The Yukon River crosses the Canada-US border, but does not form part of it.
The Niagara River, which flows north connecting Lake Erie to Lake Ontario, between the province of Ontario and the state of New York. At Niagara Falls, parts of the falls are entirely within the US, and the remainder border Canada.
The Rio Grande forms part of the border between the US and Mexico.
because it just did
The parts of the border that are not in the Rio Grande or the Colorado River are marked by a series of monuments.
mississippa arkansaw river
not generally since 1803 & not a bit since 1818
The Rio Grande or Rio Bravo del Norte.
The Mississippi River was the western border in 1787.
Several rivers form the border between Canada and the United States, with the most notable being the St. Lawrence River, which flows from the Great Lakes into the Atlantic Ocean. The Niagara River, which includes Niagara Falls, also serves as a border for a portion of the boundary. Additionally, the Rainy River and the Red River are significant waterways that delineate parts of the border between the two countries. These rivers play vital roles in trade, transportation, and ecological systems.
The Great Lakes do form a section of the border between the US and Canada, as does a short stretch of the Saint Lawrence River. However, a longer stretch of the river is wholly in Canadian territory, and one of the Great Lakes (Michigan) is entirely in US territory.
The Rio Grande forms part of the US border with Mexico, while the St. Lawrence River forms part of the border between the US and Canada.