Arkansas v. Mississippi

Background

This case is a boundary fight between the States of Arkansas and Mississippi over land cut off when the Mississippi River suddenly changed course and isolated a tract called Horseshoe Island. Arkansas says the old river channel ran through what is now Horseshoe Lake (Old River); Mississippi claims the old channel was farther north near Dustin Pond and that the boundary should be a line equidistant from the riverbanks. The differing claims are shown on a map attached to the record, and Congress’s original state admission language referring to the "middle of the main channel" is central to the dispute.

Reasoning

The Court addressed whether the boundary follows the middle of the main navigable channel as it existed before the sudden shift, or a line equidistant from the banks. Relying on earlier decisions interpreting similar admission language, the Court held that when a river forms a state line, the line is the middle of the main navigable channel (the deepest navigation route), not an equidistant line between banks. The Court found this was a clear case of a sudden change in the river’s course (an avulsion) and so the boundary remains where that navigable channel ran just before the event. The Court rejected arguments that state constitutions or local decisions change this federal rule.

Real world impact

Practically, the ruling fixes the legal principle that the boundary stays at the former navigation channel and sends the task of mapping the exact line to a three-person commission the Court will appoint. The commission will use the record and additional proof to mark the line, which will determine which state controls the island, land titles, and local criminal and civil jurisdiction.