## Facts

- The Vermejo River originates in Colorado and flows into New Mexico; its waters had long been used primarily by farm and industrial users in New Mexico.
- A 1941 New Mexico decree apportioned rights among New Mexico users.
- In 1975, a Colorado steel company obtained a conditional Colorado right to divert Vermejo water; New Mexico right holders with senior claims obtained an injunction barring the diversion.
- Colorado filed an original action seeking equitable apportionment that would allow a new Colorado diversion, alleging New Mexico’s use was wasteful and could be offset through conservation measures.
- A Special Master initially recommended allowing Colorado to divert 4,000 acre-feet per year based largely on assumed conservation and administration improvements in New Mexico and a net-benefit balancing.
- In an earlier decision, the Court remanded for more specific findings on whether water could reasonably be made available and how harms and benefits compared.
- On remand, the Special Master, relying on the same record, reaffirmed the 4,000 acre-foot recommendation; New Mexico again filed exceptions.

## Issues

1. What burden of proof applies when a state seeks equitable apportionment to initiate a new diversion from an interstate stream historically used in another state?
2. Did Colorado prove, under the applicable burden, that a 4,000 acre-foot annual diversion should be permitted?

## Decision

- The Court held that a state seeking to initiate a new diversion through equitable apportionment must prove its case by clear and convincing evidence.
- The Court sustained New Mexico’s exceptions in substantial part and rejected the Special Master’s recommendation allowing Colorado’s proposed diversion.
- The Court concluded Colorado failed to show, by clear and convincing evidence, that reasonable conservation measures in New Mexico would offset the diversion or that the benefits to Colorado substantially outweighed the risk of serious injury to existing New Mexico uses.
- Judgment was for New Mexico; Colorado’s requested apportionment allowing the diversion was denied.

## Legal Principles

- In equitable apportionment original actions, a proposed new diverter bears a heavy evidentiary burden and should bear most, though not all, of the risk of erroneous decision.
- The proponent of a new diversion must establish by clear and convincing evidence that the diversion is justified in light of existing uses and likely effects.
- Equitable apportionment requires “hard facts,” not speculation, particularly when the stream is already fully used and the requested relief would alter established patterns of use.
- Alleged availability of offsetting conservation measures must be supported by specific, feasible measures and sufficiently reliable quantification; generalized assertions about improved administration are inadequate.
- Even when multiple equitable factors are relevant (including waste, conservation feasibility, and comparative benefits and harms), the proponent must show the diversion can occur without serious injury to existing users, or that any remaining injury is justified by demonstrated benefits.

## Conclusion

The Court denied Colorado’s request to initiate a 4,000 acre-foot diversion from the Vermejo River, holding that a state seeking a new interstate diversion through equitable apportionment must prove entitlement by clear and convincing evidence and that Colorado’s showing—grounded largely in assumptions about New Mexico conservation and administration—did not satisfy that standard.


Colorado v. New Mexico, 467 U.S. 310 (1984)