Cal. Gov. Code § 23149
The boundaries of Sonoma County are as follows:
Commencing at a point in the Pacific Ocean, three miles due west of a point in the center of the channel at the mouth of the Gualala River, thence due east three miles to said point in the center of the channel at the mouth of said Gualala River; thence up the center channel of said Gualala River to a point where the center of said channel intersects the section line running east and west between Secs. 23 and 26, T. 11 N., R. 15 W., M. D. B. & M.; thence east on said section line and its continuation between Secs. 24 and 25, said township and range, to the range line between R. 14 and 15 W., M. D. M.; thence continuing east on the section line between Secs. 19 and 30, 20 and 29, 21 and 28, 22 and 27, 23 and 26, and 24 and 25, T. 11 N., R. 14 W., M. D. B. & M., to the range line between R. 13 and 14 W., M. D. M.; thence north on said range line between said R. 13 and 14 two miles more or less, to the section corner common to Secs. 12 and 13, T. 11 N., R. 14 W., M. D. B. & M., and Secs. 7 and 18, T. 11 N., R. 13 W., M. D. B. & M.;
Thence east on the section line between Secs. 7 and 18, 8 and 17, 9 and 16, 10 and 15, 11 and 14, and 12 and 13, T. 11 N., R. 13 W., M. D. B. & M., to the intersection of said section line with the range line between R. 12 and 13 W., M. D. M.; thence continuing east on the section line between Secs. 7 and 18, 8 and 17, 9 and 16, 10 and 15, 11 and 14, and 12 and 13, T. 11 N., R. 12 W., M. D. B. & M., to the intersection of said section line with the range line between R. 11 and 12 W., M. D. M.;
Thence north on said range line between R. 11 and 12, two miles, more or less, to the southwest corner of T. 12 N., R. 11 W., M. D. B. & M.; thence east on the south boundary line of said T. 12 N., R. 11 W., three miles, more or less, to the southeast corner of Sec. 33, T. 12 N., R. 11 W.; thence north on the section line between Secs. 33 and 34, one mile, more or less, to the northwest corner of said Sec. 34; and thence east on the section line between Secs. 27 and 34, 26 and 35, and 25 and 36, T. 12 N., R. 11 W., M. D. B. & M., and continuing east on the section line between Secs. 30 and 31, 29 and 32, 28 and 33, 27 and 34, 26 and 35, and 25 and 36, T. 12 N., R. 10 W., M. D. B. & M., and continuing east on the section line between Secs. 30 and 31, 29 and 32, 28 and 33, 27 and 34, and 26 and 35 to the corner common to Secs. 25, 26, 35, and 36, T. 12 N., R. 9 W., M. D. B. & M., said point lying upon the summit of the Mayacamas Ridge and constituting the common corner of Mendocino, Lake, and Sonoma; thence southerly along the Mayacamas Mountains, and on the western lines of Lake and Napa, to the westerly branch of headwaters of Huichica Creek;
Thence westerly on the line of Napa to the top of the main ridge that divides the Huichica Valley from the Sonoma Valley; thence southerly along the said dividing ridge to the tule bordering on San Pablo Bay; thence southerly to the center of Huichica Creek; thence down said creek to its mouth, which is the southwest corner of Napa; thence on the line of Solano south, 261/2 degrees E., about six and one-quarter miles distant from the mouth of Huichica Creek, to the point of intersection with the westerly line of Contra Costa, forming the common corner of Marin, Solano, Contra Costa, and Sonoma; thence following the northern boundary of Marin westerly to the mouth of Petaluma Creek; thence up said creek to the mouth of San Antonio Creek; thence up said San Antonio Creek to its head; thence in a direct line to the head of the Estero Americano, on the line surveyed and established by William Mock, under the direction of the surveyor-general, in the year 1856; thence down said Estero Americano to its mouth; thence due west three miles to a point in the Pacific Ocean; thence northwesterly by ocean shore to the point of beginning.
Ca. Gov. Code § 23149