Wash. Rev. Code § 35.92.290
Any city or town with a population over one hundred fifty thousand within the state of Washington owning an electric public utility is authorized to cooperate with any public utility district within this state in the joint acquisition, purchase, construction, ownership, maintenance and operation, within or without the respective limits of any such city or town or public utility district, of electric utility properties. The respective governing bodies of any such city or town and of any such public utility district desiring to cooperate in the joint ownership, maintenance and operation of electric utility properties pursuant to the authority contained in RCW 35.92.280 through 35.92.310, shall by mutual agreement provide for such joint ownership, maintenance and operation. Such agreement shall prescribe the rights and property interest which the parties thereto shall have in such electric utility properties, which property interest may be either divided or undivided; and shall further provide for the rights of the parties thereto in the ownership and disposition of the power and energy produced by such electric utility properties, and for the operation and management thereof.
RCW 35.92.290