Wash. Rev. Code § 47.08.040
Whenever it is necessary or desirable for the federal government or any agency thereof to acquire an interest in or in any way damage any property or interest therein owned by the state of Washington and used in connection with any highway in the state of Washington in connection with any federal project for the development of any river within or partially within the state of Washington, the department is authorized, empowered, and directed to negotiate and enter into an agreement with the proper agency of the federal government as to the rights which shall be acquired, the compensation which shall be made therefor and the character of instruments by which the rights shall be conveyed, and as to any other matters which may be necessary in order to satisfy the requirements of the federal government. If the agreement is required to be reduced to writing, the writing shall be approved as to form by the attorney general of the state of Washington.
RCW 47.08.040