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

3. 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 Approach

4. Finite set of probably relevant processes Process category ProcessesMethod Hillslope mass wasting Creep / biogenic transport Shallow and deep landsliding Numerical modeling (UCB) Field surveys / air photos Overland Flow Sheetwash and Rill erosion Field surveys Channel production Head advance / knickpoint Gullying and incision Bank erosion Field surveys / air photos Structure / vegetation ages Field surveys Road-related Cut & fill failures Surface and crossing erosion Gully formation Numerical modeling (SEDMODL2), field surveys Agriculture Surface wash, accelerated runoff, shallow landsliding, bank destabilization Field surveys / air photos Urban Fine sediment release, accelerated runoff Field surveys / air photos Channel Management Destabilization: straightening, riparian veg removal Forced storage Field surveys / air photos Routing and Storage Sediment transport Channel & overbank storage Numerical modeling (TUGS) Field / x-section data

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 Bank erosion Channel incision

9. Corroboration 1: Gauged sediment yields San Geronimo Lagunitas @ SPT Lagunitas @ PRS Gauge Location Average annual bedload (t a -1 ) Average annual suspended sediment load (t a -1 ) Average annual total sediment load (t a -1 ) Average annual unit total sediment load (t km -2 a -1 ) San Geronimo Creek at Lagunitas Rd. bridge (MMWD gauge) 1,6703,6685,337231 Lagunitas Creek at Samuel P. Taylor State Park (USGS gauge 11460400) 6413,6314,272131 Lagunitas Creek at Pt. Reyes Station (USGS gauge 11460600) 2,58414,64017,224276

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

11. Study area Contributing drainage area (km 2 ) Sediment yield into reservoir study area (t a -1 ) MMWD estimate for the period of 1961-1970/6 Stillwater Sciences bathymetry survey estimate for the period of 1961-2008 Stillwater Sciences GLU estimate for the period of 1982-2008 Nicasio/Halleck Creek arm 54.915,10025,47917,533 Entire Nicasio Reservoir (u/s of Seeger Dam) 93.232,640N/A26,595 Bathymetric yields versus GLU

12. Watershed Area Sediment yield derived from sediment rating data Unit rate Bathymetry survey estimate Unit rate Sediment delivery estimated from extrapolated field survey Unit rate km 2 WY 1983–2008 t a -1 t km -2 a -1 WY 1961–2008 a WY 1961–1976 b t km -2 a -1 WY 1983–2008 t a -1 t km -2 a -1 San Geronimo Creek 23.15,340231n/a8,850383 Lagunitas Creek at Samuel P. Taylor 32.74,270131n/a12,330377 Lagunitas Creek at Pt. Reyes Station 62.417,220276n/a19,700316 Nicasio/Halleck Creek arm 54.9n/a25,500 a 46417,550320 Whole Nicasio Reservoir 93.2n/a32,640 b 35026,600285 Comparison of sediment delivery / yield results

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

14. Sediment Budget – processes Sediment Yields t a -1 42% 26% 20% Unit Hillslope & tribs Main- stem Total Yield t km -2 a -1 SanGeron 31669385 LagCr: SGC to DG 206142348 Devils Gulch 22742269 LagCr: DG to Nicasio 214-82133 Regulated Nicasio Cr 129374503 LagCr: NC to PRS 227157384

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

16. Redwood Creek ‘Wolman Curve’

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

18. Estuarine Deposits Jon Warrick’s slides

19. Infilling: you bet! – Redwood Creek Big Lagoon Time period Measure Unit rate Pre- 1840 1841- 1920 1921- 1980 1981-2002 Total Yield to Big Lagoon m 3 a -1 454405743242376 Sedimentation rate (ave = 14%BL+50% susp intercepted) mma -1 2.60.6-4.523.35.7-40.624.76.1-43.232.33.3-56.6 Morphological rate mma -1 0.8-1.211.211.25.6-86.8 Sea-level rise mma -1 1.05-1.51.12.12.1

20. Thoughts…Sac/SJR vs. SF Bay Area Tribs? Pre-European – less “well-connected”: effective area smaller, overbank storage; Pre-European – less “well-connected”: effective area smaller, overbank storage; ~30 t km -2 a -1 ?~30 t km -2 a -1 ? Post-arrival, “resource development”, channel management, first flush: Post-arrival, “resource development”, channel management, first flush: > 350 t km -2 a -1 ; 500 t km -2 a -1 over decades?> 350 t km -2 a -1 ; 500 t km -2 a -1 over decades? Now, “normal” developed use = > 300 t km -2 a -1 Now, “normal” developed use = > 300 t km -2 a -1 Much from channelsMuch from channels Under “protective custody” = < 200 t km -2 a -1 in high relief watershed Under “protective custody” = < 200 t km -2 a -1 in high relief watershed Incision (loss of alluvial storage) is a very effective delivery mechanism Incision (loss of alluvial storage) is a very effective delivery mechanism 100% delivery; very little overbank storage100% delivery; very little overbank storage Not just volumes: changing sediment sources, and sediment caliber Not just volumes: changing sediment sources, and sediment caliber

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