Wash. Rev. Code § 85.24.280
Any district so established as aforesaid through its board of commissioners shall have the right, power and authority to straighten, deepen and improve any and all rivers, watercourses, or streams, whether navigable or otherwise, flowing through or located within the boundaries of said diking or drainage district, whenever necessary or proper in carrying out the objects of the system. The district by and through its board of commissioners shall also have the power to construct all needed auxiliary ditches, canals, flumes, locks, flood barriers, and all necessary artificial appliances in the construction of the system, and which shall be necessary and advisable to protect the land in any such district from overflow or to assist, or which may become necessary in the preservation or maintenance of such system.
RCW 85.24.280