Wash. Rev. Code § 36.04.030
Benton county shall consist of the territory bounded as follows, to wit: Beginning at the point of intersection of the middle of the main channel of the Columbia river with the township line between township thirteen north, range twenty-three east, and township thirteen north, range twenty-four east, Willamette Meridian; thence running south along the township line, being the line between range twenty-three east and range twenty-four east to the line between Yakima county and Klickitat county; thence south along the township lines, being the lines between ranges twenty-three east and twenty-four east, to the point of intersection with the middle of the main channel of the Columbia river, or to its intersection with the line between the states of Washington and Oregon; thence northeasterly, northerly and northwesterly and westerly along the middle of the main channel of the Columbia river and up said stream to the place of beginning.
RCW 36.04.030