By Jeffrey Mount and Wim Kimmerer

For decades the San Francisco Estuary, which includes San Francisco Bay and the Sacramento-San Joaquin Delta, has been routinely described as “the largest estuary on the west coast of North America.” This appeared in publications of all types, presumably to emphasize the importance and unique nature of the estuary. But this claim is wrong. While the San Francisco Estuary is quite large, with many unique features, the Salish Sea Estuary is the largest by far.

Estuaries were defined six decades ago as semi-enclosed bodies of water where rivers mix with ocean water. This definition reflected the scope of estuarine studies of that era, which were conducted mainly in large river mouths in Europe and eastern North America. This traditional view of estuaries may have influenced current thinking that the San Francisco Estuary is the largest. But, the definition of what constitutes an estuary has since been broadened to include all coastal enclosed water bodies, including those without substantial freshwater input (for example, Tomales Bay).

The San Francisco Estuary, including its historic wetlands, covers an area greater than 1,900 square miles (Figure). The Salish Sea Estuary—made up of Puget Sound, Strait of Juan de Fuca, Georgia Strait, and Desolation Sound—covers 7,200 square miles, almost four times larger. It is the biggest on the west coast and is even larger than the Chesapeake Bay Estuary.

This contrast in size stems from the geologic processes that originally formed the two estuaries.

Most large estuaries are formed by sea level rise. From around 19,000 years to 5,000 years ago, melting of continental glaciers induced a rapid rise in sea level that inundated valleys and canyons formerly occupied by rivers and streams around the globe.

Before the late Pleistocene/Holocene rise in sea level, the ancestral Sacramento River emerged from the Central Valley through the Carquinez Strait and spread out into an alluvial plain in what is now San Pablo and San Francisco Bay. The river then flowed through a narrow canyon at the Golden Gate and formed a large delta adjacent to the Farallon Islands, more than 300’ below present sea level.

By the time early Native Americans arrived, perhaps more than 10,000 years ago, there were vast floodplains, marshes, riparian forests and massive dune fields in what is San Francisco Bay. As sea level rose, brackish and freshwater tidal marsh formed along shorelines throughout the area. About 5,000 years ago, sea level rise pushed into the area near the confluence of the Sacramento and San Joaquin rivers. Here, accumulation of organic material and river sediment kept up with sea level rise—which slowed considerably—forming the Sacramento-San Joaquin Delta.

Until they were subsequently drained or filled, the marshes formed by sea level rise throughout the estuary were the unique characteristic of the relatively shallow San Francisco Estuary. They fueled what must have been one of the most productive estuarine ecosystems in North America, teeming with abundant fish and wildlife (today it has become rather unproductive owing to changes in hydrology, loss of wetlands, and the introduction of countless species from around the world).

The origins of the Salish Sea are quite different, but it is, by modern definitions, an estuary. The glaciers that covered much of the North American continent 25,000 years ago spilled over into the Salish Sea region, carving steep-walled, deep canyons. The ice in some regions was nearly a mile thick, with its base grinding out canyons well below sea level. Around 16,000 years ago these glaciers retreated rapidly, completely disappearing as the oceans began to rise and flood glacial outwash sediments in Juan de Fuca Strait, Puget Sound, and Georgia Strait. Depths in the Salish Sea today reach more than 2,000 feet in some areas. It is one of the deepest estuaries in North America: even deeper than the St. Lawrence Estuary, which is by far the largest. And unlike the San Francisco Estuary, which has one large watershed at its head, many rivers discharge into the former glacial canyons.

The San Francisco and Salish Sea Estuaries are the West Coast’s largest. Their differences in origin shape their hydrodynamics, salinities, and ecology, along with their human uses. But the differences in origins also led to big differences in size. As important as the San Francisco Estuary is to the region’s history, economy, culture, and biodiversity, it is not the biggest. That crown goes to the Salish Sea Estuary.

Jeffrey Mount is a senior fellow at the Water Policy Center, Public Policy Institute of California and founding director at the UC Davis Center for Watershed Sciences. Wim Kimmerer is Estuary and Ocean Science Center Research Professor, San Francisco State University, Romberg Tiburon Campus.

Further reading

Johnson, K. A. and G. W. Bartow, eds. 2018. Geology of San Francisco, California, United States of America. Association Engineering Geologists, Geology of Cities of the World Series.

Lasmanis, R. and E. S. Cheney, eds. 1994. Regional Geology of Washington State. Washington Division of Geology and Earth Resources, Bulletin 80.

Malamud-Roam, F., M. Dettinger, L. B. Ingram, et al. 2007. Holocene Climates and Connections between the San Francisco Bay Estuary and its Watershed: A Review. San Francisco Estuary and Watershed Science, 5 (1).

McLusky, D. S. and M. Elliott, M. 2004. The Estuarine Ecosystem: Ecology, Threats and Management. New York: Oxford University Press.