Saint Clair, Lake, c.490 sq mi (1,270 sq km), 27 mi (43 km) long, on the U.S.-Canadian border, between SW Ont. and SE Mich. The St. Clair River (41 mi/66 km long) flows into the lake from Lake Huron; the Detroit River drains it S into Lake Erie. The lake is one of the busiest sections of the Great Lakes Waterway. The lake region is a popular summer vacation area.