Wisconsin v. Illinois

Background

The State of Illinois, its cities and water agencies, and the Metropolitan Sanitary District of Greater Chicago are party to a 1967 court decree limiting how much water Illinois may divert from Lake Michigan. The Court amended Paragraphs 3 and 5 of that decree to change how diversions are counted, to add measurement and oversight rules, and to set conditions tied to Illinois passing specific state legislation about dilution and allocation priorities.

Reasoning

The central question was whether and how Illinois could manage its allotted 3,200 cubic feet per second (cfs) of Lake Michigan water while making more domestic allocations and coping with variable storm runoff. The Court authorized using a forty-year running average to judge compliance, allowed an annual average up to 3,680 cfs (and up to 3,840 cfs in two years of any forty-year period for extreme conditions), and capped the first thirty-nine years’ cumulative exceedance at 2,000 cfs‑years. The order requires measurements and computations by Illinois or the U.S. Army Corps of Engineers under Corps supervision, periodic audits, annual Corps reports, and creation of a three-member technical committee to set measurement standards. The amendment to take effect is conditioned on Illinois changing its Level of Lake Michigan Act to limit dilution to 320 cfs and prioritize domestic allocations while aiming to reduce groundwater pumping from the Cambrian‑Ordovician aquifer.

Real world impact

The ruling lets Illinois plan and allocate more Lake Michigan water for homes and cities while imposing yearly caps and federal oversight to monitor totals. The change aims to reduce deep aquifer pumping, but it is conditional on state legislation and a short objection period before it becomes effective.