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San Francisco Bay Delta

SF Bay Projects

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San Francisco Bay and Delta

San Francisco Bay (Bay) comprises four major subembayments: Suisun Bay, San Pablo Bay, Central San Francisco Bay, and South San Francisco Bay (see map). This region experiences a mixed semi-diurnal tide—a daily pattern of two high tides of different heights and two low tides of different heights1. The semi-diurnal tidal range varies due to the two-week spring-neap cycle, with maximum range during spring tide and minimum range during neap tide.

The watershed of the Bay begins in the mountains of the Sierra Nevada and ends at the Golden Gate, covering an area of 63,000 mi2 (162,000 km2) or about 40 percent of California's land area. Almost all (95 percent) of the area of this watershed is drained by two major rivers-Sacramento and San Joaquin-which flow through the Central Valley of California. The region surrounding their confluence is called the Delta, which was historically a fresh water tidal marsh. About 90 percent of the fresh water discharge into the Bay is from these two major rivers1; on an annual basis this discharge is greatest following the snowmelt in the Sierra Nevada, which has historically occurred during spring months.

In terms of watershed-sourced sediments entering the Bay, the Central Valley rivers previously supplied the majority, but researchers have observed a decreasing trend over the last 150 years. Current estimates suggest that only 39 percent of suspended sediments entering the Bay between 1995 and 2010 were from the Sacramento and San Joaquin Rivers, down from 86 percent during 1909-19662,3. This decrease is due in part to the cessation of hydraulic mining and the construction of dams4. This trend highlights the increased importance of local Bay tributaries-defined as tributaries that enter the Bay seaward of Mallard Island-in supplying sediment to the Bay.

The bottom sediments in South San Francisco Bay and in the shallow water areas (water depth less than 12 ft or 3.7 m) of Central, San Pablo, and Suisun Bays are composed of mostly silts and clays. Silts and sands are present in the deeper parts of Central, San Pablo, and Suisun Bays.

The landscape of the Delta has changed dramatically in response to human development. The extensive fresh water tidal marsh was leveed and drained during the 1800s to create fertile farmland. Presently, the Delta has a highly complex geometry with canals and waterways adjacent to islands of farmland. Many of these islands have subsided to elevations less than the neighboring waterways; several of the islands are permanently flooded following levee breaches. The Delta is the home of major pumping plants that divert fresh water to southern areas of California. Several endemic species of fish are threatened or endangered. Management of the water flow in the Delta system is challenging because of these competing needs for farming, fresh water exports, and ecosystem habitat.


1 Conomos, T. J., R. E. Smith, and J. W. Gartner, 1985. Environmental setting of San Francisco Bay. Hydrobiologia 129, 1-12.

2 McKee, L. J., M. Lewicki, D. H. Schoellhamer, and N. K. Ganju, 2013. Comparison of sediment supply to San Francisco Bay from watersheds draining the Bay Area and the Central Valley of California. Marine Geology 345, 47-62.

3 Porterfield, G., 1980. Sediment transport of streams tributary to San Francisco, San Pablo, and Suisun Bays, California, 1909-1966. U.S. Geological Survey Water-Resources Investigations 80-64, 91 pp.

4 Wright, S. A. and D. H. Schoellhamer, 2004. Trends in the sediment yield of the Sacramento River, California, 1957 – 2001, San Francisco Estuary and Watershed Science, 2(2).

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