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

Abstract

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 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 twi