ALONG THE COLORADO RIVER – It’s detective time on the Colorado River. There’s been a massive thievery. Who or what has purloined the agua?

Precipitation in the seven-state Colorado River Basin during 2000-2014 averaged 6.1 percent less than in the 20th century, but flows declined 19.3 percent. In other words, despite a few sour winters of marginal snow, most notably 2002 and 2012, it’s mostly been business as usual at elevations of 9,000 to 11,000 feet.

That elevation band, the money belt for Colorado’s ski industry, provides 85 percent of the river’s water when it reaches Lee’s Ferry at the entrance to the Grand Canyon. Colorado alone is responsible for 70 percent of the river’s flows.

Where did this water go? Rising temperatures, say researchers Brad Udall and Jonathan Overpeck in a new study, explain roughly a third of reduced flows.

Drought alone has been the conventional explanation for reduced flows in the river. The poster for the drought has been the ever-more bleached rocks of the rivers’ two giant reservoirs, Powell and Mead, looking like a swimsuit tugged indecently below the tan line.

But precipitation has declined relatively little in the 21st century. However, temperatures in this century have increased 1.6 degrees Fahrenheit as compared with the 20th-century record. Therein lies the major difference, according to Udall and Overpeck.

They believe that flows will almost certainly decline further, by 35 percent or more during this century, as temperatures inexorably rise due to increasing heat-trapping greenhouse gases in the atmosphere.

“Our work demonstrates that flows are unlikely to return to the 20th century averages if we only wait,” they write in a study being published in a journal called Water Resources Research.

Things could really turn strange if rising temperatures pile on top of natural droughts that last several decades. Tree rings document several such multi-decadal droughts between 900 A.D. and 1400 A.D. in the Southwest.

Might heat-induced drought coupled with natural drought also result in empty reservoirs such as, say, Ruedi Reservoir above Basalt on the Fryingpan River?

An empty Ruedi is far out on the limb of hypotheticals, but it’s among the possibilities that water managers should contemplate as they examine an ever-changing human-driven climate, said Udall, who is affiliated with the Colorado Water Institute at Colorado State University. Overpeck is with the University of Arizona.

Udall said the major contribution of this new study is that it examines temperature as driving changes in how much water flows in Colorado River tributaries, including the Roaring Fork River, apart from precipitation changes.

Previous studies have stepped up to the same conclusion but then walked away. Instead, they amalgamated temperature with precipitation in an effort to project how accumulating greenhouse gases will change the climate in the American Southwest. Amalgamation, said Udall, hides the true risk, because uncertain increases in precipitation shown in some climate models obscure the known and dramatic temperature-induced declines from all models.

“Combining the two effects is not the right way to look at risk,” Udall said. “Future temperatures are known, but precipitation is not. We have to prepare for this downside risk and not pretend that it doesn’t exist.”

Eric Kuhn, the general manager for the Glenwood Springs-based Colorado River Water Conservation District, said he started wondering in 2010 whether rising temperatures were at least partly responsible for the reduced flows being observed. If the Udall and Overpeck conclusions don’t surprise him, he finds them “troubling.”

“The No. 1 thing is there is no return to normal,” he said. “Everything is going to be different in the future.”

A dam and reservoir on he headwaters of the Yampa River, a tributary of the Green and Colorado rivers.

Is storage the answer?

The study says nothing about new water storage projects. And when asked what the study says about the conditional water rights held by the city of Aspen for potential reservoirs on Castle and Maroon creeks, or about storage in other mountain towns, Udall declined to talk about new dams in general.

Kuhn said he sees the need for additional storage “but it will be specific to the location. The need for storage for lower-elevation demands will be greater than the higher-elevation demands.”

At the other end of the Colorado River, in places like Arizona’s Yuma Valley, growing seasons — and water demand — are expected to expand. The region produces 80 to 90 percent of the lettuce, broccoli and other produce found in grocery stores in the United States during the winter. But any additional reservoirs will also be vulnerable to increased evaporation due to rising temperatures.

