About the Bay

SF Bay/Delta Primer•SF Bay Then & Now•Glossary

From the Sierra To the Sea:
A San Francisco Bay/Delta Primer

The San Francisco Bay-Delta Estuary is the largest estuarine system on the west coasts of both North and South America.

A rich national treasure, it encompasses 1,600 square miles, ranging from the salty waters of San Francisco Bay to the brackish waters of the Sacramento-San Joaquin Delta, and contains over 90 percent of the state’s remaining coastal wetlands. The Estuary’s vast upstream watershed drains more than 40 percent of California’s land mass, including the freshwater streams of the Sierra Nevada and Coast Ranges.

More than one million shorebirds and waterfowl make their home on bay marshes while millions of others use it as a migratory destination or a rest and reenergizing food stop. The Delta is a major nursery for fish and invertebrates as well as a highway for migrating salmon and steelhead.

The system supports some 750 species of fish, animals, and birds. Unfortunately, 18 species of those have been designated by state and federal agencies as threatened or endangered. In addition, hundreds of non-native invasives, including algae, marsh plants, invertebrates, and dozens of fish species, have a major negative impact on an already damaged eco-system.

Most of the fresh water creating the estuary flows from the rugged western slopes of the Sierra Nevada and Cascade mountain ranges. Winter rains average from 40 to 80 inches a year south to north, allowing sparkling streams to carry runoff and snowmelt to the great Central Valley below. Unfortunately, on average only about 60 percent of this freshwater flow reaches San Francisco Bay: the rest is captured upstream of the Delta by thousands of dams and diversions or exported from the Delta to San Joaquin agribusiness and Southern California cities by the giant pumps of the federal and state water projects. This massive water mining of the estuary severely degrades water quality and habitat conditions and interferes with spawning, rearing, and migration of many aquatic species.

In the cold, clear upper waters of the Sierra Nevada, 40 species of native fish, including sculpins, cutthroat, golden, rainbow trout and mountain whitefish, make their home. Downstream, in the oak studded foothills, native species include large suckers, hardhead, and rainbow trout. Massive runs of chinook salmon and steelhead formerly populated the San Joaquin; now, only fall run salmon are seasonally found in large numbers in the Sacramento River.

The Delta, once a 400,000 acre tidal marsh with extensive uplands at the confluence of the Sacramento and San Joaquin rivers, was diked and channelized in the late 1800s and early 1900s, and converted into agricultural islands. Today, only 2 percent of the original marsh habitat remains, and many of the islands have subsided to below sea level behind their protective levees.

San Francisco Bay covers 400 square miles with an average depth of 14 feet with depths plunging to 360 feet at the Golden Gate. The Bay has shrunk by a third in the last 150 years, and only about 25 percent of its original wetland, riparian, and tidal mudflat habitat remains. Even so, the bay still supports a commercial bait shrimp, herring, and Dungeness crab fishery, the only urban commercial fisheries in the nation.

At the same time, San Francisco Bay is also the largest harbor on the U.S. Pacific Coast, with more than 67,000,000 tons of cargo passing annually through the Golden Gate. Unfortunately, much of this cargo is crude oil, making spills and other industrial accidents a constant threat to estuarine fish and wildlife.