Wash. Rev. Code § 36.04.040
Chelan 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 fifth standard parallel north, thence running west along said fifth standard parallel north to the point where said fifth standard parallel north intersects the summit of the main divide between the waters flowing northerly and easterly into the Wenatchee and Columbia rivers, and the waters flowing southerly and westerly into the Yakima river, thence in a general northwesterly direction along the summit of said main divide between the waters flowing northerly and easterly into the Wenatchee and Columbia rivers and the waters flowing southerly and westerly into the Yakima river, following the course of the center of the summit of the watershed dividing the said respective waters, to the center of the summit of the Cascade mountains, at the eastern boundary line of King county; thence north along the east boundary lines of King, Snohomish and Skagit counties to the point upon the said east boundary of Skagit county, where said boundary is intersected by the watershed between the waters flowing northerly and easterly into the Methow river and the waters flowing southerly and westerly into Lake Chelan, thence in a general southeasterly direction along the summit of the main divide between the waters flowing northerly and easterly into the Methow river and the waters flowing westerly and southerly into Lake Chelan and its tributaries; following the course of the center of the summit of the watershed dividing said respective waters, to the point where the seventh standard parallel north intersects said center of the summit of said watershed; thence east along the said seventh standard parallel north to the point of intersection of the middle of the main channel of the Columbia river with said seventh standard parallel north; thence down the middle of the main channel of the Columbia river to the point of beginning.
RCW 36.04.040