Wash. Rev. Code § 36.04.390
Yakima county shall consist of the territory bounded as follows, to wit: Commencing at the northwest corner of township six north of range twelve east; thence east along the north boundary of township six north until said line intersects the range line between range twenty-three east and range twenty-four east; thence north along said range line to the Columbia river; thence north up the midchannel of said river to the southeast corner of Kittitas county; thence along the southern boundary of Kittitas county to the summit of the Cascade mountains; thence southerly to the southeast corner of Lewis county; thence west along the line of said county to the northeast corner of Skamania county; thence along the east line of Skamania county to the line between townships six and seven north; thence east along said line to the place of beginning.
RCW 36.04.390