Colorado River Basin reservoir storage.

Jack Schmidt* and John Fleck**

*Center for Colorado River Studies, Utah State University
**Utton Transboundary Resources Center, University of New Mexico School of Law
1 June 2025

We now begin June, when the Colorado River’s two largest reservoirs, Lake Mead and Lake Powell, should be swelling with melting snow for use later this year and beyond, but that is not happening. Although Lake Powell is our reservoir and Lake Mead is theirs (or vice versa), the two reservoirs are effectively one very large facility located downstream from Upper Basin consumptive users and upstream from Lower Basin users. At least 60% of the total storage in 46 reservoirs tracked by Reclamation is in Lake Powell and Lake Mead. The total contents of the two reservoirs have been steadily declining since early July 2024 and continued to decline through at least 31 May 2025. Never in the past 15 years has the decline in total storage of Powell and Mead extended so late into spring. Current reservoir storage data are showing us, in real time, an ominous pattern familiar from past dry years: upstream use of water before it has a chance to get to Lake Powell combined with releases from Lake Mead to users further downstream is outpacing the melting snowpack’s ability to replenish the two reservoirs.

While the normal tools we use for measuring and managing use of Colorado River water – the Consumptive Uses and Losses Reports and the Lower Basin decree accounting reports – lag by weeks or even years, reservoir storage, which is the net difference between stream flow into reservoirs and what is released downstream or is lost to evaporation, provides the closest thing we have to an accurate, real-time measure of the Colorado River basin’s water budget. Right now, we are not doing well.

• The duration of time this year during which total storage in Lake Powell and Lake Mead has declined is unprecedented in the past 15 years. In a typical year, the steady decrease in the combined contents of Powell and Mead that begins the preceding summer ends in early May when Rocky Mountain snowmelt becomes significant. However, inflows to Lake Powell this year have yet to exceed releases from Lake Mead , and the total contents continue to decline, suggesting that this year’s recovery in storage will be minimal.
• Data from other years also suggests that reservoir recovery this year will be relatively small. This year, total unregulated inflow to Lake Powell is predicted to be 55% of normal. Based on past trends, net increase in total reservoir storage of the 46 reservoirs tracked by Reclamation will be ~1.2 million acre feet (af). By July, we are likely to resume draw down the basin’s reservoirs until the 2026 snowmelt season begins.
• Presently, storage in the watershed’s reservoirs is comparable to conditions in late summer and fall 2021 when water managers expressed significant concern. The very wet conditions of 2023 averted a major crisis, but the system remains depleted. In 2024, total basin reservoir storage climbed by 2.5 million af, but subsequent drawdown of those reservoirs was 3.6 million af during the following 10 months. Although the net difference between reservoir gain and subsequent drawdown of 1.1 million af might be considered “balanced” in the context of the last 15 years, there is no question that we have begun to mine the bounty of 2023, and we are likely to continue to do so until at least spring 2026 unless we greatly reduce consumptive uses.

For too long, we have hoped that big wet years will occur with sufficient frequency to avert true crisis, but there have been too few of those wet years during the 21st century. Only three of the last 15 years have been sufficiently wet to result in a significant increase in reservoir storage given the magnitude of the basin’s consumptive uses. We can’t continue with a water management policy that hopes for another wet year. The basin’s water managers have no choice but to further reduce consumptive uses to sustainably manage the dwindling water supply.

In response to a previously posted mini-white paper on reservoir storage, a supportive friend commented, “Nobody cares.” Another friend said, “I don’t see how we can get agreement about recovering storage. Let’s hope for more wet years.” We should care, and we need to try harder.

These mini-white papers seek to demonstrate that reservoir storage data, analyzed in aggregate, provide timely and accurate data relevant to understanding the reliability and security of the Colorado River’s water supply. These data are more precise, accurate, and timely than estimates of natural runoff, reservoir inflow, consumptive uses, or evaporation. Reservoir storage data provided by Reclamation are a significant contribution to transparency in water management. However, these data are under-utilized and under-analyzed and are typically reported without long-term context. We can do better.

These data can be used to develop an excellent correlation between April-July unregulated inflow to Lake Powell, forecast by the Colorado Basin River Forecast Center, and anticipated increase in basin-wide storage. Such an analysis strongly indicates that the 2025 snowmelt runoff will yield only a small increase in basin storage and necessitate greater reductions in consumptive use so as to better position the basin’s water users should next year also be dry.

Click here for our full report.