Cal. Gov. Code § 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 261/2 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.
Ca. Gov. Code § 23107