The St. Lawrence is in Ontario.
Ontario . Almost all of Ontario 's 2700 km border with the United States follows inland waterways : from the west at Lake of the Woods , eastward along the major rivers and lakes of the Great Lakes/Saint Lawrence River drainage system . The Saint Lawrence River ( Fleuve Saint-Laurent Tuscarora : Kahnawáʼkye ; Mohawk : Kaniatarowanenneh , meaning `` big waterway '' ) is a large river in the middle latitudes of North America . It traverses the Canadian provinces of Quebec and Ontario , and is part of the international boundary between Ontario , Canada , and the U.S. state of New York .