The Never Summer Mountains tower almost 13,000 feet above sea level on the west side of Rocky Mountain National Park, the regal headwaters of the Colorado River. Snowmelt and rainfall trickle southwest from the peaks through jumbles of scree and colorful deposits of silicic rock, formed some 27 to 29 million years ago, then plunge into Gore Canyon. There, the river gallops downstream, absorbing other tributaries from Arizona, New Mexico, Utah and Wyoming on its way to California. More than 40 million people from seven states and Mexico depend on water from the Colorado River Basin to drink, irrigate crops, generate electricity and recreate, a demand that is greater than the river system can bear. 

Historically, variations in snowpack would correlate with the amount of available water in the river come summertime. But since 2000, less and less snowmelt has been making its way into the Colorado River, and water levels in the river have not tracked as closely with variations in precipitation. A new study from the University of Washington, published today in the journal Geophysical Research Letters, offers a clue as to why this may be: increased evaporation and decreased springtime rainfall is leading parched plants and trees to suck up much of the snow melt before it ever reaches the river. 

“These headwater areas provide around 70 to 80 percent of the Colorado River’s water,” said Daniel Hogan, a PhD student at the University of Washington who worked on the study. “Snowy peaks and all those high mountain rivers are really the linchpin of the system. So if less water is coming from there, then you can expect less water in the entire river.”

Hogan and a team of scientists used precipitation and streamflow data from 26 upper Colorado River basins—a large sample of the eventual river’s supply, accounting for about a quarter of the Colorado River’s streamflow—to study why there was a growing disparity between snowpack and water levels. 

They found that the upper Colorado River basin had experienced a 9 percent decrease in annual spring rainfall compared with precipitation levels prior to 2000. Over half of the 26 basins they surveyed had “significant annual precipitation decreases,” they wrote. Spring had the most severe dropoff in rain, with a 14 percent decline compared to pre-2000 data. “Lower and middle elevation headwater basins were particularly affected,” with 12 of 17 showing “significant decreases,” they wrote.

This drop-off in spring precipitation appears to be especially detrimental to water levels in the summer. Though the researchers did find evidence of decreased rainfall in other seasons, spring rains accounted for 56 percent of the water-level variance.

“Spring precipitation decreases alone fall short of explaining observed streamflow deficits,” the team concluded, but when combined with other forms of water loss, like evaporation and nearby vegetation soaking up the moisture, that explained 67 percent of the variance.

Among the tens of millions of people the Colorado River is overpromised to are farmers irrigating about 5 million acres of agricultural land. But theirs aren’t the only plants impacting Colorado River levels. In their study, the research team worked under the assumption that trees and vegetation in forests ringing the Rockies need springtime precipitation to grow; in its absence, snowmelt becomes the plants’ primary source of water—and they have first dibs. 

“It’s a very sound study,” said Tanya Petach, a climate science fellow with the Aspen Global Institute, which helps connect academics with outside organizations that can make use of their work. Petach, who was not involved in the University of Washington study, is a hydrologist who got her Ph.D. in environmental engineering from the University of Colorado. “It helps fill out part of the missing puzzle piece” as to why high levels of winter snowpack haven’t translated to large stream flow numbers in some recent years, she said.

The group’s findings read “like two knockout punches,” said Hogan. “You have less precipitation, so that leads to less streamflow, just inherently. And then, you also have a consequence of the trees and plants that still need their water,” which leads to “uncertainty in how much water we think we have.” He hopes this study helps water modelers understand the importance of using spring precipitation in addition to winter snowpack to predict how much water will be available in the river. 

This study “puts a lot of momentum” behind improving spring forecasts for Colorado River stream flows, Petach said. 

Hogan could not say for sure whether climate change has played a role in the decreasing springtime precipitation levels across the upper Colorado River basin as no part of their study was designed to investigate that possible connection. But other studies have already suggested climate change is driving droughts in the Colorado River’s upper basin. 

Decreasing water levels across the Colorado River “could very well be linked to climate change directly,” Hogan said. “And if that is the case, then we can expect these declines to continue.”