Cal. Government Code § 23134

The boundaries of Sacramento County are as follows: Beginning on the northern line of the county, at a point 10 miles north of a point which was, on the thirtieth of March, 1857, the mouth of the American River; thence easterly to the junction of the north and south forks of said river; thence up the principal channel of the south fork to a point one mile above Mormon Island, so as to include said island in Sacramento County, forming the northeast corner; thence southerly to a point on the Cosumnes River, eight miles above the house of William Daylor; thence south to Dry Creek, forming the southeast corner; thence down said Dry Creek to its entrance into the Mokelumne River; thence down the Mokelumne River to a point where said river divides into east and west branches; thence down the west branch to its junction with the east branch; thence down said river to its junction with the San Joaquin River; Thence down the San Joaquin River to the mouth of the Sacramento River, at the head of Suisun Bay, forming the southwest corner; thence up the Sacramento River to the mouth of Steamboat Slough, formerly called Merritt Slough; then up said slough to the mouth of Sutter Slough; thence up said Sutter Slough to the Sacramento River; then up the Sacramento River to a point west of the place of beginning, forming the northwest corner of Sacramento; thence east to the place of beginning.