Do the Great Lakes and St. Lawrence River Form the U.S.–Canada Border?

Yes. The Great Lakes–St. Lawrence system constitutes a massive natural boundary between the United States and Canada, stretching over 2,300 miles (3,700 km) from western Lake Superior to the Atlantic Ocean.

The Water Boundary (West to East)

1. Lake Superior – Border runs through the lake's center.
2. Lake Michigan – Entirely within the U.S.; not an international border.
3. Lake Huron – Border through the lake (including Georgian Bay).
4. Lake Erie – Border through the lake.
5. Lake Ontario – Border through the lake.
6. St. Lawrence River – From Lake Ontario downstream to east of Cornwall/Massena, the international boundary follows the river's thalweg (deep channel). Below that, the river lies wholly in Canada.

Key Points

• Four of the five Great Lakes are binational; only Lake Michigan is wholly U.S.
• The 1909 Boundary Waters Treaty governs shared use and dispute resolution.
• Major binational infrastructure: Niagara Falls power projects, St. Lawrence Seaway locks, Sault Ste. Marie locks.

This freshwater border is the world's longest and most peaceful international boundary—facilitating $300B+ in annual trade.