Colorado v. New Mexico

Background

Colorado sued New Mexico seeking to divert water from the small Vermejo River for future industrial and other uses. The Vermejo begins in Colorado and flows about 55 miles into New Mexico, where farmers and industry already use all available water. A Special Master recommended allowing Colorado a 4,000 acre‑foot diversion, but New Mexico objected and challenged the factual basis for that recommendation.

Reasoning

The core question was whether the Court may consider conservation, efficiency, and a balance of harms and benefits when one State seeks water for future use while another State has established users under prior appropriation. The Court held that equitable apportionment is flexible: priority is an important guide but not the sole rule. The Court said it is appropriate to consider reasonable conservation measures and weigh harms and benefits, reject any automatic rule based solely on headwaters, and require clear factual support before upsetting existing uses. It also explained the burden shift: New Mexico must show likely substantial injury, and Colorado must then prove by clear and convincing evidence that a diversion is justified.

Real world impact

The decision means the Special Master must make specific factual findings about existing uses, available supply, possible conservation measures, the exact uses Colorado proposes, and the likely injury to New Mexico. No diversion is authorized yet. The ruling changes how courts will evaluate interstate water claims by allowing more flexible balancing while protecting established users unless strong factual proof supports a change.

Dissents or concurrances

Chief Justice Burger joined, stressing equal footing for the States. Justice O’Connor (joined by Justice Powell) concurred in the judgment but warned courts to be cautious weighing speculative future benefits against established local economies and demanded clear evidence before altering uses.