A wall bleached, and stained, in Lake Powell. Credit: Brent Gardner-Smith/Aspen Journalism

Not like the past

During the 20th century, water managers and virtually everybody else presumed that past was prelude to the future. Droughts occurred, yes, and sometimes lasted for years, as occurred in the 1930s and then, more deeply yet in Western Colorado, during the 1950s. Again in 1976-77 and 1980-81, winters were time for rock skis.

Then things would get back to normal. But climate scientists point to a constant shift in temperatures and perhaps in precipitation as accumulated greenhouse gases drive changes that overlay natural variability.

“We’re going to be reacting to unforeseen (or unpredictable) conditions, not just more severe droughts,” says Kuhn. “It might be something like California is experiencing this year. As temperatures increase, the atmosphere carries more moisture and storms can be much more intense. It will be different.”

Some water agencies, including Denver Water, are trying to plot their futures, given the large uncertainties that remain, using a process called scenario planning. Scenario planning contemplates a wide array of futures, but also a great many options.

Assumptions of climate stability led to massive investments in water plumbing in the 20th century. These include Ruedi Reservoir, built to benefit the Western Slope for the federally financed Fryingpan-Arkansas Project’s diversions to farms along the Arkansas River. To the north, Green Mountain Reservoir was similarly built to benefit the Western Slope as part of the federally financed Colorado-Big Thompson Project.

Between are the tunnels, both at Winter Park and from Summit County, used by Denver Water to draw the water for use by residents of that city and many of its suburbs under the Continental Divide. Other cities, including Aurora and Colorado Springs, have other diversions—including Homestake Reservoir and Tunnel, located partially in the northeast corner of Pitkin County.

All these dams, reservoirs and water tunnels have one thing in common: They were constructed after 1922. That’s the year that the seven basin states struck an agreement about how to apportion what was then presumed to be 16.5 million acre-feet of average flows on the Colorado River. In fact, the river has mostly fallen short.

But with the projections by Udall and Overpeck of at least 20 percent declines in Colorado River flows by mid-century, all that infrastructure could be the equivalent of a dry dock. That’s true especially if deep, lingering drought overlays the new temperature-induced drought.

“It could mean a lot of dried up stuff,” says Udall, ticking off Ruedi, Green Mountain, Dillon, and other dams built in the 1930s and since to create reservoirs.

Silt walls in upper Lake Powell. Credit: Bernt Gardner-Smith/Aspen Journalism

Change the compact?

Those with pre-1922 compact water rights are more secure. Palisade peaches and Olathe corn look safer, and also a lot of hay pastures. Irrigators tend to have older, more senior rights. For that matter, Aspen has mostly very senior rights, too.

Here’s Udall’s thinking: The 1922 compact has somewhat vague language about the entitlements of water by Arizona, California, and Nevada to water from Colorado and other headwater states. Does that mean that, in the event of extended drought, that those with post-1922 water rights in Colorado must allow water to flow downstream to the deserts? This scenario is called a compact curtailment.

By one debatable interpretation of the Colorado River compact, Colorado and other upper-basin states would have half as much water as California, Arizona, and Nevada.

“That puts us into a gray area where we have no historical precedent for sharing a shortage with the lower basin,” says Udall. “There’s nothing in the compact that addresses that. We have no roadmap for how to do it.”

A rethinking of the Colorado River Compact and other agreements is needed, says Udall. Water management laws, agreements, and policies adopted over the last 100 years never expressly included risk management for climate change nor contained provisions for how to handle the relentless flow reductions that he and Overpeck foresee.

Editor’s note: Aspen Journalism and the Aspen Daily News are collaborating on the coverage of rivers and water. The Daily News published this story on Friday, Feb. 24, 2017.

Brent Gardner-Smith, the founder of Aspen Journalism, and who served as AJ’s executive director until August 2021 and as editor from 2011-2020, is the news director at Aspen Public Radio. He's also been...