The San Francisco Bay Area is dominated by one of the world’s great estuaries, an interconnected series of three major bays through which the non-diverted waters of the Sierra Nevada and the Central Valley flow on their journey to the Golden Gate and the Pacific Ocean. The two principal rivers are the Sacramento and San Joaquin, which flow into the shallow waters of Suisun Bay. This bay is, in turn, connected to the San Pablo Bay by the Carquinez Strait and then the main San Francisco Bay.
The margins of the estuary, once characterized by extensive marshes and wetlands, have been modified dramatically during the last one hundred and sixty years. Beginning with the City of San Francisco’s waterfront at the time of the Gold Rush, much of the shoreline has been diked or filled with rubble, shrinking the once extensive flood lands to a small fraction of their former area. Where built upon by the surrounding metropolis, these areas pose a serious liquefaction problem during major earthquakes.
The are four distinctive islands in the estuary. These are the naturally occurring remnants of three hills that once stood above the low valley floor and are now isolated by the invading floodwaters of the Pacific Ocean’s rising sea level: Alcatraz, Angel Island, and Yerba Buena Island. They were joined by the construction of the artificial Treasure Island in the 1930s for the Golden Gate International Exposition. Many more, marginal islands may be found along the still marshy shorelines where the tidal flows through sloughs and creeks surround now diked areas.
Almost 8,000,000 people reside in the nearly one hundred separate cities surrounding the estuary.

A collection of unique aerial panoramas of this region may be seen in the San Francisco Bay section of the California Atlas of Panoramic Aerial Images.