Cal. Government Code § 23139

The boundaries of San Joaquin County are as follows: Beginning at the junction of the San Joaquin and Mokelumne Rivers on the line of Sacramento; thence up the latter to the mouth of Dry Creek; thence up Dry Creek to the southeast corner of Sacramento; thence southeasterly to a point on Mokelumne River, being the point of beginning of the survey of Boucher and Wallace of the line between San Joaquin and Calaveras, May, 1864; thence southeasterly, on the line of said survey, to the extreme northern corner of Stanislaus, on the north side of and near to Calaveras River, at a point on the western line of R. 10 E., Mount Diablo meridian, as established by the survey of George E. Drew, approved May, 1860, shown on the map of said survey; thence south, on said range line, to the Stanislaus River; thence down said river to its confluence with the San Joaquin; thence southwest, to the summit of the Coast Range, as shown on the survey and map of Wallace and Stakes, May, 1868, and forming the common corner of San Joaquin, Stanislaus, Santa Clara, and Alameda, as shown also on the map of Boardman and Stakes, July, 1868; thence northwesterly and northerly along the eastern boundary of Alameda to the corner common to Alameda, Contra Costa, and San Joaquin; thence due east to the center of the west channel of the San Joaquin River; thence down the said west channel to its confluence with the main river; thence down said river to the place of beginning.