Wash. Rev. Code § 36.04.310
Snohomish county shall consist of the territory bounded as follows, to wit: Commencing at the southwest corner of Skagit county; thence east along the eighth standard parallel to the summit of the Cascade mountains; thence southerly along the summit of the Cascade mountains to the northeast corner of King county, it being a point due east of the northeast corner of township twenty-six north, range four east; thence due west along the north boundary of King county to Puget Sound; thence northerly along the channel of Puget Sound and Possession Sound to the entrance of Port Susan, including Gedney Island; thence up the main channel of Port Susan to the mouth of the Stillaguamish river; thence northwesterly through the channel of the slough at the head of Camano Island, known as Davis Slough; thence northerly to the place of beginning.
RCW 36.04.310