Does the St. Lawrence River Flow from Lake Ontario to the Atlantic?

Yes. The St. Lawrence River flows northeast from Lake Ontario to the Atlantic Ocean — a 1,197 km (744 mi) international waterway.

Course Summary

• Source: Outlet of Lake Ontario at Kingston, Ontario.
• Upper River: Widens into Lake St. Francis, Lake St. Louis (Montreal archipelago).
• Montreal: Head of ocean navigation; St. Lawrence Seaway locks begin here (bypassing Lachine Rapids).
• Freshwater → Brackish: Near Quebec City — tidal influence begins.
• Estuary: Widens dramatically; Saguenay Fjord joins.
• Gulf of St. Lawrence: Opens to Northwest Atlantic near Anticosti Island.

Key Facts

• Binational: Canada (QC, ON) / USA (NY) — Great Lakes–St. Lawrence Basin.
• Drainage: 1.34M km² — Great Lakes + Quebec watersheds.
• Seaway (1959): 15 locks (7 US, 8 CAN) — enables 740-ft ships to Great Lakes.
• Hydropower: Moses-Saunders, Beauharnois-Les Cèdres — 20+ GW capacity.
• Ecology: Belugas (year-round), migratory birds, wetlands, fish ladders.

The St. Lawrence is North America’s great inland seaway — connecting heartland to ocean.