Lake Superior drains into Lake Huron through the St. Marys River and receives the waters of many short, swift-flowing streams including the Nipigon, Kaministikwia, St. Louis, and Pigeon rivers. The largest islands are Isle Royale, Isle St. Ignace, and Simpson and Michipicoten. The shoreline is irregular (with many large bays, inlets, and peninsulas) and in places is high and rocky. The waters of Lake Superior are generally purer than those of the lower lakes and are minimally polluted; a U.S.-Canadian pact (1972) was established to prevent pollution and to maintain and improve the water's quality.
Lake Superior is part of the Great Lakes–St. Lawrence Seaway system, and it is reached by oceangoing and lake vessels through the Sault Sainte Marie Canals, which bypass rapids in the St. Marys River. The principal cargoes are grain, flour, and iron ore. The lake does not freeze completely, but ice impedes navigation from mid-December to the end of March at the lake's outlet and from early December to the end of April in harbors on the south shore. Fog and rough water are hazards.
The chief Canadian cities on the lake are Michipicoten and Thunder Bay. The principal cities on the U.S. shore are Marquette, Superior, Ashland, and Duluth. Commercial and sport fishing are important; and tourism is popular in the lake area. Recreational facilities are found on Isle Royale (part of a U.S. national park), in Pukaskwa National Park (Ontario), and at state and provincial parks on the lake's shores and islands; the U.S. Apostle Islands and Pictured Rocks national lakeshores are there. Étienne Brulé, the French explorer, probably visited the lake in 1616; Pierre Radisson and the sieur des Groseilliers explored it in 1659–60; Father Allouez established (1665) a mission near Ashland; and the sieur Duluth visited the lake in 1678–79.
See bibliography by Water Resources Scientific Information Center (1972).
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