Wash. Rev. Code § 35.21.210
Any city or town shall have power to provide for the sewerage, drainage, and water supply thereof, and to establish, construct, and maintain a system or systems of sewers and drains and a system or systems of water supply, within or without the corporate limits of such city or town, and to control, regulate, and manage the same. In addition, any city or town may, as part of maintaining a system of sewers and drains or a system of water supply, or independently of such a system or systems, participate in and expend revenue on cooperative watershed management actions, including watershed management partnerships under RCW 39.34.210 and other intergovernmental agreements, for purposes of water supply, water quality, and water resource and habitat protection and management.
RCW 35.21.210
Finding-Intent-2003 c 327: See note following RCW 39.34.190.