New climate data that shows a north/south split in streamflow declines in the Colorado River basin could have implications for water managers as they navigate how to address water shortages.

This month, Brad Udall, senior water and climate research scientist at Colorado State University, presented data that shows when comparing records from the past 20 years to those from most of the 20th century, rivers in the southern half of the upper Colorado River basin have lost a larger percentage of flows than rivers in the northern part of the basin.

For example, flows on the San Juan River near Bluff, Utah, have declined by 30% and flows on the Dolores River near Cisco, Utah, have declined by 21%. Flows on the Yampa River near Maybell and the Colorado River near Glenwood Springs have each lost just 6% of flows.

“We do think it’s going to dry more in the south and less in the north and we should at some point see a gradient, and sure enough, that has popped up at some of these gauges,” Udall said.

Udall presented his findings at the University of Colorado Getches-Wilkinson Center’s 41st annual Colorado Law Conference on Natural Resources, which was simulcast as part of the Colorado River Water Conservation District’s annual seminar on Oct. 1.

Udall analyzed the Bureau of Reclamation’s natural flow data, which is an estimate of the flow that would have been observed at a stream gauge if there were no reservoirs or diversions present. Then he compared 1906-99 data to 2000-19 data to see how much the flows have declined.

Udall said the data was just an initial look; more research needs to be done, and there is at least one outlier that bucks the trend: The White River, which flows through the northwest corner of Colorado, has experienced a 19% decline in flows at the Watson gauge, which is just over the state line in Utah.

“Are the natural flows conveying accurately what’s going on? I just don’t know,” Udall said. “When you begin to do science, you come up with these results that bear more digging.”

Lake Powell is seen in a November 2019 aerial photo from the nonprofit EcoFlight. Data that shows streamflows are declining more in the southern part of the upper Colorado River basin could influence how the state of Colorado develops a program that would pay water users to use less and send the saved water downstream to Lake Powell. Credit: EcoFlight

Warming temperatures

A 2017 paper co-authored by Udall and Jonathan Overpeck found that an average of one-third of the Colorado River’s flow loss could be attributed to warming temperatures. Higher temperatures may even cancel out any increases in precipitation.

“The hotter it is, the thirstier the air is for water, so it’s going to pull more moisture out of the soil or the crops or the reservoir or whatever the case may be,” CSU climatologist Russ Schumacher said. “As the snow starts to melt, it has to go back into recharging the soil. That’s the first place it goes, and not as much ends up in the rivers.”

Schumacher said the north/south flow-loss differential is consistent with what climate predictions have shown.

“The interior Southwest of the United States is a place that is especially vulnerable because it’s a dry place to begin with,” he said. “Adding more heat into the system, you get more evapotranspiration and everything else, and we are seeing rivers decline there.”

The dividing line between drier and wetter is somewhere near the middle latitude of Colorado and bisects the state into northern and southern halves. Also, it is roughly where the main stem of the Colorado River flows through the state. But as the impacts of climate change worsen, high pressure over the deserts of the Southwest could creep northward and expand the more intense streamflow losses already happening in the southwestern part of the state to the northern half of Colorado.

“The difficulty with climate change is that it’s changing,” Udall said. “This is a moving target throughout the 21st century, and every time you think you have it figured out, something else is going to happen.”

This map shows the south-to-north flow losses in the upper Colorado River basin, based on natural flow data from the Bureau of Reclamation. The White River, which flows through northwestern Colorado, is an outlier that climate scientist Brad Udall says needs more study.

Equity in demand management

The north/south difference in flow declines could have implications for how Colorado water managers develop a potential water-savings plan. State officials are currently investigating a program known as demand management that would pay water users to cut back and send the saved water to a special storage pool in Lake Powell. The water would be an insurance policy against a Colorado River Compact call.

A compact call could occur if the upper-basin states (Colorado, Wyoming, Utah and New Mexico) can’t deliver the 7.5 million acre-feet of water per year to the lower-basin states (Arizona, California and Nevada), as required by a nearly century-old binding agreement. Colorado water managers desperately want to avoid a compact-call scenario, which could result in mandatory water cutbacks.

A major topic of demand-management discussions has been proportionality and how to design a program that ensures that no particular river basin experiences more negative economic or environmental impacts than another. Another question is: If there is a compact call, how would state engineers administer it so that already water-short basins aren’t forced to cut back even more?

“(The north/south flow-loss differential) would be an interesting input into that, especially in the area of equity,” Udall said. “If the upper part of the (Colorado River) main stem is actually not suffering very much but the San Juan is really suffering, what does that mean for who should help contribute to the shortfall?”

Ken Curtis, general manager of the Dolores Water Conservancy District, said he has noticed a warming trend in southwest Colorado and that irrigators have seen an increasing number of shortages over the past 20 years. He has been monitoring the demand-management discussions and questions surrounding proportionality.

“This year, we were substantially worse than the rest of Colorado,” Curtis said. “You overlay the question, if this is a real pattern, how does this play into equitability? You can’t really get blood out of a turnip. There wasn’t any water to demand manage this year.”

State officials say they are striving to avoid disproportionate impacts on certain basins or water users as they continue their investigation into a demand-management program. Colorado Water Conservation Board Deputy Section Chief Amy Ostdiek said climate change and drought factor into everything the organization does.

“Talking about the southwest corner of the state, just what we are seeing on the ground is that it has been heavily impacted by drought and that has a number of implications,” she said. “In terms of demand management, there are going to be issues and concerns that are specific to each area of the state, for sure.”

But regardless of how the flow loss breaks down among the different tributaries of the upper basin, the overall streamflow trend is downward. According to Udall’s data, the Colorado River at the all-important Lee Ferry — just downstream from Lake Powell near the Arizona-Utah border, which is the dividing line between the upper and lower basins and the point at which upper-basin water deliveries to the lower basin are measured — has lost 17% of its flow. Despite a near-average snowpack, 2021 saw the second-worst unregulated inflow into Lake Powell, at 31% of average. This summer federal officials began emergency releases from upper-basin reservoirs to prop up levels in Lake Powell to maintain the ability to make hydroelectric power.

Eric Kuhn, former general manager of the River District and co-author of “Science Be Dammed: How Ignoring Inconvenient Science Drained the Colorado River,” said it is this bigger picture that should have upper-basin water managers worried, especially when it comes to plans for future water projects.

“We are thinking it, but we aren’t saying it out loud: There’s no more water,” Kuhn said. “There’s just not a lot of water for development. It’s an obvious conclusion right now. It’s the elephant in the room in the upper basin.”

Aspen Journalism covers water and rivers in collaboration with The Aspen Times and other Swift Communications newspapers. This story ran in the Oct. 16 edition of The Aspen Times, the Glenwood Springs Post-Independent, the Oct. 17 edition of the Vail Daily and Oct. 18 edition of Sky-Hi News.

Heather Sackett is the managing editor at Aspen Journalism and the editor and reporter on the Water Desk. She has also reported for The Denver Post and the Telluride Daily Planet. Heather has a master’s...