Wash. Rev. Code § 36.89.030
Counties are authorized to establish, acquire, develop, construct, and improve open space, park, recreation, and community facilities, public health and safety facilities, stormwater control facilities, and highways or any of them pursuant to the provisions of this chapter within and without the cities and towns of the county and for such purposes have the power to acquire lands, buildings and other facilities by gift, grant, purchase, condemnation, lease, devise, and bequest, to construct, improve, or maintain buildings, structures, and facilities necessary for such purposes, and to use and develop for such purposes the air rights over and the subsurface rights under any highway. The approval of the state department of transportation shall be first secured for such use and development of any state highway. For visual or sound buffer purposes the county shall not acquire by condemnation less than an owner's entire interest or right in the particular real property to be so acquired if the owner objects to the taking of a lesser interest or right.
RCW 36.89.030
Acquisition of interests in land for conservation, protection, preservation, or open space purposes by counties: RCW 64.04.130.
Flood control, county powers: RCW 86.12.020.