ST CLAIR, a lake and river of North America, forming part of the boundary between the state of Michigan, U.S.A., and the province of Ontario, Canada. The lake is 29 m. long and 20 broad. It contains numerous islands, receives from the Canadian side several rivers, the largest of which is the Thames, and is drained into Lake Erie by the Detroit river. At its foot are the cities of Detroit (Michigan) and Windsor (Ontario). On the north it receives the St Clair river, the outlet of Lake Huron. The shores of both lake and river are flat, and their waters shallow, but, owing to the enormous traffic which passes through, they have been in great part canalized, and can accommodate the largest steamers.