Wash. Rev. Code § 36.04.280
San Juan county shall consist of the territory bounded as follows, to wit: Commencing in the Gulf of Georgia at the place where the boundary line between the United States and the British possessions deflects from the forty-ninth parallel of north latitude; thence following said boundary line through the Gulf of Georgia and Haro Strait to the middle of the Strait of Fuca; thence easterly through Fuca Straits along the center of the main channel between Blunt's Island and San Juan and Lopez Islands to a point easterly from the west entrance of Deception Pass, until opposite the middle of the entrance to the Rosario Straits; thence northerly through the middle of Rosario Straits and through the Gulf of Georgia to the place of beginning.
RCW 36.04.280