Trends in Suspended Sediment Input to the San Francisco Bay from Local Tributaries Presented by Setenay Bozkurt Philip Williams & Associates, Ltd. San Francisco, CA December 2005

PROJECT OBJECTIVE To detect spatial and temporal trends in suspended sediment records No emphasis on the absolute amounts of sediment entering the Bay McKee et al, 2003 for sediment delivery to the Bay

DISCUSSION TOPICS CONCEPTUAL MODEL – GEOMORPHIC PROCESSES – SPATIAL VARIABILITY – TEMPORAL VARIABILITY SUSPENDED SEDIMENT RECORDS – SPATIAL TRENDS – TEMPORAL TRENDS CONCLUSIONS

GEOMORPHIC PROCESSES

Dominant sediment sources: Landslides, gullies, bed/bank erosion, rills and sheetwash

DISCUSSION TOPICS CONCEPTUAL MODEL – GEOMORPHIC PROCESSES – SPATIAL VARIABILITY – TEMPORAL VARIABILITY SUSPENDED SEDIMENT RECORDS – SPATIAL TRENDS – TEMPORAL TRENDS CONCLUSIONS

CONCEPTUAL MODEL FOR SPATIAL VARIABILITY Drainage area Topography Climate Hydrology Geology/Soils Land use/ Land Cover Processes

DISCUSSION TOPICS CONCEPTUAL MODEL – GEOMORPHIC PROCESSES – SPATIAL VARIABILITY – TEMPORAL VARIABILITY SUSPENDED SEDIMENT RECORDS – SPATIAL TRENDS – TEMPORAL TRENDS CONCLUSIONS

Native American Life in California

Arrival of the Spanish… and their cows

All of this occurred before the invention of the modern low impact cow

California in the 20 th Century

California in the 20 th Century: Urban and suburban growth

Hydrograph Modification Impacts Runoff Time Pre-Development Urbanization tends to increase stormwater runoff: peak flows volume frequency Post-Development

Hydrograph Modification Impacts Erosion impacts: small, frequent flows move most sediment hydromodification increases frequency of small flows increased sediment transport = erosion

Incision in Bay Area streams due to hydromod

Sediment Yield: Agricultural sources

CONCEPTUAL MODEL FOR TEMPORAL VARIABILITY DisturbanceExpected effect on sediment yield GrazingIncrease Urban/suburban developmentInitial increase, then decrease ChannelizationIncrease Bed and bank protectionDecrease Dams and reservoirsDecrease Channel dredgingDecrease Other factors (agricultural development and logging) Increase Modified from Wright and Schoellhamer, 2004

DISCUSSION TOPICS CONCEPTUAL MODEL – GEOMORPHIC PROCESSES – SPATIAL VARIABILITY – TEMPORAL VARIABILITY SUSPENDED SEDIMENT RECORDS – SPATIAL TRENDS – TEMPORAL TRENDS CONCLUSIONS

SPATIAL TRENDS IN SUSPENDED SEDIMENT RECORDS 19 watersheds Look for correlation of suspended sediment yield with: – drainage area – annual runoff – relief – % sand in soils – % area with landslides – mean annual precipitation – rainfall threshold statistics – % area with steep slopes – % sandy soils – % area of rangeland and barren land – % area of rangeland with steep slopes – % impervious area

SPATIAL CORRELATIONS Significant correlations between sediment yield and – Drainage area: r 2 = 0.79 – Mean annual runoff: r 2 = 0.78 – Relief: r 2 = 0.68

SPATIAL CORRELATIONS No multiple regression possible due to small data set Several parameters weakly correlated: % sand in soils % landslide area % impervious area

DISCUSSION TOPICS CONCEPTUAL MODEL – GEOMORPHIC PROCESSES – SPATIAL VARIABILITY – TEMPORAL VARIABILITY SUSPENDED SEDIMENT RECORDS – SPATIAL TRENDS – TEMPORAL TRENDS CONCLUSIONS

TEMPORAL TRENDS IN SUSPENDED SEDIMENT RECORDS Do sediment loads change over time? Compare sediment concentrations for a given discharge range over long term

TEMPORAL TRENDS IN SUSPENDED SEDIMENT RECORDS

PREDICTING SUSPENDED SEDIMENT INPUT TO THE BAY Sediment rating curves: Q s vs. Q Sediment loads correlated well for all stations r 2 > 0.70 Scatter due to: – Inherent random changes in any system at any given time – Nonlinear relation between discharge and concentration – Hysteresis and sediment lag of floods Better correlation and predictability if low discharges are excluded

SEDIMENT RATING CURVES

DISCUSSION TOPICS CONCEPTUAL MODEL – GEOMORPHIC PROCESSES – SPATIAL VARIABILITY – TEMPORAL VARIABILITY SUSPENDED SEDIMENT RECORDS – SPATIAL TRENDS – TEMPORAL TRENDS CONCLUSIONS

SUMMARY and CONCLUSIONS Suspended sediment records are spatially and temporally limited Significant functional relationship between discharge and sediment load for all stations rating curves can be used to estimate sediment yields and to extrapolate data for prediction

SUMMARY and CONCLUSIONS Decreasing sediment yields in Alameda Creek at Niles Colma Creek at South San Francisco Cull Creek above the Reservoir Drainage area, mean annual runoff, and relief are the best variables to predict sediment yield from local watersheds

SUMMARY and CONCLUSIONS Significance of decreasing suspended sediment trends implications of decreasing sediment yields for large scale restoration projects around the Bay Need for more quantitative measurements on: dominant geomorphic process e.g. gullies, landslides, channel erosion Need for more suspended sediment measurements