Cal. Gov. Code § 23120
The boundaries of Madera County are as follows:
Beginning at a point where the third standard line south of Mount Diablo base line crosses the San Joaquin River; thence up the middle of said river, following the meanderings thereof southeasterly and northeasterly, to the point where said river crosses the south boundary line of T. 6 S., R. 24 E., M. D. B. & M.; thence running northeast to the boundary line of Mono; thence following the western line of Mono and southern line of Tuolumne to the corner common to Tuolumne, Mariposa, and Madera; thence following the southern line of Mariposa, to the southeast corner of Merced; thence westerly, following the southern line of Merced to a point where said line is intersected by the San Joaquin River; thence following up the middle of said river to the point of beginning.
Ca. Gov. Code § 23120