The Anglo-American Convention of 1818 between the…
October 1818 CE
The Anglo-American Convention of 1818 between the U.S. and the United Kingdom establishes the boundary between the U.S. and British North America as the 49th parallel, from the Lake of the Woods to the "Stony Mountains", also creating the Northwest Angle; the treaty is signed on October 20, 1818.
Thus, the southern portion of Selkirk's grant goes to the United States.