The St. Croix River starts in the northwestern corner of Wisconsin. It flows out of Upper St. Croix Lake in Douglas County. The lake is near Solon Springs, approximately south of Lake Superior. The St. Croix flows south into Gordon, then southwest. It is joined by the Namekagon River in northern Burnett County. A few miles downstream the St. Croix meets the boundary between Minnesota and Wisconsin. It marks that boundary for another until its confluence with the Mississippi River.