Wash. Rev. Code § 36.75.160
The board of county commissioners of any county may erect and construct or acquire by purchase, gift, or condemnation, any bridge, trestle, or any other structure which crosses any stream, body of water, gulch, navigable water, swamp or other topographical formation requiring such structure for the continuation or connection of any county road if such topographical formation constitutes the boundary of a city, town, another county or the state of Washington or another state or a county, city or town of such other state.
The board of such county may join with such city, town, other county, the state of Washington, or other state, or a county, city or town of such other state in paying for, erecting, constructing, acquiring by purchase, gift, or condemnation any such bridge, trestle, or other structure, and the purchase or condemnation of right-of-way therefor.
The board of any county may construct, maintain, and operate any county road which forms the boundary line between another county within the state or another county in any other state or which through its meandering crosses such boundary; and acquire by purchase or condemnation any lands or rights within this state, either within or without its county, necessary for such boundary road; and enter into joint contracts with authorities of adjoining counties for the construction, operation, and maintenance of such boundary roads. The power of condemnation herein granted may be exercised jointly by two counties in the manner provided in RCW 36.75.170 for bridges, or it may be exercised by a single county in the manner authorized by law.
RCW 36.75.160