Schoellhamer D, Marineau M, McKee L, Pearce S, Kauhanen P, Salomon M, Dusterhoff S, Grenier L, Trowbridge P (2018) Sediment Supply to San Francisco Bay, Water Years 1995 through 2016: Data, trends, and monitoring recommendations to support decisions about water quality, tidal wetlands, and resilience to sea level rise. San Francisco Estuary Institute; 2018 Jun 11.
Knowledge of the status and trends of sediment supply to San Francisco Bay is critically important for management decisions about dredging, marsh restoration, flood control, contaminants, water clarity (in relation to primary production), and sea level rise. Several sitespecific studies of sediment supply to San Francisco Bay have been conducted, but no synthesis of recent studies is available. The purpose of this report is to synthesize the best available data and knowledge to answer a few of the key study questions related to sediment supply to the Bay (listed below). This synthesis report was prepared jointly by the Regional Monitoring Program for Water Quality in San Francisco Bay (RMP) and the U. S. Geological Survey (USGS) with funding from both organizations. The project is meant to be a step in the development of a more comprehensive sediment management and monitoring strategy for the Bay.
What are the magnitudes and sources of fine and coarse sediment transported to San Francisco Bay?
Net sediment supply to San Francisco Bay from terrestrial sources during the most recent 22- year period (water years [WY] 1995-2016) was 1.9+/-0.8 Mt/yr (1 Mt is one million metric tonnes or 1 billion kilograms). Sixty-three percent of the sediment supply was from small tributaries that drain directly to the Bay. Net supply from the Central Valley (measured at Mallard Island) was 37% of the total supply. Bedload supply, after accounting for dredging, removals, storage in flood control channels, and errors in measurements was indistinguishable from zero. For a 30-year “climate normal” reference period of WY 1981-2010 (a period assumed to be representative of current climatic conditions), we estimate the total sediment supply would be 2.0 Mt/yr of which 70% would come from small tributaries. The delivery points are Mallard Island for sediment from the Delta and the head of tide of each small tributary or outfall for sediment from the small tributaries. The finding that, on average, small tributaries have supplied more sediment to the Bay than the Delta is important but not new (McKee et al., 2013). During the Gold Rush and perhaps through to the 1980s, 80% or more of the supply was estimated to be from the Central Valley (Porterfield, 1980). But land and water management have continued to evolve (Krone, 1996) and the sediment wave associated with the Gold Rush has diminished (Schoellhamer, 2011). In addition, the coastal mountains of California and around the Bay are steep, tectonically active and composed of relatively erodible marine sedimentary and metasedimentary rocks, in contrast to the Central Valley watershed that is dominated by highly indurated granitic, metasedimentary, and metavolcanic rocks in the western-facing slopes of the Sierra Nevada Mountains (McKee et al., 2013). Also, water management is quite different between the Central Valley rivers and small tributaries. About 48% of the Central Valley watershed is upstream from dams ix that are designed to capture, delay and diminish discharge from spring snowmelt and so eliminate or damp many of the peak flows that are normally crucial for sediment transport. Another factor contributing to the importance of small tributaries for sediment supply is the way that they deliver sediment. Annual discharge from small tributaries is very small in comparison to the volume of the Bay (around one-fifth of a Bay volume on average), and the load that small tributaries supply is delivered through hundreds of channels and outfalls via wetland sloughs to the mudflats on the margin of the Bay. Therefore, the majority of this sediment delivered from Bay Area small tributaries is more likely to be trapped in these tidal channels or the margins of the Bay. In contrast, supply from the Central Valley enters the Bay through one large river channel at the head of the estuary (functionally adjacent to Mallard Island, near Pittsburg, CA) with an average annual discharge volume that is more than twice that of the Bay.
What are the present temporal trends of fine and coarse sediment supply to San Francisco Bay?
From the Delta. Since the step decrease in suspended-sediment concentrations in WY 1999 (Schoellhamer et al., 2011), there has been no statistically significant trend in sediment supply from the Delta to the Bay. After WY 1999 there appears to be a slight downward trend that is not statistically significant. However, the possibly downward trend in load was largely driven by decreasing discharge associated with the drought during the latter part of the study period.
From Bay Area Watersheds. At this time, we are unable to conclude whether there has been a trend in sediment supply from Bay Area small tributaries collectively. Trends in sediment loads for individual tributaries could not be determined due to sparse and incomplete datasets, the spatial heterogeneity of anthropogenic changes in watersheds, and the strong influence of variable climate on erosion in the tectonically active coastal area of California. Consistent monitoring of sediment loads at a subset of representative watersheds could be used in the future to answer this question.
Between Subembayments. Erosion in Suisun Bay has been declining as it likely approaches a state of dynamic equilibrium. Over the past 15 years (WY 2002-2016), Suisun Bay was net depositional. Sediment is exported during wet years and imported during dry years. WY 2012-2016 were dry, so the period of record ends with a prolonged dry period and net sediment deposition. The higher discharges of WY 2017 (not quantified for this report) likely exported a large amount of sediment, so the WY 2002-2017 period was likely erosional. Erosion would be consistent with our estimate of mean annual long-term erosion under current conditions. Unfortunately, a similar exercise cannot be performed for the Lower South Bay. Suspendedsediment flux data at the Dumbarton Bridge are not available over a long enough period to x evaluate trends in transport. Given a 50-year plan for wetland restoration, evidence of contamination by both PCBs and Hg, challenges associated with algal productivity, and the overall linkage of sediment dynamics to these issues, we recommend further data collection to provide a better understanding of fluxes in the Lower South Bay.
What are scenarios for future sediment supply to San Francisco Bay?
From the Delta. We hypothesize that future changes in sediment supply from the Delta will be much smaller than those observed since 1850 that resulted from hydraulic mining and dam construction. Ongoing changes associated with climate change and invasive species do not appear to be significantly affecting net sediment supply at present and foreseeable sudden and rapid landscape changes are unlikely. For example, deposition in Suisun Bay was large after hydraulic mining and was followed by extensive erosion of those deposits. The rate of erosion has since been decreasing as the sediment dynamics in the Bay become more stable and this stability, compared to the late 19th and early 20th century, is expected to continue. A record flood, however, has the potential to greatly alter sediment supply. The flood of WY 2017 was rather significant and will provide an interesting reference point for trends once data are published in the spring of 2018. In addition, if the proposed use of tunnels to divert Sacramento River water to export pumps in the southern Delta is implemented, sediment supply to San Francisco Bay is predicted to decrease downstream by 8-9% (p.5.3-24, CDWR 2013). [Link to publication]