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
The Rio Grande or Rio Bravo del Norte.
not generally since 1803 & not a bit since 1818
The Mississippi River was the western border in 1787.
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.
Rio Grande forms some of the border.
Rainy River, Pigeon River, St Marys River, St Clair River, Detroit River, Niagara River, St Lawrence River, St John River and St Croix River all form part of the International Boundary between Canada and the US.