## Facts

- The Arkansas River originates in Colorado and flows into Kansas; both States rely on its water supply.
- Congress approved the Arkansas River Compact in 1949, including a provision barring “future development” in either State that would “materially deplete the usable quantity” of water available to the other State.
- Kansas alleged that Colorado’s increased post-Compact groundwater pumping from wells hydrologically connected to the river materially depleted river flows into Kansas, violating the Compact and harming Kansas water use.
- The Supreme Court, exercising original jurisdiction, appointed a Special Master who concluded that post-1949 pumping in Colorado violated the Compact and caused compensable injury to Kansas.
- In a damages-phase report, the Special Master recommended monetary relief (rather than water reallocation) measured by Kansas’s losses attributable to the violations, and recommended prejudgment interest beginning in 1969 based on Colorado’s knowledge of its violations.
- The Court considered exceptions to the Special Master’s third report concerning damages mechanics and prejudgment interest.

## Issues

1. Whether the Eleventh Amendment barred a monetary award against Colorado because Kansas’s damages calculation incorporated injuries suffered by individual Kansas water users.
2. Whether prejudgment interest could be awarded on unliquidated damages in an interstate compact enforcement action, and what start date was equitable.
3. Whether it was proper to award retrospective monetary damages (rather than water) using an economic model that converted depleted flows into quantified losses.

## Decision

- The Court overruled Colorado’s Eleventh Amendment objection, holding Kansas was the real party in interest and was vindicating a sovereign interest in enforcing the Compact, even if private-user losses informed the damages measure.
- The Court held that the unliquidated character of Kansas’s damages did not bar prejudgment interest; interest could be necessary to make Kansas whole.
- The Court largely accepted the Special Master’s damages methodology and the use of monetary relief for past depletions that could not practicably be restored.
- The Court modified the temporal scope of prejudgment interest from the Special Master’s proposed start date, applying equitable adjustments rather than adopting the recommendation in full.

## Legal Principles

- A State enforcing a congressionally approved interstate compact may seek retrospective monetary relief from another State in the Supreme Court’s original jurisdiction.
- The Eleventh Amendment does not bar a damages award to a plaintiff State merely because the damages calculation uses private economic losses as a metric for the State’s injury; the State remains the real party in interest when it controls the litigation and disposition of any recovery.
- Prejudgment interest may be awarded in interstate compact disputes to achieve full compensation, even where damages are not fixed in amount, subject to equitable limits on the interest period.
- Monetary damages can be an appropriate remedy for historic water depletions where in-kind restoration or reallocation is impracticable.

## Conclusion

The Court enforced the Arkansas River Compact by sustaining a monetary damages remedy against Colorado and permitting prejudgment interest as part of full compensation, while rejecting an Eleventh Amendment bar based on the use of private-user losses in the damages measure and adjusting interest timing on equitable grounds.


Kansas v. Colorado, 533 U.S. 1 (2001)