Cal. Gov. Code § 23145
The boundaries of Shasta County are as follows:
Beginning at the northern line of Tehama, at the head of Bloody Island, in the Sacramento River; thence to and down the eastern channel to the mouth of Battle Creek; thence easterly, up Battle Creek, by the main channel, to the mouth of the middle fork, known as Digger Creek; thence up Digger Creek to its head; thence east to a point south of Black Butte Mountain, forming the southeast corner; thence north, on the western line of Lassen, to a rock mound forming the northeast corner, on the southern line of Siskiyou; thence west, on said southern line, to Castle Rock, forming the northwest corner; thence southerly along Trinity Mountain to the head of Bee Gum Creek, forming the southwest corner; thence easterly down Bee Gum, Middle Fork, and Cottonwood Creeks to the western channel of the Sacramento River; thence, by direct line, to the point of beginning.
Ca. Gov. Code § 23145