Government Code section 23107

The boundaries of Contra Costa County are as follows:
Beginning in the Bay of San Francisco at the northwest point of Red Rock, being the common corner of Marin, Contra Costa, and San Francisco; thence up the Straits and Bay of San Pablo, on the eastern boundary of Marin, to the point of intersection with a line bearing south 2612 degrees E., and about six and one-quarter miles distant from the southwest corner of Napa, forming the common corner of Marin, Solano, Sonoma, and Contra Costa; thence to the Straits of Carquinez; thence up said straits and Suisun Bay, to the mouth of the San Joaquin River; thence up said river, to the confluence of the west and main channels thereof, as laid down on Gibbe’s map; thence up the said west channel, to a point about 10 miles below Moore and Rhodes’ ranch, at a bend where the said west channel, running downward, takes a general course north, the point being on the westerly line of San Joaquin, and forming the northeast corner of Alameda and southeast corner of Contra Costa; thence westerly on the northern line of Alameda to the easterly line of San Francisco; thence due northwest, along said easterly line of San Francisco, four and one-half miles, more or less, to the place of beginning.

Source: Section 23107, https://leginfo.­legislature.­ca.­gov/faces/codes_displaySection.­xhtml?lawCode=GOV§ionNum=23107.­ (updated Jan. 1, 1947; accessed Jun. 25, 2025).

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Verified:
Jun. 25, 2025

§ 23107's source at ca​.gov