St. Lawrence River
Geography

With the draining of the Champlain Sea, due to a rebounding continent from the Last Glacial Maximum, the St. Lawrence River was formed. The Champlain Sea lasted from about 13,000 years ago to about 10,000 years ago and was continuously shrinking during that time, a process that continues today. The head of the St. Lawrence River, near Lake Ontario, is home to the Thousand Islands.

Today, the St. Lawrence River begins at the outflow of Lake Ontario and flows adjacent to Gananoque, Brockville, Morristown, Ogdensburg, Massena, Cornwall, Montreal, Trois-Rivières, and Quebec City before draining into the Gulf of St. Lawrence, often given as the largest estuary in the world. The estuary begins at the eastern tip of Île d'Orléans, just downstream from Quebec City. The river becomes tidal around Quebec City.

The St. Lawrence River runs 3,058 kilometres (1,900 mi) from the farthest headwater to the mouth and 1,197 km (743.8 mi) from the outflow of Lake Ontario. These numbers include the estuary; without the estuary, the length from Lake Ontario is c. 500 km (c. 300 mi). The farthest headwater is the North River in the Mesabi Range at Hibbing, Minnesota. Its drainage area, which includes the Great Lakes, the world's largest system of freshwater lakes, is 1,344,200 square kilometres (518,998.5 sq mi), of which 839,200 km (324,016.9 sq mi) is in Canada and 505,000 km (194,981.6 sq mi) is in the United States. The basin covers parts of Ontario and Quebec in Canada, parts of Minnesota, Wisconsin, Illinois, Indiana, Ohio, Pennsylvania, New York, Vermont, and nearly the entirety of the state of Michigan in the United States. The average discharge below the Saguenay River is 16,800 cubic metres per second (590,000 cu ft/s). At Quebec City, it is 12,101 m /s (427,300 cu ft/s). The average discharge at the river's source, the outflow of Lake Ontario, is 7,410 m /s (262,000 cu ft/s).

The St. Lawrence River includes Lake Saint Francis at Salaberry-de-Valleyfield, Lake Saint-Louis south of Montreal and Lake Saint Pierre east of Montreal. It encompasses four archipelagoes: the Thousand Islands chain near Alexandria Bay, New York and Kingston, Ontario; the Hochelaga Archipelago, including the Island of Montreal and Île Jésus (Laval); the Lake St. Pierre Archipelago (classified a biosphere world reserve by the UNESCO in 2000) and the smaller Mingan Archipelago. Other islands include Île d'Orléans near Quebec City and Anticosti Island north of the Gaspé. It is the second longest river in Canada.

Lake Champlain and the Ottawa, Richelieu, Saint-Maurice, Saint-François, Chaudière and Saguenay rivers drain into the St. Lawrence.

The St. Lawrence River is in a seismically active zone where fault reactivation is believed to occur along late Proterozoic to early Paleozoic normal faults related to the opening of the Iapetus Ocean. The faults in the area are rift -related and comprise the Saint Lawrence rift system.

According to the United States Geological Survey, the St. Lawrence Valley is a physiographic province of the larger Appalachian division, containing the Champlain section. However, in Canada, where most of the valley is, it is instead considered part of a distinct St. Lawrence Lowlands physiographic division, and not part of the Appalachian division at all.
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