Northwest Angle - Origin

Origin

The Treaty of Paris, concluded between the United States and Great Britain at the end of the American Revolutionary War, stated that the boundary between U.S. territory and the British possessions to the north would run "...through the Lake of the Woods to the northwestern most point thereof, and from thence on a due west course to the river Mississippi..."

The parties did not suspect that the source of the Mississippi, Lake Itasca (then unknown to European explorers), was south of that point, and that thus the entire Mississippi was too far south to be intersected by a line running west from the Lake of the Woods. A factor in this mistake was the use of the Mitchell Map during the treaty negotiations; that map showed the Mississippi extending far to the north. In the Anglo-American Convention of 1818, the error was corrected by having the boundary run directly from the northwest point of the lake to the 49th parallel and then westward along it.

When a survey team led by David Thompson finally located the northwesternmost point of the lake and surveyed this north-south line, it was found to intersect other bays of the lake and therefore form the boundary of a section of U.S. territory to its east, now known as the Northwest Angle.