## Facts

- The Vermejo River originates in Colorado and flows primarily through New Mexico; existing uses in New Mexico fully appropriated the river.
- Water rights on the Vermejo had developed entirely in New Mexico, where a 1941 decree allocated rights among New Mexico users under prior appropriation.
- Both Colorado and New Mexico follow prior-appropriation doctrines for intrastate water rights.
- In 1975, a Colorado steel company obtained a conditional Colorado right to divert Vermejo water, but New Mexico users obtained relief in state proceedings based on senior New Mexico rights.
- In 1978, Colorado filed an original action seeking equitable apportionment to allow a Colorado diversion for proposed future uses, asserting New Mexico used water wastefully and inefficiently.
- A Special Master recommended allowing Colorado to divert 4,000 acre-feet per year, reasoning New Mexico could offset losses through reasonable conservation and that benefits to Colorado would outweigh injury to New Mexico.
- New Mexico excepted, arguing that priority principles barred any Colorado diversion.

## Issues

1. Whether equitable apportionment applies to a state’s request to divert interstate waters for future uses, and how prior-appropriation priority should be weighed within that analysis.
2. What burden of proof applies when an upstream state seeks to reallocate water away from existing, fully appropriated downstream uses, and whether the Special Master’s findings were sufficient to evaluate that burden.

## Decision

- The Court held that equitable apportionment applies to claims for diversions for future uses in interstate water disputes.
- The Court approved, in general, consideration of conservation possibilities and comparative harms and benefits as consistent factors in equitable apportionment.
- The Court imposed a heightened evidentiary requirement: an upstream state seeking a future-use diversion that would impair existing uses must prove by clear and convincing evidence that the diversion’s benefits substantially outweigh the resulting harms.
- The Court concluded the Special Master’s findings were too incomplete to permit review of how those standards were applied to the facts.
- The case was remanded to the Special Master for additional, specific factual findings; no final apportionment or authorization of Colorado’s diversion was ordered at this stage.

## Legal Principles

- Interstate water disputes are governed by equitable apportionment, a flexible federal doctrine aimed at a fair allocation between states.
- When both states recognize prior appropriation, priority is a guiding principle in equitable apportionment but not the only criterion.
- Established, senior uses receive substantial equitable protection, reflecting reliance interests in long-standing uses.
- Equitable apportionment may consider available, reasonable conservation measures and the comparative balance of harms and benefits from a proposed diversion.
- A state seeking to divert water for future uses from a fully appropriated stream must prove by clear and convincing evidence that the benefits of the diversion substantially outweigh the harms to existing uses.
- A Special Master’s recommendations must include sufficiently detailed factual findings to permit meaningful judicial review, including findings on feasible conservation measures and quantified harms and benefits.

## Conclusion

The Court confirmed that equitable apportionment governs interstate water disputes even when an upstream state seeks water for future uses, but required the upstream state to meet a clear-and-convincing, benefits-substantially-outweigh-harms standard and remanded for fuller factual findings before any reallocation could be approved.


Colorado v. New Mexico, 459 U.S. 176 (1982)