The Colorado River's shrinking reservoirs have become a familiar sign of the West's water challenges. But a new study finds groundwater that helps sustain the river system is also disappearing.
Researchers at Virginia Tech found the Upper Colorado River Basin lost an estimated 4.6 trillion gallons of groundwater during a severe three-year drought from 2020 to 2022. That's enough water to halfway fill Lake Powell, stretched along the Utah-Arizona border.
The losses were greater than those in the drier Lower Basin, according to thestudy published in Water Resources Research.
"In the Mountain West, groundwater is an important source of supply," said Esther Oyedele, who led the study. "So our study has been able to show that groundwater cannot simply just be treated as an unlimited backup anymore."
A surprising shift in groundwater losses
The Upper Basin includes parts of Colorado, Wyoming, Utah and New Mexico, where mountain snowpack helps replenish water supplies.
Using satellite data, researchers tracked groundwater changes across the Colorado River Basin from 2004 through 2023. They found losses have accelerated since 2019, with the typically wetter Upper Basin experiencing especially significant declines during the recent drought.
Increasingly dry conditions, rising temperatures and growing water demand are putting more pressure on underground supplies.
Susanna Werth, a Virginia Tech geosciences professor and study co-author, said the connection between groundwater and surface water means losses underground could also affect rivers and reservoirs.
"And that, of course, will have lots of consequences for farmers and water users," Werth said, "even if they don't pump groundwater."
Growing pressure on Colorado River supplies
The findings come as Western states grapple with new cuts to Colorado River water deliveries.
In August, the federal government announced reductions for Lower Basin states, including Arizona, California and Nevada, amid declining reservoir levels and disagreements over how to share the river's shrinking supply.
The Colorado River provides water to more than 40 million people across seven Western states and Mexico, supporting farms, cities and ecosystems.
While the new study doesn't evaluate those cuts, it highlights another challenge for water managers: groundwater reserves are also declining, including in the river system’s mountainous headwaters.
Oyedele said the findings highlight the need for water managers to think beyond immediate shortages and plan for the river system’s long-term health.
"We need to really manage the water based on not just how much is available today, but also on whether those resources can actually be sustained in the future," she said.
This story was produced by the Mountain West News Bureau, a collaboration between Boise State Public Radio, Wyoming Public Media, Nevada Public Radio, KUNR in Nevada, KUNC in Northern Colorado, KANW in New Mexico, Colorado Public Radio and KJZZ in Arizona as well as NPR, with support from affiliate newsrooms across the region. Funding for the Mountain West News Bureau is provided in part by the Corporation for Public Broadcasting and Eric and Wendy Schmidt.
