Wash. Rev. Code § 89.12.040
In connection with a district contracting or intending to contract with the United States under this chapter, the secretary for the purpose of administering the federal reclamation laws and of providing for the delivery of water thereto, the method thereof, and the turnout therefor may segregate such lands, or any part thereof, into units and/or legal subdivisions, having in mind the character of soil, topography, method or methods of irrigation best suited therefor, location with respect to the irrigation system, type of irrigation system, and such other relevant factors as enter into the determination of the area and boundaries thereof and the method or methods of irrigating the same. Plats or revisions thereof showing the units and/or the legal subdivisions and the exclusive method or methods of irrigating such units and/or legal subdivisions or portions thereof when approved, may be filed by the United States for record with the auditor of the county in which the land is located. Lands in excess of the acreage in the amount specified by applicable federal law as not being excess lands held by any one landowner shall be deemed excess land.
RCW 89.12.040