Lake St Clair, 1114 km², elev 175 m, 6 m deep, the smallest of the GREAT LAKES, is bordered by the province of Ontario to the E and the state of Michigan to the W. Almost circular in shape, it has a length of 42 km and maximum width of 38 km. Sulpician missionaries Dollier de Casson and Bréhant de Galinée traversed the lake in 1670. In 1679 LA SALLE, becalmed in the lake on the feast day of Ste-Claire, christened the lake (and river) in her honour.

Lake St Clair is connected to Lake HURON by the ST CLAIR RIVER and is drained into Lake ERIE by the DETROIT RIVER. Its most important Canadian tributary is the Thames River. The farmlands surrounding the lake are among the most productive in N America. The cities of WINDSOR, Ont, and Detroit, Mich, are located at the SW end of the lake.

As part of the ST LAWRENCE SEAWAY, Lake St Clair and the river serve as a major transport route for commercial shipping vessels. Because of the many industries and large population in the area, pollutants have markedly influenced the quality of the lake's water. Its once-prosperous commercial fisheries are closed because of toxic contaminants in the fish. However, the lake remains an important recreational facility and has the largest concentration of boats and harbours of any of the Great Lakes.