Illinois v. Kentucky

Background

In 1986 Illinois asked the Court to decide where its southern border with Kentucky lies along the Ohio River, asking the Court to name the low-water mark on the river’s north shore as it existed in 1792. Kentucky answered that the border should instead follow the river’s current low-water mark and raised defenses including long-standing possession, acquiescence by Illinois, laches, and rules about changing riverbanks. A Special Master gathered years of evidence and recommended that the boundary be the 1792 low-water mark and that Kentucky had not proved its defenses.

Reasoning

The central question was whether the boundary is the historic 1792 line or a shifting shoreline today. Relying on earlier cases and the Special Master’s factual findings, the Court agreed the legal boundary is the low-water mark on the north shore as it was in 1792. The Court found Kentucky failed to prove long, continuous acts of sovereign control or Illinois’ long acquiescence. The Court noted examples said by Kentucky’s own officials and the limited taxing of structures that undercut Kentucky’s claim. The Court also refused to apply laches and said ordinary river-change rules only matter if Kentucky had proved its factual defense. The Court did not accept the Special Master’s separate finding about modern dams raising the river and sustained Kentucky’s exception on that point.

Real world impact

The decision fixes the legal rule that the 1792 low-water mark defines the border and sends the case back to the Special Master to map and prepare a formal decree locating that line. The Court left open further factual work about how modern dams or river changes may affect the exact placement of the line, so additional surveying and hearings will follow.