Which Great Lake drains into the Atlantic Ocean through the St. Lawrence River?

Lake Ontario drains into the Atlantic Ocean through the St. Lawrence River.

The St. Lawrence River begins at Lake Ontario’s northeastern outlet and flows eastward through Quebec before reaching the Gulf of St. Lawrence and the Atlantic Ocean. This makes Lake Ontario the final downstream lake in the Great Lakes system.

Water reaches Lake Ontario mainly through the Niagara River from Lake Erie, along with smaller tributaries. The lake’s eastern outlet has supported navigation for centuries and now forms part of the St. Lawrence Seaway, which allows ocean-going and lake freighter traffic to travel between the interior and the Atlantic.

Lake Ontario is not the source of the Great Lakes system. Water generally moves from the upper lakes toward Lake Erie, then through the Niagara River, and finally through Lake Ontario to the St. Lawrence.