Wash. Rev. Code § 35.88.010
For the purpose of protecting the water furnished to the inhabitants of cities and towns from pollution, cities and towns are given jurisdiction over all property occupied by the works, reservoirs, systems, springs, branches and pipes, by means of which, and of all the lakes, rivers, springs, streams, creeks, or tributaries constituting the sources of supply from which the cities and towns or the companies or individuals furnishing water to the inhabitants thereof obtain their supply of water, or store or conduct it, and over all property acquired for any of the foregoing works or purposes or for the preservation and protection of the purity of the water supply, and over all property within the areas draining into the lakes, rivers, springs, streams, creeks, or tributaries constituting the sources of supply whether they or any of them are within the city or town limits or outside.
RCW 35.88.010