Wash. Rev. Code § 36.04.110
Franklin county shall consist of the territory bounded as follows, to wit: Beginning at a point where the midchannel of the Snake river intersects that of the Columbia river, and running thence up the Columbia river to a point where the section line between sections twenty-one and twenty-eight, township fourteen north, range twenty-seven east, Willamette Meridian, strikes the main body of the Columbia river, on the east side of the island; thence east on said section line to range line between ranges twenty-seven and twenty-eight east; thence north on said range line to the north boundary of township fourteen; thence east on said north boundary of township fourteen to the Palouse river; thence down said river to midchannel of Snake river; thence down Snake river to place of beginning.
RCW 36.04.110