St. Lawrence River, a major river of eastern North America. It drains an area of about 500,000 square miles (1,300,000 km2) and forms the outlet to the Atlantic Ocean from the Great Lakes. The St. Lawrence begins at the eastern end of Lake Ontario in the Thousand Islands area. It flows northeastward 745 miles (1,200 km), past the cities of Montreal and Quebec, and empties into the Gulf of St Lawrence near Anticosti Island. For the first 120 miles (190 km) of its course the river forms the boundary between Canada and the United States.

Between Lake Ontario and Montreal the rapids that once hindered navigation on the river have been bypassed by the canals of the St Lawrence Seaway project. Downstream from the city of Quebec, the river widens until it is more than 75 miles (120 km) across at Anticosti Island. The river drops 250 feet (76 m) from Lake Ontario to the sea, almost all of the descent occurring above Montreal. Tides are felt as far upstream as the St. Maurice River. Flooding is seldom a problem except when ice causes the river to back up during spring.

Several major tributaries supplement the large volume of water from the Great Lakes. The largest enter from the north. They include the Ottawa, St. Maurice, Saguenay, Outardes, and Manicouagan rivers. The principal tributary entering from the south is the Richelieu River, which drains Lake Champlain.

The St. Lawrence was discovered and explored in 1535–36 by Jacques Cartier, who sailed as far as the site of Montreal. In 1608 Quebec became the first permanent French settlement in Canada, and in 1642 Montreal was founded. In addition to the thriving agricultural and manufacturing economy that has developed in the St. Lawrence Valley, the river itself has achieved great significance as a major route to the continent's interior and as a source of hydroelectric power.