Cal. Gov. Code § 23146
The boundaries of Sierra County are as follows:
Beginning at the south corner of Plumas, in the center of Slate Creek; thence easterly on the southern line of Plumas to the range line between T. 21 N., R. 13 E., and T. 21 N., R. 14 E., M. D. B. & M.; thence north on said range line, to the northwest corner of T. 21 N., R. 14 E., M. D. B. & M.; thence east on the line between T. 21 and 22 N., M. D. B., to the state line forming the northeast corner; thence south on said state line to the northeast corner of Nevada, a point east of the Bent Monument; thence west to the Bent Monument; thence down the south fork of the middle Yuba River and down the middle Yuba River to a point 10 miles above the mouth of the latter; thence in a straight line northerly to a point on the north fork of the Yuba River known as Cuteye Foster's Bar; down said river to the mouth of Big Canon Creek, then up said creek four miles; thence in a straight line to the point of beginning.
Ca. Gov. Code § 23146