak

• box elder

• Gambel oak

• introduced/native tree mix

• ornamental mix

• other native trees orshrubs

• Siberian elm

• understory

• water birch




Table 3.5. Bridges and culvert crossings in the study area.

CROSSINGLOCATION/DESCRIPTION

REACHNUMBER(S)

APPROXIMATECULVERTLENGTH

(feet)

CROSSINGTYPE

SIZE/DESCRIPTION

CONDITION RECOMMENDATIONS

Red Footbridge between UCC_R10Band UCC_10C

N/A a pre-fabricatedmetal footbridge

full span, highclearance

good none

Wood PlankFootbridge

between UCC_11B andUCC_11C N/A a

wood plank onmetal I-beam

supports

low clearance; doesnot span entire

floodplain

planks missing;in disrepair

remove or repair toeliminate safety

concern

BonnevilleBoulevard

between UCC_R11Cand lower debris

basin96

concrete culvertpipe 6-foot diameter good none

Canyon Road between lower debrisbasin and LCC_R01A

72 concrete culvertpipe

6-foot diameter good none

Brown Footbridge middle of LCC_R01B N/A a pre-fabricatedmetal footbridge


good none

Stream Gage between LCC_R01Band LCC_R01C

N/A a footbridge acrossconcrete flume

high clearance,spans width

between concreteflume walls

good improve bed protectionbelow concrete flume

Concrete utilitycrossing

LCC_R01C 6 concrete/stonearch culvert

6-foot-wide arch;3.5-foot clearance

evidence ofsediment

deposition onupstream side

monitor condition; iffeasible replace withfull-span structure

Concrete utilitycrossing LCC_R01D_02A 6

concrete/ stonearch culvert

7-foot-wide arch;3.5-foot clearance

fair; crack incement

monitor condition; iffeasible replace withfull-span structure

ConcreteFootbridge

LCC_R01D_02A N/A a concrete bridgeand wingwalls

3-foot clearance;spans width

between concretewingwalls

fair; some scourat wingwalls

remove bridge/wingwallsand replace with bridge

that spans entirefloodplain

Rockwork Bridge middle of LCC_R02B N/A a arched rockworkfootbridge


good none

Concrete Bridge inMemory GrovePark

LCC_R02B N/A aflat concrete

footbridge withmetal railing

high clearance;spans width

between groutedrock walls

good none

a Not applicable.




Table 3.6. Existing significant streambed protection structures in the study area.

REACH a DESCRIPTION POTENTIAL FISHBARRIER

ESTIMATED VERTICALDROP IN WATER

SURFACE ATSTRUCTURE b

LATERAL CHANNELCONFINEMENT

UCC_R11C concrete sill/old gage yes 3 feet yes (concrete wall)

UCC_R11C concrete and rubble yes two drops, each 2 feet yes (concrete walls)

UCC_R11C narrow concrete sill no 0.5 foot yes (concrete piers)

LCC_R01B narrow concrete sill no 0.8 foot yes (concrete wing walls;gabions)

LCC_R01B cemented bed yes 2.5 to 3 feet no

LCC_R01B boulder weir unlikely 2 feet total no

LCC_R01Bconcrete block, boulders,

rubble unlikely several drops, each 2 feetnot confined at structure,but confined just upstream

by stone block wall

LCC_R01B two long concrete pieces no 0.5 foot no

LCC_R01B concrete flume(stream gage)

yes two drops; 3.5 and 2.5 feet yes (concrete walls)

LCC_R01C grouted rock dropstructure

yes 4 feet yes (grouted rockwing walls)

LCC_R01D_02A grouted rock dropstructure

yes 10 feet yes (narrow grouted rockwing walls)

LCC_R01D_02A grouted rock dropstructure

yes 3.5 feet yes (grouted rock wingwalls)

LCC_R02B grouted rock dropstructure

yes 5 feet yes (grouted rock channel)





a Structures listed in upstream to downstream order.b Assessed at low flow.




