Which river is the main outlet of Lake Ontario?

The Saint Lawrence River is the main outlet of Lake Ontario. It carries the lake’s water eastward toward the Atlantic Ocean through the Great Lakes–Saint Lawrence system.

Lake Ontario is the last lake in the Great Lakes chain. Its major inflow is the Niagara River, which brings water from Lake Erie; its outflow is the Saint Lawrence River. The river begins at the northeastern end of Lake Ontario and continues through Quebec before reaching the Gulf of Saint Lawrence.

A frequent mix-up is the Niagara River, because it is strongly associated with the Great Lakes and Niagara Falls. However, Niagara is Lake Ontario’s principal inlet, not its outlet. The Saint Lawrence also became a major transportation route through the Saint Lawrence Seaway, linking inland North America with the Atlantic.