A new report from a team of water law scholars, engineers, and policy experts argues that the 80-year-old framework governing how the United States delivers Colorado River water to Mexico is no longer fit for purpose — and that a fundamental redesign is both possible and urgent.
Since 1945, the US has been obligated under a 1944 treaty to deliver a fixed 1.5 million acre-feet of water to Mexico annually. That figure made sense when it was negotiated, but the treaty's authors were working with incomplete hydrological data during an unusually wet period in the river's history. Decades of aridification have since reduced average flows significantly, meaning Mexico's fixed share now represents a steadily growing percentage of what little water actually flows — and the burden of that shrinkage falls disproportionately on the US.
The report's central proposal is to replace the fixed delivery obligation with an allocation tied directly to the Colorado River's actual natural flow, measured at Lees Ferry. The authors suggest a figure around 11% of the five-year rolling average of natural flow, with an annual cap of 1.7 million acre-feet. Their modelling shows this would have produced roughly the same historical average deliveries to Mexico since 1945, while automatically scaling down during drought years — sharing the pain more equitably between the two countries.
The suggestion is timely because the treaty minutes currently governing US-Mexico water cooperation — Minutes 323 and 330 — expire at the end of 2026. Domestic negotiations among the seven US Basin States have collapsed without agreement, raising the prospect of either unilateral federal action or interstate litigation. The report argues that a percentage-based agreement with Mexico can and should move forward independently of that domestic stalemate, and could actually serve as a stabilizing model: under the current "alignment" approach, Mexico is effectively treated as a fourth Lower Basin state, meaning any future Supreme Court litigation among US states could directly affect Mexico's water supply.
