E+E Leader Team
A new 2025 assessment of the Colorado River Basin concludes that the Southwest’s long-running drying trend is being driven not only by rising temperatures but also by declining precipitation linked to human-caused climate change. The finding heightens urgency for a basin that supplies water to more than 40 million people and underpins an economy estimated at over $1.4 trillion.
The analysis by Jonathan Overpeck of the University of Michigan and Brad Udall of the Colorado Water Center at Colorado State University appears in Colorado River Insights, 2025: Dancing With Deadpool. Their chapter, Think Natural Flows Will Rebound in the Colorado River Basin? Think Again, updates a set of long-standing hydrologic indicators—natural flows, precipitation, temperatures, and storage levels—originally developed after their 2017 “hot drought” study.
That 2017 work showed the then-18-year drought had already reduced natural flows by roughly 17%, with higher temperatures responsible for as much as half of the decline. The new report shows conditions have worsened. Twenty-first-century natural flow at Lees Ferry now averages 12.3 million acre-feet, compared with 15.0 million acre-feet in the 1990s.
What changes the scientific picture in 2025 is attribution. Overpeck and Udall find growing evidence that declining upper-basin precipitation—both the century-long trend and the sharp drop during the 26-year megadrought—is also driven primarily by human-caused warming rather than natural variability. They point to recent research showing that temperature and precipitation patterns now fall outside the range of past megadroughts over the last two millennia, with future decades likely to see more extended dry periods in the headwaters.
The storage analysis provides the clearest measure of the basin’s vulnerability. In 1999, the major reservoirs held 59.5 million acre-feet of active storage. By November 15, 2025, Lake Powell and Lake Mead together held 14.9 million acre-feet, but only 6.2 million acre-feet—about 42%—is considered realistically accessible because of engineering and hydropower constraints. Meanwhile, natural flows from 2020–2025 averaged 10.8 million acre-feet, matching the record-low period of 2000–2005.
With both inflows and storage diminished, the margin for error has disappeared. A scenario modeled in the report shows that if 2026 repeats 2025’s natural flow of 8.5 million acre-feet, the basin would face a 3.6 million acre-foot supply gap. Drawing down storage to cover most of that deficit would push accessible water in Powell and Mead to about 3.5 million acre-feet, slightly below the low point reached in 2023. If the winter of 2025–2026 is similarly weak, the report warns that less than 4 million acre-feet may remain available heading into 2027—insufficient under current operating rules to prevent a new water-supply crisis.
Overpeck and Udall frame the conclusion directly: human-caused warming is reducing Colorado River flows by increasing temperatures and suppressing precipitation, and those impacts will intensify as emissions rise. Because of this, climate mitigation is no longer separate from water planning—emissions now directly shape the future reliability of the Colorado River itself.