The Saint 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 Saint Lawrence, 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 Saint 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 ca. 500 km (ca. 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 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 Ontario and Quebec in Canada, parts of Illinois, Indiana, Minnesota, New York, Ohio, Pennsylvania, Vermont, and Wisconsin, 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 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 Saint Lawrence 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 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 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.
The Saint 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 Iapetus Ocean. The faults in the area are rift related and are called the Saint Lawrence rift system.
According to the United States Geological Survey, the Saint Lawrence Valley is a physiographic province of the larger Appalachian division, containing the Champlain and Northern physiographic section. However, in Canada, where most of the valley is, it is instead considered part of a distinct Saint Lawrence Lowlands physiographic division, and not part of the Appalachian division at all.
Though the Norse explored the Gulf of Saint Lawrence in the 11th century and were followed by fifteenth and early sixteenth century European mariners, such as John Cabot, the brothers Gaspar, Miguel Corte-Real, and Alonso Sanchez. The first European explorer known to have sailed up the Saint Lawrence River itself was Jacques Cartier. At that time, the land along the river was inhabited by the St. Lawrence Iroquoians; at the time of Cartier's second voyage in 1535. Because Cartier arrived in the estuary on Saint Lawrence's feast day, he named it the Gulf of Saint Lawrence. The Saint Lawrence River is partly within the U.S. and as such is that country's sixth oldest surviving European place-name.