Cal. Gov. Code § 23106
The boundaries of Colusa County are as follows:
Beginning at the southeast corner, being the northeast corner of Yolo, in the Sacramento River, at its intersection with the south line of T. 13 N., M. D. B.; thence west, on said township line to the ridge dividing the waters flowing into Bear Creek and Stony Creek, from those flowing west into the north fork of Cache Creek and Clear Lake; thence northerly, along said ridge to the summit line of the Coast Range, being the eastern line of Lake, forming the southwest corner of Colusa and northwest corner of Yolo; thence northerly on said eastern boundary of Lake, to the southwest corner of Glenn; thence easterly on the southern line of Glenn to Butte Creek; thence down Butte Creek to Butte Slough; thence up Butte Slough to the Sacramento River; thence down the Sacramento River to the place of beginning.
Ca. Gov. Code § 23106