The Saint Lawrence River is born at the outflow of Lake Ontario at Kingston, Ontario. From there, it passes Brockville, Cornwall, Montreal, Trois-Rivières, and Quebec City before draining into the Gulf of Saint Lawrence, the largest estuary in the world. It runs 1900 miles (3058 kilometers), and together with the Great Lakes which it drains, and their tributaries, forms the world's largest fresh-water system.
The river includes Lac Saint-Louis south of Montreal and Lac Saint-François at Salaberry-de-Valleyfield, Quebec. It surrounds such islands as the Thousand Islands near Kingston, the Island of Montreal, Île Jésus (Laval), Île d'Orléans near Quebec City, and Anticosti Island north of the Gaspé.
The St. Lawrence was formerly continuously navigable only as far as Montreal due to the Lachine Rapids. The Lachine Canal was the first to allow ships to pass the rapids; the Saint Lawrence Seaway, an extensive system of canals and locks, now permits ocean-going vessels to pass all the way to Lake Superior.