Wash. Rev. Code § 54.16.300
A public utility district by resolution may combine two or more of its separate utility functions into a single utility and combine its related funds or accounts into a single fund or account. The separate utility functions include electrical energy systems, domestic water systems, irrigation systems, sanitary sewer systems, and storm sewer systems. All powers granted to public utility districts to acquire, construct, maintain, and operate such systems may be exercised in the joint acquisition, construction, maintenance, and operation of such combined systems. The establishment, maintenance, and operation of the combined system shall be governed by the public utility district statutes relating to one of the utility systems that is being combined, as specified in the resolution combining the utility systems.
RCW 54.16.300