California Code, Streets and Highways Code - SHC § 1670

The board of supervisors of any county may enter into cooperative agreements with the Secretary of Agriculture of the United States, or other properly authorized officer of the United States, for the survey, construction, and maintenance of highways or trails within such county, or for the survey, construction, and maintenance of highways or trails outside of its county limits, including highways or trails lying partly within and partly without the boundaries of the State, where authorization therefor is given by ordinance or resolution of the board of supervisors or other properly authorized agency of the county in which such highways or trails or a portion thereof are to be constructed.