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

Comparison between 2003 & 2005 Identify similarity in features Identify differences Discuss Implications

Deposition Erosion

revetments river left revetments river right

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