## Facts

- Great Lakes states sued the U.S. Army Corps of Engineers and the Metropolitan Water Reclamation District of Greater Chicago over operation and maintenance of the Chicago Area Waterway System (CAWS), a man-made connection between the Mississippi River system and Lake Michigan.
- Plaintiffs alleged defendants were creating and maintaining a public nuisance by allowing a pathway for invasive Asian carp to reach the Great Lakes.
- Plaintiffs presented evidence that Asian carp were near the CAWS connection, reproduce rapidly, consume foundational organisms in the food chain, and could cause major ecological and economic damage if established in the Great Lakes.
- Plaintiffs sought a preliminary injunction requiring additional measures (e.g., more barriers, separation measures, or operational changes) to prevent carp from entering Lake Michigan.
- Defendants presented evidence that plaintiffs’ proposed measures would disrupt navigation and commerce, block boat traffic, increase flood risk, and affect public safety.
- Defendants also showed that multiple agencies were already implementing monitoring, electric barriers, and related efforts to prevent or slow carp migration.

## Issues

1. Whether plaintiffs showed a likelihood of success on their federal common-law public nuisance claim sufficient to justify preliminary injunctive relief.
2. Whether plaintiffs demonstrated irreparable harm and the inadequacy of legal remedies absent a preliminary injunction.
3. Whether the balance of harms and the public interest favored the requested injunction, including consideration of navigation, commerce, flood control, and public safety.

## Decision

- The Seventh Circuit affirmed the district court’s denial of a preliminary injunction.
- The court accepted that plaintiffs showed a non-trivial risk of severe harm if Asian carp entered the Great Lakes.
- The court held plaintiffs did not show the specific injunction requested would significantly reduce that risk beyond ongoing mitigation efforts.
- The court held the requested relief posed substantial harms to navigation, commerce, and flood control, and those harms outweighed the uncertain incremental benefit of the injunction.
- The court noted that coordinated agency actions were already addressing the problem and cautioned against judicially imposed, technically complex reconfiguration of a major integrated waterway system on a preliminary record.

## Legal Principles

- A preliminary injunction is an extraordinary remedy; the movant bears the burden to justify immediate equitable relief.
- In environmental-risk cases, showing a serious potential harm is insufficient; the movant must show the requested injunction will materially reduce the risk of the feared injury.
- Courts must weigh the incremental benefit of the requested relief against countervailing harms, including impacts on the public interest.
- Where technical, multi-agency mitigation efforts are underway, uncertainty about the efficacy of additional judicially ordered measures can defeat a request for preliminary relief, particularly when the proposed measures carry significant collateral consequences.

## Conclusion

The Seventh Circuit left ongoing anti-carp measures in place and declined to order additional preliminary relief because the states did not show their requested injunction would meaningfully reduce the invasion risk and because the likely harms to navigation, commerce, and flood management outweighed the uncertain benefits.


Michigan v. U.S. Army Corps of Eng'rs, 667 F.3d 765 (7th Cir. 2011)