Flows in the southern half of the upper Colorado River basin are declining faster


Rafters run down the Colorado River to Glenwood Canyon.
Chelsea Self / Post Independent

New climate data that shows a north / south divide in flow declines in the Colorado River basin could have implications for water managers as they strive to address water shortages.

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

For example, the flows of the San Juan River near Bluff, Utah, decreased by 30% and the flows of the Dolores River near Cisco, Utah, decreased by 21%. The Yampa River flows near Maybell and the Colorado River near Glenwood Springs each lost only 6% of the flows.



Nothing

“We think it’s going to dry out more in the south and less in the north and at some point we should see a gradient, and of course, that’s appeared at some of these gauges,” Udall said.

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



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


Udall said the data was only a first glimpse; more research needs to be done, and there is at least one outlier that goes against the trend: The White River, which runs through the northwestern corner of Colorado, has seen a 19% drop in flows at the Watson gauge, which is just above the state line in Utah.

“Do natural flows accurately reflect what is happening? I just don’t know, ”Udall said. “When you start doing science, you get these results that are worth exploring. “

Warming temperatures

A 2017 article co-authored by Udall and Jonathan Overpeck found that an average of a third of the Colorado River’s flow loss could be attributed to warming temperatures. Higher temperatures can even offset any increase in precipitation.

“The hotter it is, the more the air thirsts for water, so it’s going to extract more moisture from the soil or the crops or the tank or whatever,” said Russ Schumacher, CSU climatologist. “When the snow starts to melt, it has to start recharging the ground again. This is the first place he goes, and not so much ends up in rivers.

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

“The interior of the southwestern United States is a particularly vulnerable place because it’s a dry place to start,” he said. “By adding more heat to the system, you get more evapotranspiration and everything, and we see the rivers shrink in it.”

The line between driest and wettest is somewhere near Colorado’s mid-latitude and divides the state into northern and southern halves. Also, this is roughly where the main stem of the Colorado River flows through the state. But as the impacts of climate change worsen, the high pressure on the southwestern deserts could shift northward and extend the more intense flow losses already occurring in the southwestern part of the country. State to the northern half of Colorado.

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

Ice forms in a shallow puddle of water inside river rocks along the Colorado River near Two Rivers Park on a cloudy, dreary day in Glenwood.
Chelsea Self / Glenwood Springs Post Independent

Fairness in demand management

The north / south difference in flow drops could have implications for how Colorado water managers come up with a potential water-saving plan. State officials are currently investigating a program known as demand management that would pay water users to reduce and send the saved water to a special storage pool in Lake Powell. 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) cannot provide 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’s water managers are desperate to avoid a compact call scenario, which could result in mandatory water cuts.

A major topic in the demand management discussions has been proportionality and how to design a program that ensures that no particular river basin suffers more negative economic or environmental impacts than another. Another question is, if there is a compact appeal, how would the engineers in the state administer it so that already running out of water ponds don’t have to shrink even more?

“(The north / south flow loss differential) would be an interesting contribution to this, especially in the area of ​​equity,” Udall said. “If the upper main arm (of the Colorado River) isn’t actually in much pain but the San Juan is in really pain, what does that mean for who should be contributing to the deficit?” “

Lake Powell is seen in a November 2019 aerial photo from the nonprofit EcoFlight. Keeping enough water in the reservoir to support downstream users in Arizona, Nevada, and California is complicated by climate change. (Photo courtesy of EcoFlight)

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

“This year we were considerably worse than the rest of Colorado,” Curtis said. “You ask the following question: if this is a real model, how does that affect fairness? You can’t really get blood on a turnip. There was no water to manage on demand this year.

State officials say they are working to avoid disproportionate impacts on certain basins or water users as they continue to investigate a demand management program. Colorado Water Conservation Board deputy section chief Amy Ostdiek said climate change and drought were a factor in everything the organization does.

“Speaking of the southwest corner of the state, what we are seeing on the ground is that it has been badly affected by the drought and that has a number of implications,” he said. she declared. “In terms of demand management, there will certainly be issues and concerns specific to each region of the state.”

But whatever the distribution of the flow loss between the different tributaries of the upper basin, the overall flow trend is downward. According to Udall data, the Colorado River at the all-important Lee Ferry – just downstream of Lake Powell near the Arizona-Utah border, which is the dividing line between the upper and lower basins and the point where shipments of water from the upper basin to the lower basin are measured – lost 17% of its flow. Despite a near-average snowpack, 2021 saw the second unregulated influx into Lake Powell, at 31% of average. This summer, federal authorities began urgently releasing reservoirs in the upper basin to raise levels in Lake Powell to maintain the capacity to generate hydroelectric power.

Eric Kuhn, former chief executive of the River District and co-author of “Science Be Dammed: How Ignoring Inconvenient Science Drained the Colorado River,” said it’s this larger picture that should worry basin water managers. higher, especially when it comes to plans for future water supply projects.

“We think about it, but we don’t say it out loud: there is no more water,” said Kuhn. “There just isn’t a lot of water for development. This is an obvious conclusion at the moment. It’s the elephant in the upper basin room.

Aspen Journalism covers rivers and water in collaboration with The Aspen Times and other Swift Communications newspapers. For more information, visit http://www.aspenjournalism.org.