The U.S. Supreme Court case State of Illinois v. Commonwealth of Kentucky in 1990 was a dispute over the boundary line between the two states where they meet at the Ohio River. The controversy arose from differing interpretations of an 1818 act by Congress admitting Illinois to the Union, which specified that its border with Kentucky would be "at low-water mark on...the northwestern shore" of the river. Over time, changes in water levels and course had moved this shoreline significantly southward; however, both states claimed jurisdiction up to their original borders as defined by landmarks rather than current waterlines. Kentucky argued for a fixed boundary based on historical markers while Illinois contended that it should follow present-day low-water marks along what is now entirely within Kentucky's territory due to shifts in river flow since statehood was granted. In a unanimous decision delivered by Justice Byron White, the court ruled against Illinois' claim and upheld Kentucky's interpretation: namely that boundaries were intended to remain constant once established regardless of natural changes or ambiguities in language used when defining them initially.

In the dissenting opinion for the case of State of Illinois v. Commonwealth of Kentucky, Justice Stevens argued that the majority's decision to award sovereignty over a portion of the Ohio River to Kentucky was incorrect. He contended that historical documents and treaties clearly indicated that when Virginia ceded its land claims in 1784, it intended for all states bordering rivers to share jurisdiction over those waters equally. This would mean both Illinois and Kentucky should have joint control over this part of the Ohio River. Furthermore, he disagreed with how much weight was given by his colleagues on previous court decisions which were not directly related or applicable to this case at hand.