Saint Lawrence, one of the principal rivers of North America, 744 mi (1,197 km) long. It issues from the northeastern end of Lake Ontario and flows northeast, first along the U.S.-Canadian border, then into S Que., Canada, past Montreal and Quebec City, to the Gulf of St. Lawrence, N of Cape Gaspé. It is the outlet of the Great Lakes and together with them forms a c.2,300-mi (3,700-km) waterway from the western end of Lake Superior to the Atlantic Ocean. The river is an integral part of the Saint Lawrence Seaway (opened 1959).
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.