San Pedro Creek:A Longitudinal Profile Study

Andrew Georgeades

Anne Jurek

Mary Snow

Introduction

• Longitudinal study of 1403 foot lowest reach of San Pedro Creek

• Data on water surface elevation (base flow) and of stream bed elevation

• Constructed a longitudinal profile from this data 

Purpose & Justification

• Compare our data and profile to data from similar study in 2003 to provide preliminary assessment of depositional and/or erosional changes  

• Erosion/deposition of sediment can degrade steelhead trout habitat, cause loss of property and unsafe conditions for creekside residents

  

  

Landscape & Development History of San Pedro Creek

• Prior to mid-1800s, lower reach of SPC comprised of large wetland and lagoon 

• Large willow thicket near Linda Mar Shopping Center

• Intenstive agriculture began in 1850s

• Suburban development in 1950s

Current Conditions Of SPCW

• Dense residential and commercial development along valley floor of mainstem

• North Fork sub-watershed extensively culverted• Middle and South Forks used for water supply at• Uplands are designated as county, state, and

federal recreation and parklands, thus protected from development

• Flood Control Project: downstream portion of San Pedro Creek

Hillslope Morphology of San Pedro Creek

• Hillslopes susceptible to gulley formation due to impervious surface area from agriculture and roads and highly weathered Franciscan mélange and granite – Resultant fine sediment in creek

• Hillslopes and the sediment yield from the tributaries are the main contributors of sediment to the mainstem channel

• Landslides contribute sediment due to increased gradient from development combined with inherently unstable hillslope system

• Hillside terraces constructed in the North Fork to counteract landslides,

Hillslope Morphology

Channelization

• Deep entrenchment of the creek due to increased peak discharge and decreased lag time from impervious surfaces

• As much as 16 feet near Capistrano bridge, thus losing access to its historic floodplain

• 217 years ago, downstream reach of stream no more than 5 feet deep

• Most significant sediment source is from streambank failure, enhanced by downcutting .

• Significant undercutting of banks due to riprap, gabions

Channelization, Entrenchment & Undercutting

Steelhead Habitat of SPCW

• San Pedro Creek is only creek within 30 miles of S.F. that provides habitat that supports migrating steelhead trout.

• Good habitat for spawning located throughout Mainstem and Middle Fork – Middle Fork provides the best spawning – Mainstem provides the best conditions for rearing steelhead to

smolt size• Sediments from upland hillslopes are a main contributing factor to

steelhead habitat (gravel and fines)• Steelhead more vulnerable to siltation in mainstem more than Middle

Fork• Destruction of riparian vegetation and bridge obstacles impact habitat

Materials and Methods

• Longitudinal Study (Harrelson, et al 1994)– level, stadia rod, field tape measure

– Determined water surface and elevation and bed elevation at thalweg

– Plotted data in MS tm Excel, – Compared to data of 2003 study

Materials and Methods

Results and Analysis

• Graphical interpretation of data– Influences on Longitudinal Profiles (Leopold et

al 1964)

• Comparisons between 2003 & 2005 longitudinal profiles– Compare only sections that can convincingly be

correlated

Factors that Influence a Longitudinal Profile

• Discharge• Sediment Load• Grain size• Flow resistance

(Manning’s n)

• Velocity • Width • Depth• Slope

San Pedro Creek Flood Control Project Longitudinal Profile

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Distance from Hwy 1 Bridge [ft]

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CUM ELEV CUM WS

San Pedro Creek Flood Control Project Longitudinal Profile

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CUM ELEV CUM WS Linear (CUM ELEV)

Comparison between 2003 & 2005

• Identify similarity in features

• Identify differences

• Discuss Implications

Long Profile 2005

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0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000 2100

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Thalweg 2005 WS 2005

Long Profile 2003

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0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000 2100

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Thalweg 2003 WS 2003

Long Profile 2005

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66 266 466 666 866 1066 1266

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Thalweg 2005 WS 2005 Thalweg 2003 WS 2003

Long Profile 2005

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66 266 466 666 866 1066 1266

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Thalweg 2005 Thalweg 2003

Long Profile 2005

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66 266 466 666 866 1066 1266

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Thalweg 2005 Thalweg 2003

DepositionDeposition

ErosionErosion

Long Profile 2005

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45 245 445 645 845 1045 1245

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Thalweg 2005 Thalweg 2003 Revetments 2005 Revetments 2003

revetments river left

revetments river right

Long Profile 2005

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Thalweg 2005 Revetments 2005 Thalweg 2003 Revetments 2003

Average Slopes of Longitudinal Profiles 2003 & 2005

Slope 2003 y = 0.0027x + 5.8886

Slope 2005 y = 0.0042x + 5.3807

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45 145 245 345 445 545 645 745 845 945 1045 1145 1245 1345

Distance from Hwy 1 Bridge [ft]

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Next Steps for Longitudinal Profile Analysis

• Slope

• Width Cross Sections

• Depth Cross Sections

• Velocity

• Discharge

• Sediment Load

• Grain size Pebble counts

• Flow resistance Use photos &/or descriptions to (Manning’s n) infer n

Summary

• In regard to steelhead habitat, San Pedro Creek is very vulnerable to siltation and sedimentation, compared to other tributaries due land development, increases in impervious areas, increases runoff with resulting incision and bank erosion

Important Field Observations

• Noted thalweg migrating northward for much of measured reach– Change in sinuosity

• Vegetation increase add to measurement difficulties– Anticipate vegetation density to increase with time

• Needed more field expertise prior to collecting vital data

Conclusion

• Further ongoing longitudinal studies like ours are in order

• Strongly recommended that when doing future studies, provide and maintain consistency by identifying and using the same monitoring stations

• Since the tributaries upstream contribute sediment supply to the mainstream, the rest of the watershed should be regularly assessed for sources of sediment