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 Periods : European arrival & resource development : regulation and grazing : 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 = km 2

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

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

Discrete hillslope sources From air photo analysis From field surveys

Extrapolation via Geomorphic Landscape Units

Road-related erosion – SEDMODL2

Channel-related erosion Bank erosion Channel incision

Corroboration 1: Gauged sediment yields San Geronimo SPT 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, Lagunitas Creek at Samuel P. Taylor State Park (USGS gauge ) 6413,6314, Lagunitas Creek at Pt. Reyes Station (USGS gauge ) 2,58414,64017,224276

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

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

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, Lagunitas Creek at Samuel P. Taylor 32.74,270131n/a12, Lagunitas Creek at Pt. Reyes Station ,220276n/a19, Nicasio/Halleck Creek arm 54.9n/a25,500 a 46417, Whole Nicasio Reservoir 93.2n/a32,640 b 35026, Comparison of sediment delivery / yield results

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

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

Implications for the SF Bay…or “So What?” Lagunitas Creek (62.4 km 2 ) ~ 330 t km -2 a ~ 330 t km -2 a -1 –(Gauges = 131 – 276; bathymetry = 350 – 464; GLUs = 285 – 383) Southern Tomales bay sedimentation ( km 2 ): Rooney & Smith (1999) : 325 t km -2 a -1 (prograded primarily ) : 325 t km -2 a -1 (prograded primarily ) : 290 t km -2 a : 290 t km -2 a : 190 t km -2 a -1 (dry period; not efficient trapping) : 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 : 304 t km -2 a -1 – Euro Arrival and Resource Development: : 304 t km -2 a -1 – Euro Arrival and Resource Development: : 324 t km -2 a -1 – Engineering as Management: : 324 t km -2 a -1 – Engineering as Management: : 198 t km -2 a -1 – Recovery & Restoration – reduces with incision : 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

Redwood Creek ‘Wolman Curve’

Pre Redwood Creek

Estuarine Deposits Jon Warrick’s slides

Infilling: you bet! – Redwood Creek Big Lagoon Time period Measure Unit rate Pre Total Yield to Big Lagoon m 3 a Sedimentation rate (ave = 14%BL+50% susp intercepted) mma Morphological rate mma Sea-level rise mma

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

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