Page "Louisiana Purchase" Paragraph 44

If the territory included all the tributaries of the Mississippi on its western bank, the northern reaches of the Purchase extended into the equally ill-defined British possession — Rupert's Land of British North America, now part of Canada.

The Purchase originally extended just beyond the 50th parallel.

However, the territory north of the 49th parallel ( including the Milk River and Poplar River watersheds ) was ceded to the UK in exchange for parts of the Red River Basin south of 49th parallel in the Anglo-American Convention of 1818.