Stream Stability

A number of different issues werenoted as affecting stream stabilitywithin the City Creek ripariancorridor. Specific issues arediscussed in the subsectionsbelow.

Fill EncroachmentSteep, poorly vegetated fill slopesassociated with roads affectstability in portions of the lowerreaches of City Creek and areparticularly prevalent in reachLCC_R01A. Because theseslopes lack vegetation and theassociated resistive strength itprovides, they are vulnerable toerosion during flood events. Fillslopes also restrict floodplainwidth and laterally confine highflows, causing increased flowvelocities and shear stress onstreambanks. Fill encroachmentalso limits the ability of floodplainsurfaces to act as sponges toabsorb flood water.

Road Runoff and StormDrain/Pipe OutfallsErosion associated with poorlycontrolled road runoff wasobserved in several studyreaches. In a number ofinstances, road runoffconcentrates at the edges ofinstalled gabion basket bankarmoring, causing rilldevelopment at the gabion/natural bank interface. Rills werealso observed at severalunprotected pipe outfalls thatconvey tributary and storm

flows under roads and into CityCreek. The outfalls lackadequate outlet protection todissipate runoff velocities andlack stabilized conveyancechannels between the outlet andthe main City Creek channel. Clogged roadside storm draininlet grates were also observed inseveral locations, and could leadto further rill creation when flowsbypass the designed storm drainpipe system.

Factors limiting shruband understory cover:

• oversteepened slopes

• inadequate revegetationefforts followingmaintenance orconstruction work

• soil compaction fromheavy foot traffic

• poorly controlled runofffrom upland areas

Top left: Steep bare slope adjacent to Bonneville Boulevard in reachLCC_R01A. Note trees, grass, and large rocks at base of slopeindicating toe stability. Top right: Rill erosion below pipe outfalls.Bottom left: Rill erosion where road runoff concentrates at gabion/natural bank interface. Bottom right: Bare bank and compacted soilassociated with heavy recreational foot traffic.




Compaction from FootTrafficWithin the downstream reachesof the study area that receive thegreatest recreational use,trampled bank and floodplainareas were commonly observed. Steep, poorly stabilized accesstrails also occur locallythroughout the study area andare commonly associated withestablished picnic sites. Theseareas lack vegetation and aresusceptible to erosion duringstorm and flood events.

Bank ErosionLateral erosion of streambanks isa natural process in streamchannels, which are dynamicsystems. Erosion and sedimenttransport are necessary for thecreation and maintenance ofimportant habitat features suchas scour pools, undercut banks,and spawning gravels. Deposition of sediment onfloodplain areas is alsoimportant, as it provides freshsubstrate for the growth of willowand cottonwood seedlings thatare needed to maintain nativeriparian forests. However,excessive amounts of erosion ordeposition can degrade habitatand water quality and threateninfrastructure and homes.

In some reaches of City Creektall, vertical, bare banks arepresent where the creek hasmigrated laterally into steep, fine-grained alluvial or tuffformations. This is a naturalerosion process that is relativelycommon within confined canyonsettings where less-resistantgeologic formations are present. Low bank/root zone erosion isanother type of bank erosion thatis common in urbanized channelswhere hydrology is dominated by

flashy, frequent, erosive runoffevents during storms. Becausethe City Creek watershedremains relatively natural,evidence of this type of erosion isminimal in the study area. Thefact that nearly 1 mile of banklength in the study area has beenartificially hardened with gabionbaskets and other structures alsolimits the potential for this type ofbank erosion. As discussedpreviously, in the areas wheregabions were installed after the1983 flood, the present-daystreambed generally remainsflush with the lowest tier ofgabions, suggesting that bedelevations have remained stablesince the 1980s. On otherstreams assessed as part of theRCS, bed lowering andassociated bank erosion causedby the toe of the slope beingundermined was a commonlyobserved process. This does notappear to be occurring to asignificant degree within CityCreek.

Above: Steep vertical bankwhere stream has migratedlaterally into a fine-grainedgeologic formation.

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