In its upper course the river cuts through a part of the Canadian Shield; there, just downstream from Lake Ontario, are the Thousand Islands . Below Cornwall, Ont., the river widens into Lake St. Francis. Shortly after, it widens again into Lake St. Louis then descends through the Lachine Rapids to Montreal, head of navigation for very large oceangoing vessels. Between Sorel and Trois Rivières is Lake St. Peter. Below the city of Quebec the river is tidal. It gradually increases in width to c.90 mi (140 km) at its mouth. The river's principal tributaries are the Richelieu (linking the St. Lawrence with Lake Champlain and the Hudson River), St. Francis, Ottawa, St. Maurice, and Saguenay rivers.
The St. Lawrence River is an important source of hydroelectric power; one of the world's largest facilities is the Beauharnois power plant near Montreal. Agreements between the United States and Canada govern power distribution and navigation in the international section of the river. The river's valley is an agricultural region; potatoes, grains, hay, vegetables, and dairy cattle are raised. The most important cities and ports along the St. Lawrence are Ogdensburg, N.Y.; Kingston, Brockville, and Cornwall, Ont.; and Montreal, Sorel, Trois Rivières, Quebec City, and Lévis, Que.
Canals have been constructed around the river's rapids, making the entire river navigable; however, the upper part is unnavigable during the winter months because of ice accumulation. The many bridges that cross the St. Lawrence River include the Thousand Islands International Bridge (1938), the Roosevelt International Bridge (1934), and the Seaway Skyway Bridge (1960), all between Ontario and New York; the Victoria Bridge (remodeled 1898) at Montreal; and the Quebec Bridge (1917), near Quebec City.
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.
See more Encyclopedia articles on: Canadian Physical Geography