Wash. Rev. Code § 36.04.270
Pierce county shall consist of the territory bounded as follows, to wit: Commencing at the mouth, midchannel, of the Nisqually river; thence following the main channel of said river to its head; thence due east to the summit of the Cascade mountains; thence northerly along the summit to the head of the Green Water; thence westerly down said river to its confluence with White river; thence down the main channel of White river to the intersection of the fifth standard parallel; thence west along said line to the southeast corner of section thirty-one, township twenty-one north, range four east of Willamette Meridian; thence north along the east line of said section thirty-one to its intersection with the northerly line of the Puyallup Indian reservation; thence northwesterly on said line of the Puyallup Indian reservation, projected northwesterly in a straight line, to its intersection with the center line of Puget Sound; thence southwesterly and westerly following the channel of Dalco Passage to the south entrance of Colvo's Passage; thence down the channel of said passage to the northeast corner of section sixteen, in township twenty-two north, range two east; thence west to the northeast corner of section sixteen, in township twenty-two north, range one west; thence southerly along the channels of Case's Inlet and Puget Sound, to the middle of the mouth of the Nisqually river and place of beginning.
RCW 36.04.270