San Francisco Bay, a sheet of water in California, connected with the Pacific ocean by a strait 5 m. long and 1 m. wide, called the Golden Gate, in lat. 37° 48' N., lon. 122° 30' W. It extends S. S. W. about 40 m., being separated from the Pacific by a peninsula from 6 to 15 m. wide, on the N. extremity of which is the city of San Francisco. The bay opposite the city is about 7 m. wide, and in its widest part 12 m. The shores of the Golden Gate are bold and rocky, rising on the north in some places nearly 2,000 ft., while on the south the hills are from 300 to 400 ft. high, many of them being covered with shifting white sand. On the bar there is 30 ft. of water at low tide; within there is a much greater depth and good anchorage. The principal islands are Alca-traz, in the middle of the channel, about 4 m. from the entrance; Angel, the largest, containing 800 acres; and Yerba Buena or Goat island, opposite the city. There is a fortification on Alcatraz island, and another at Fort point on the S. side of the Golden Gate. San Francisco bay is connected on the north by a strait 3 m. wide with San Pablo bay, which is nearly round and about 10 m. in diameter; and this receives from the east through Carquinez strait (1 m. wide) the waters of Suisun bay, which is about 8 m. long from E. to W. and 4 m. wide.

San Pablo and Suisun bays are deep, but Carquinez strait has only 16 ft. of water at low tide. At the head of San Pablo bay is Napa or Mare island, on which is a United States navy yard. The Sacramento and San Joaquin rivers, which drain the great valley between the Sierra Nevada and Coast mountains, discharge into Suisun bay. The name San Francisco bay is often extended over San Pablo bay.