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Trends in Sediment Delivery from Bay Area Watersheds determined from sediment budget analysis

Trends in Sediment Delivery from Bay Area Watersheds determined from sediment budget analysis. Peter W. Downs Stillwater Sciences, Berkeley, CA. Lagunitas Creek – sediment budget since 1983. Total area = 281.4 km 2. = 93.3 km 2. = 19.1 km 2. = 64.3 km 2. Periods

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Trends in Sediment Delivery from Bay Area Watersheds determined from sediment budget analysis

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  1. Trends in Sediment Delivery from Bay Area Watersheds determined from sediment budget analysis Peter W. Downs Stillwater Sciences, Berkeley, CA

  2. Lagunitas Creek – sediment budget since 1983 Total area = 281.4 km2 = 93.3 km2 = 19.1 km2 = 64.3 km2 Periods 1850-1918: European arrival & resource development 1919-1945: regulation and grazing 1946-1982: intensive damming 1983-present: raising of Peters Dam, planning, mitigation = 49.0 km2 = 55.7 km2

  3. Approach • Data sources • Establishing rates • existing quantitative sediment source inventories • analysis of sequential aerial photographs • hillslope and in-channel reconnaissance • digital terrain modeling of GLUs for extrapolation • soil production / diffusion model application • road sediment model application • Corroborating rates • literature review of nearby process rates estimates • analysis of gauging records for sediment yields • bathymetric surveys of Nicasio Reservoir • Management scenarios • sediment transport modeling

  4. Finite set of probably relevant processes

  5. Discrete hillslope sources From air photo analysis From field surveys

  6. Extrapolation via Geomorphic Landscape Units

  7. Road-related erosion – SEDMODL2

  8. Channel-related erosion Channel incision Bank erosion

  9. Corroboration 1:Gauged sediment yields San Geronimo Lagunitas @ SPT Lagunitas @ PRS

  10. Corroboration 2:Bathymetric yields Section 12B-12A Section 6A-6B

  11. Bathymetric yields versus GLU

  12. Comparison of sediment delivery / yield results

  13. SedimentBudget – sub-watershed Sediment Yields t a-1 M = mainstem T = tributary & hillslope

  14. Sediment Yields t a-1 SedimentBudget – processes 26% 42% 20%

  15. Implications for the SF Bay…or “So What?” Lagunitas Creek (62.4 km2) • 1983-2008 ~ 330 t km-2 a-1 • (Gauges = 131 – 276; bathymetry = 350 – 464; GLUs = 285 – 383) Southern Tomales bay sedimentation (226-132 km2): Rooney & Smith (1999) • 1861-1931: 325 t km-2 a-1 (prograded primarily 1862-1918) • 1931-1957: 290 t km-2 a-1 • 1957-1994: 190 t km-2 a-1 (dry period; not efficient trapping) Redwood Creek (22.7 km2) • Pre 1840: 34 t km-2 a-1 • 1841-1920: 304 t km-2 a-1 – Euro Arrival and Resource Development: • 1921-1980: 324 t km-2 a-1 – Engineering as Management: • 1981-2002: 198 t km-2 a-1 – Recovery & Restoration – reduces with incision • Future: 154 t km-2 a-1 – because of legacy destabilization of tributaries

  16. Redwood Creek ‘Wolman Curve’

  17. Redwood Creek Pre- 1840 1841 - 1920 1921 - 1980 1981 - 2002

  18. Estuarine Deposits Jon Warrick’s slides

  19. Infilling: you bet! – Redwood Creek Big Lagoon

  20. Thoughts…Sac/SJR vs. SF Bay Area Tribs? • Pre-European – less “well-connected”: effective area smaller, overbank storage; • ~30 t km-2 a-1 ? • Post-arrival, “resource development”, channel management, first flush: • > 350 t km-2 a-1; 500 t km-2 a-1 over decades? • Now, “normal” developed use = > 300 t km-2 a-1 • Much from channels • Under “protective custody” = < 200 t km-2 a-1 in high relief watershed • Incision (loss of alluvial storage) is a very effective delivery mechanism • 100% delivery; very little overbank storage • Not just volumes: changing sediment sources, and sediment caliber

  21. Conceptual Model Yields proportional to: • Relief • Relative erodibility (geology) • Frequency of large floods / high intensity precipitation events – ENSO • Fire / earthquakes triggering landslides • Effective contributing area – tributary disconnection • (Availability of marsh / lagoon / estuary habitat ) • Land use history: agriculture vs. conservation management • Road and rail • Channel management impacts on disconnecting floodplains: straightening (drainage or flood control) • Dams: existing and former – supply reduction them increase • Urban development – supply increase then reduction then increase… • Aggregated mining – interception possibly, more likely narrowing and baseline related incision • 1 / Area

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