Idaho Code § 50-324
All cities of this state are empowered by ordinance to negotiate for and purchase or lease, and to maintain and operate, in cooperation with adjoining cities of states bordering this state, the out of state water distribution system, plant and equipment of privately owned utilities used for the purpose of supplying water to the purchasing or leasing cities from an out of state source; provided, the legislature of the state in which such water distribution system, plant, equipment and supply are located, by enabling legislation, authorizes its cities to join in such purchase or lease, maintenance and operation. The city council of the cities acting jointly under this section shall have authority, by mutual agreement, to exercise jointly all powers granted to each individual city in the purchase or lease, maintenance and operation of a water supply system.
Idaho Code § 50-324