Wyoming v. Colorado

Background

The State of Wyoming sued the State of Colorado and two Colorado irrigation companies to stop a proposed Colorado diversion of part of the Laramie River into a different watershed for irrigation more than fifty miles away. The Laramie starts in Colorado, flows into Wyoming, and both States long have relied on diverting stream water for farming. Wyoming argued the Colorado project would take water Wyoming needs and would impair older Wyoming irrigation uses; Colorado argued it could dispose of water within its borders and that the planned diversion fit an equitable share.

Reasoning

The Court addressed whether the same “first-in-time” water rule used inside each State (the prior-appropriation doctrine) should control rights on an interstate stream, and whether Colorado could divert water regardless of harm. The Court rejected Colorado’s claim of unfettered control and held that prior appropriations on the river must be respected across the state line. The Court analyzed measured flows, seasonal and year-to-year variation, and practical storage limits. It found the practical available supply at relevant points and totaled 288,000 acre-feet per year, determined Wyoming’s senior appropriations reasonably require 272,500 acre-feet, and fixed the Colorado tunnel project’s priority as of October 1909. Because only 15,500 acre-feet remained available, the Court barred the defendants from diverting more than that amount annually.

Real world impact

The decision protects long-established Wyoming irrigation projects and limits how much water Colorado’s tunnel project may take. It applies the familiar first-in-time water rule across state lines and enforces a concrete annual cap of 15,500 acre-feet on the contested diversion, stopping the project from taking most of the proposed supply.