The Thousand Islands constitute a North American archipelago of 1,864 islands that straddles the Canada–US border in the Saint Lawrence River as it emerges from the northeast corner of Lake Ontario. They stretch for about 50 miles (80 km) downstream from Kingston, Ontario. The Canadian islands are in the province of Ontario and the U.S. islands in the state of New York.

The St. Lawrence River is a large international river in the middle latitudes of North America connecting the Great Lakes to the North Atlantic Ocean. Its waters flow in a northeasterly direction from Lake Ontario to the Gulf of St. Lawrence, traversing Ontario and Quebec in Canada and New York in the United States. A section of the river demarcates the Canada–U.S. border.