Cal. Sts. & Hy. Code § 1626
Power hereby is granted to the board of supervisors of any county, to construct, maintain, improve and repair any county highway which in whole or in part forms the boundary line between said county and another county in this State or any other state, or which in its meanderings crosses and recrosses such boundary line; and to acquire by purchase or condemnation any lands or rights-of-way within this State, necessary for such boundary highway; and to enter into joint contracts with authorities of adjoining counties within or without the State, for the construction, maintenance, improvement and repair of such highways. Any such highway shall be considered a county highway and expenditures may be made thereon from funds received by the county from the Highway Users Tax Fund to the extent provided by law for county highways.
Ca. Sts. and High. Code § 1626