Wash. Rev. Code § 86.09.004
Such flood control districts shall be organized to provide for the ultimate necessary control of the entire part, or all, of the stream system of any stream or tributary, or for the protection against tidal or any bodies of water, within this state and may include all or part of the territory of any county and may combine the territory in two or more such counties, in which any of the lands benefited from the organization and maintenance of a flood control district are situated.
A district established wholly within the boundaries of any city or town may also provide for the collection, control, and safe and suitable conveyance over and across the district, of intermittent surface and drainage water, originating within or without its boundaries, to suitable and adequate outlets.
RCW 86.09.004