St. Lawrence River

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Related to St. Lawrence River: St. Lawrence Seaway

St. Lawrence River


a river in eastern North America; the outlet for the entire Great Lakes system. The St. Lawrence River proper—from Lake Ontario to the Gulf of St. Lawrence—is approximately 1,200 km long. The entire system—from the upper St. Louis River, which empties into Lake Superior—is 3,350 km long.

The St. Lawrence drains an area of 1,269,000 sq km. Its largest left tributaries are the Ottawa, St. Maurice, and Saguenay rivers; its largest right tributary is the Richelieu, which is linked by canal with the Hudson River in the USA.

The upper St. Lawrence, from Lake Ontario to the city of Cornwall, forms the border between the USA and Canada; the lower St. Lawrence is entirely within Canada. Together with the Great Lakes, the St. Lawrence is the most important natural route between interior North America and the Atlantic Ocean.

The St. Lawrence River valley lies in a broad tectonic basin between the Canadian Shield and the Appalachian Mountains. Between the cities of Prescott and Montreal the river cuts through a narrow spur of the shield; here, a 175-km stretch of rapids, with a gradient of up to 70 m, is bypassed by canals. Below the city of Quebec, the St. Lawrence forms an estuary, approximately 400 km long and up to 50 km wide, at the head of the Gulf of St. Lawrence. In this area, the riverbanks are high and steep.

The St. Lawrence is fed by snow and rain. The mean long-term flow rate at Lake Ontario is 6,750 cu m per sec, and below the mouth of the Ottawa River, approximately 7,800 cu m per sec. The St. Lawrence is icebound from December through April. The estuary does not freeze, but navigation ceases for a period during the spring to allow upstream ice to pass through. Ocean tides reach as far as the city of Trois-Rivières and have a range of 5.5 m at Quebec.

Since 1959, with the completion of the St. Lawrence Seaway, large oceangoing vessels can travel as far as Lake Superior. Between Montreal and Lake Ontario there is a network of power plants, canals, and reservoirs, in addition to the bypass canals. The largest hydroelectric power plant is the St. Lawrence, which furnishes 1.9 gigawatts of electric power to the USA and Canada. The Beauharnois (1.6 gigawatts) and Robert H. Saunders (1.7 gigawatts) plants are Canadian. The chief cities and ports on the St. Lawrence are Kingston, Cornwall, Montreal, Sorel, Trois-Rivières, and Quebec.


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