San Francisco Bay is the hub for downtown San Francisco, California. On land it is called the San Francisco Bay Area. The bay itself is a shallow estuary where much of the water flows into the Sacramento and San Joaquin Rivers. Roughly 40 percent of the estuary flows into the rivers from the Sierra Nevada Mountain Range. The estuary carries the water into the Pacific Ocean. Also, both rivers bring water to Suisun Bay close to the Carquinez Strait before meeting the Napa River and San Pablo Bay.
The Bay is a maximum length of 60 miles and 12 miles wide. There is a surface area of 400 to 1,600 square miles depending on the time of year. San Francisco Bay is connected with several bays, which are technically a part of San Francisco Bay. The bay is large enough to be part of Oakland and San Jose. The bay is made up of several acres of wetlands. From the late 19th to 20th century the bay was filled in and built on by the use of dredging. However, liquefaction and earthquakes made the area unstable. Geologically, San Francisco Bay lies on the San Andreas Fault to the west and Hayward Fault to the east.