Wisconsin v. Illinois

Background

The dispute involves the State of Illinois, many of its cities and water agencies (including Chicago and nearby suburbs), and the flow of water from Lake Michigan into the Illinois waterway and the Sanitary and Ship Canal. A Special Master prepared a report whose factual findings the Court adopted. The Court then issued a decree that controls how much water Illinois and its local governments may divert from Lake Michigan and how that amount must be measured and allocated.

Reasoning

The Court accepted the Special Master’s findings of fact and set a concrete limit on diversions: an average combined maximum of 3,200 cubic feet per second from Lake Michigan and its watershed. The decree explains how to calculate diversions, what counts as domestic pumpage or storm runoff, and who is credited or excluded in the accounting. It requires measurements under the general supervision of the U.S. Army Corps of Engineers, allows a five-year averaging approach with a 110% annual cap, and sets an interim limit of 1,600 cubic feet per second for the early 1970 period. Illinois may later ask the Court to modify the limit if regional needs cannot be met despite conservation and pollution-control efforts.

Real world impact

The decree restricts how much Lake Michigan water Illinois and its municipalities can withdraw and prescribes detailed monitoring and allocation. It affects municipal water suppliers, regional water planning, and the Sanitary and Ship Canal’s operations. The Court keeps jurisdiction to modify the decree, and one local commission’s separate complaint was dismissed without preventing that commission from sharing in the allowed water.