San Francisco Bay - Size

Size

The Bay covers somewhere between 400 and 1,600 square miles (1,040 to 4,160 square kilometers), depending on which sub-bays (such as San Pablo Bay), estuaries, wetlands, and so on are included in the measurement. The main part of the Bay measures 3 to 12 miles (5 to 20 km) wide east-to-west and somewhere between 48 miles (77 km)1 and 60 miles (97 km)2 north-to-south. It is the largest Pacific estuary in the Americas.

The bay was navigable as far south as San Jose until the 1850s, when hydraulic mining released massive amounts of sediment from the rivers that settled in those parts of the bay that had little or no current. Later, wetlands and inlets were deliberately filled in, reducing the Bay's size since the mid-19th century by as much as one third. Recently, large areas of wetlands have been restored, further confusing the issue of the Bay's size. Despite its value as a waterway and harbor, many thousands of acres of marshy wetlands at the edges of the bay were, for many years, considered wasted space. As a result, soil excavated for building projects or dredged from channels was often dumped onto the wetlands and other parts of the bay as landfill.

From the mid-19th century through the late 20th century, more than a third of the original bay was filled and often built on. The deep, damp soil in these areas is subject to liquefaction during earthquakes, and most of the major damage close to the Bay in the Loma Prieta earthquake of 1989 occurred to structures on these areas.

The Marina District of San Francisco, hard hit by the 1989 earthquake, was built on fill that had been placed there for the Panama-Pacific International Exposition (1915), although liquefaction did not occur on a large scale. In the 1990s, San Francisco International Airport proposed filling in hundreds more acres to extend its overcrowded international runways in exchange for purchasing other parts of the bay and converting them back to wetlands. The idea was, and remains, controversial. (For further details, see the "Bay Fill and Depth Profile" section.)

There are five large islands in San Francisco Bay. Alameda, the largest island, was created when a shipping lane was cut in 1901. It is now predominately a bedroom community. Angel Island was known as "Ellis Island West" because it served as the entry point for immigrants from East Asia. It is now a state park accessible by ferry. Mountainous Yerba Buena Island is pierced by a tunnel linking the east and west spans of the San Francisco – Oakland Bay Bridge. Attached to the north is the artificial and flat Treasure Island, site of the 1939 Golden Gate International Exposition. From the Second World War until the 1990s, both islands served as military bases and are now being redeveloped. Isolated in the center of the Bay is Alcatraz, the site of the famous federal penitentiary. The federal prison on Alcatraz Island no longer functions, but the complex is a popular tourist site. Despite its name, Mare Island in the northern part of the bay is a peninsula rather than an island.