Wash. Rev. Code § 36.04.360
Walla Walla county shall consist of the territory bounded as follows, to wit: Commencing at a point where the boundary line between Washington and Oregon intersects the Columbia river; thence up the main channel of the Columbia to the mouth of the Snake river; thence up the main channel of said river to where the range line between ranges thirty-six and thirty-seven intersects said point; thence south on said range line to the northwest corner of township nine north, range thirty-seven east; thence east on the north boundary line of township nine north, range thirty-seven east, to the northeast corner of said township; thence south on the line between ranges thirty-seven and thirty-eight east, of the Willamette Meridian, to the northwest corner of township eight north, range thirty-eight east; thence along the north boundary line of township eight north, range thirty-eight east, to the northeast corner of said township; thence due south to the line dividing the state of Washington from the state of Oregon; thence due west on said dividing line to the place of beginning.
RCW 36.04.360