St Clair County

County in southwestern Illinois, bordered to the west by the Mississippi River, opposite St Louis, Missouri; area 1,740 sq km/672 sq mi; population (1990) 262,900. Its capital is Belleville. In the northwest, industrial East St Louis is the centre of one of Illinois's largest urban areas outside Chicago. Nearby factory and coal-related cities include Centreville, Cahokia, O'Fallon, Belleville, and Swansea. Farther east and south, in the valleys of Silver Creek and the Kaskaskia River, market gardening is the mainstay of the economy.

St Clair County

County in eastern Michigan, bounded by Lake Huron and the St Clair River, which form the borders with Ontario, Canada, and Anchor Bay of Lake St Clair; area 1,901 sq km/734 sq mi; population (1990) 145,600. Its capital is Port Huron. The county has commercial fishing, salt mines, and resorts, and manufacturing at Port Huron and Marysville. It makes dairy products and raises livestock, grain, vegetables, and sugar beet.

It is drained by the Belle and Black rivers.