The Saint Lawrence River is a large river flowing approximately from southwest to northeast in the middle latitudes of North America, connecting the Great Lakes with the Atlantic Ocean. It is the primary drainage conveyor of the Great Lakes Basin. The river traverses the Canadian provinces of Quebec and Ontario and forms part of the international boundary between Ontario, Canada, and New York State in the United States. This geographic entity is coterminous with the commercial Saint Lawrence Seaway.
The St. Lawrence River originates at the outflow of Lake Ontario between Kingston, Ontario, on the north bank, Wolfe Island in mid-stream, and Cape Vincent, New York. From there, it passes Gananoque, Brockville, Morristown, Ogdensburg, Massena, Cornwall, Montreal, Trois-Rivières, and Quebec City before draining into the Gulf of Saint Lawrence, one of the largest estuaries in the world. The estuary portion begins at the eastern tip of Île d’Orléans, just downstream from Quebec City. The river becomes tidal in the vicinity of Quebec City.
The 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. The farthest headwater is the North River in the Mesabi Range at Hibbing, Minnesota. Its drainage area, which includes the Great Lakes and hence the world’s largest system of freshwater lakes, has a size of 1,344,200 square kilometres (518,998.5 sq mi), of which 839,200 km2 (324,016.9 sq mi) is in Canada and 505,000 km2 (194,981.6 sq mi) is in the United States. The basin covers parts of the provinces of Ontario and Quebec, and the states of Illinois, Indiana, Michigan, Minnesota, New York, Ohio, Pennsylvania, Vermont, and Wisconsin. 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 m3/s (427,300 cu ft/s). The average discharge at the river’s source, the outflow of Lake Ontario, is 7,410 m3/s (262,000 cu ft/s).
The river includes Lake Saint-Louis south of Montreal, Lake Saint Francis at Salaberry-de-Valleyfield and Lac Saint-Pierre east of Montreal. It encompasses four archipelagoes: the Thousand Islands chain near Kingston, Ontario; the Hochelaga Archipelago, including the Island of Montreal and Île Jésus (Laval); the Lake St. Pierre Archipelago (Classified 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, Saguenay, and Saint-François 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 Palaeozoic normal faults related to the opening of Iapetus Ocean. The faults in the area are rift related and are called the Saint Lawrence rift system.
The St. Lawrence Valley is a physiographic province of the larger Appalachian division, containing the Champlain and Northern physiographic section.
Note : The above story is based on materials provided by Wikipedia