North America’s second-mightiest river drains the Great Lakes to the sea
The St. Lawrence carries Great Lakes water northeast from Lake Ontario to the Gulf of St. Lawrence, forming part of the Canada–United States border. Its discharge ranks second in North America after the Mississippi and sixteenth worldwide, feeding what many call the planet’s largest estuary.
From the farthest Mesabi Range headwater the system runs 3,058 kilometres; from Lake Ontario’s outlet it is 1,197 kilometres including the estuary, or about 500 kilometres without it. The basin spans 1,344,200 square kilometres across Ontario, Quebec, and eight U.S. states. Average flow below the Saguenay reaches 16,800 cubic metres per second. Jacques Cartier named a bay for Saint Lawrence on the saint’s 10 August feast in 1535; Samuel de Champlain fixed Fleuve sainct Laurens on a 1604 map. Indigenous names abound—Kaniatarowanénhne in Mohawk, Magtogoek among Algonquin speakers, Micta sipi for the Atikamekw.
Landmarks pack the corridor: 1,864 Thousand Islands, the Mingan limestone monoliths, and Saguenay–St. Lawrence Marine Park whales. Ice begins between Montreal and Quebec City in December and cloaks the gulf by January–February. The river formed as the Champlain Sea drained after the last glacial maximum roughly 13,000 to 10,000 years ago. Archipelagoes include Hochelaga (Montreal and Laval) and UNESCO-listed Lake Saint-Pierre. Tributaries include the Ottawa, Richelieu, and Saguenay.
Montreal and Quebec City grew on this Indigenous highway; today the St. Lawrence Seaway’s locks let ocean freighters reach inland Great Lakes ports. Seismicity follows the Saint Lawrence rift system’s old Iapetus faults. Second-longest river in Canada, it remains both border ditch and continental drain—freshwater oceans pouring toward salt. Winter ice can calm spray enough to protect hulls even as it blankets the gulf; tides begin near Quebec City, and the estuary officially starts at the eastern tip of Île d’Orléans.
Source: St. Lawrence River
