Wash. Rev. Code § 8.08.090
Every county in this state is hereby, for the purposes of RCW 8.08.090 through 8.08.130, declared to be a body corporate and is authorized and empowered by and through its board of county commissioners whenever said board shall judge it to be clearly for the general welfare and benefit of the people of the county, and so far as shall be in harmony with the Constitution of this state and the provisions of RCW 8.08.090 through 8.08.130, to condemn and appropriate as hereinafter in RCW 8.08.090 through 8.08.130 provided and to dispose of for public use such lands, properties, rights and interests as are hereinafter in RCW 8.08.090 through 8.08.130 mentioned, whenever the government of the United States or of this state is intending or proposing the construction, operation or maintenance of any public work situated or to be situated wholly or partly within such county, or the expenditure of money or labor for the construction, operation or maintenance of any such work, and such condemnation or appropriation will enable the county to aid, promote, facilitate or prepare for any such construction, operation, maintenance or expenditure by either or both such governments, or to fulfill or dispose of any condition upon which such construction, operation, maintenance or expenditure is by law or from any cause contingent, and no property shall be exempt from such condemnation, appropriation or disposition by reason of the same having been or being dedicated, appropriated or otherwise reduced or held to public use.
RCW 8.08.090