Wash. Rev. Code § 85.06.070
All drainage districts organized or that may hereafter be organized under the provisions of this chapter or the acts amendatory thereof shall have the right of eminent domain, with the power by and through its board of commissioners, to cause to be condemned and appropriated private property for the use of said corporation in the construction and maintenance of a system or systems of drainage, and make just compensation therefor, and such right of eminent domain may be exercised either within or without the boundaries of such districts, and may be exercised with respect to rights-of-way for ditches, drains, dams, outlets or any other necessary appliances or structures and whether for the original system or any additions, enlargements or extensions thereof or for additional outlets or systems of drainage: PROVIDED, That the property of private corporations may be subjected to the same rights of eminent domain as that of private individuals: PROVIDED, FURTHER, That the said board of commissioners shall have the power to acquire by purchase all the real property necessary to make the improvements herein provided for.
RCW 85.06.070