San Francisco Bay
San Francisco Bay, the largest estuary on North America’s west coast, consists of four smaller, interconnected bays. One of these, Suisun Bay, receives fresh water drained from about 40 percent of California’s land area. This water flows into San Pablo Bay and then Central Bay, where it mixes with salt water that has entered deep down from the Pacific Ocean via the Golden Gate channel. From Central Bay, there is little flow of fresh water to the largest body of water, South San Francisco Bay, but there is some surface outflow of brackish water to the Pacific. San Francisco Bay is a tectonic estuary—one caused by movement at tectonic faults (lines of weakness) in Earth’s crust, of which there are several in the area, notably the San Andreas Fault.
During the past 150 years, human activity has resulted in the loss of 90 percent of the bay’s surrounding marshy wetland, a greatly reduced flow of fresh water (which has been diverted for agricultural purposes), and contamination by sewage and effluent. Nevertheless, the bay remains an important ecological habitat. Its waters are home to large numbers of economically valuable marine species, such as Dungeness crab and Pacific halibut, and millions of geese and ducks annually use the bay as a refuge.