The U.S. Supreme Court case Wisconsin, Minnesota, Ohio and Pennsylvania v. Illinois and the Sanitary District of Chicago et al., 1940 was a dispute over water diversion from Lake Michigan to the Mississippi River system by the city of Chicago for sanitation purposes. The plaintiff states argued that this diversion lowered lake levels, negatively impacting their commercial shipping industries and violating their rights under common law principles governing interstate bodies of water. They sought an injunction to limit or stop the flow entirely. However, Illinois countered that stopping or reducing the flow would cause serious public health issues in Chicago due to inadequate sewage treatment facilities at that time. The Supreme Court ruled in favor of Illinois on grounds that it had not been conclusively proven whether such diversions significantly impacted lake levels or if natural factors were more influential; hence they could not justify granting an injunction based on speculative harm alone against certain public health risks posed by halting sewage disposal into Mississippi river system via Lake Michigan.

The dissenting opinion in the case of Wisconsin, Minnesota, Ohio, and Pennsylvania v. Illinois and The Sanitary District of Chicago et al., 1940 argued that the majority's decision to allow Illinois to continue diverting water from Lake Michigan was a violation of states' rights. They contended that this diversion significantly altered natural resources shared by several states without their consent or compensation for any potential harm caused. This action not only threatened these states' access to clean drinking water but also had significant implications on commercial fishing industries due to changes in lake levels affecting fish populations. Furthermore, they expressed concern about setting a dangerous precedent where one state could unilaterally make decisions impacting shared resources without considering other affected parties’ interests or obtaining their agreement.