The river issues from Lake Ontario, and is fed by the Great Lakes. In its upper course the St Lawrence includes the scenic Thousand Islands and forms the boundary between Ontario and New York State, USA. It then marks the Ontario-Québec border, before entering Québec province, where it flows past Montréal and Québec City. The river widens from about 3 km/2 mi below Québec to a maximum width of 145 km/90 mi as it empties into the Gulf of St Lawrence. The river had become polluted by the 1970s, and despite clean-up programmes there are continuing problems with chemical pollutants and invasive species.

Lakes along its course include St Francis; St Louis, at Montréal, and St Peter, below Sorel.

The St Louis River, which flows into Lake Superior near Duluth, is considered the furthest headwater of the St Lawrence River system; the length from its source in Minnesota, USA, to the Gulf of St Lawrence is 3,060 km/1,915 mi. River links between the Great Lakes, are known as the St Mary, the St Clair, the Detroit, and the Niagara. The main tributaries feeding the St Lawrence River are the Ottawa, Saint-Maurice, and Saguenay flowing from the north; and the Richelieu, St Francis, and Chaudière which join from the south. The total drainage area, including the Great Lakes system, is over 1.3 million sq km/500,000 sq mi. Half the world's fresh water is contained in the Great Lakes basin.

Economy

Water power is heavily exploited, the largest development being at Beauharnois, above Montréal, with a capacity of over 1.5 million kW. The fisheries of the lower St Lawrence are rich, and the valleys contain fertile agricultural land, chiefly devoted to mixed and dairy farming, and to fruit-growing. There is an aluminium smelting company on Sept Iles.

History

The French explorer Jacques Cartier explored the river in 1535, naming it for St Lawrence because he entered its course on the saint's feast day (August 10). Québec City, established in 1608 by Samuel de Champlain, was the first permanent colonial settlement on its banks.