Cal. Gov. Code § 23128
The boundaries of Napa County are as follows:
Beginning at the southwestern corner, at a point in Huichica Creek where the said creek empties into San Pablo Bay; thence east to the mountains dividing Napa Valley from Suisun Valley, forming the southeastern corner; thence northerly along the summit line of said mountains to its intersection with the first standard north, Mount Diablo base, marked by a rock monument erected by Ralph Norris; thence east along said standard line seven and three-fourths miles to the Vaca Mountains, which divide the Vaca and Suisun Valleys; thence northerly along the main ridge of said Vaca Mountains to Putah Creek, at a point called the Devil's Gate; thence northerly across said creek to and along the mountains dividing Berryessa Valley from the Sacramento Valley to the southeast corner of Lake on the western line of Yolo; thence southwesterly along the southern line of Lake to its intersection with the eastern line of Sonoma; thence southeasterly on said line of Sonoma to the western branch of the headwaters of Huichica Creek; thence westerly to 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 the Huichica Creek; thence down said creek to its mouth, the place of beginning.
Ca. Gov. Code § 23128