The water level on Lake Powell, shown in 2013, dropped by about 100 feet from its high mark. That distance is indicated by the white marks on the canyon wall, often likened to a bathtub ring.

Federal management of the Colorado River’s reservoirs is draining Lake Powell while keeping Lake Mead propped up out of shortage territory, says a team of scientists studying the river.

Since 2015, Lake Mead — the source of Central Arizona Project water serving Tucson and Phoenix — has typically finished each year barely above the level where CAP cutbacks would be required.

A key reason shortages were avoided is that for four straight years, federal officials sent the embattled lake an above-normal release of water from Lake Powell, the giant reservoir at the Utah border that’s separated from Mead by the Grand Canyon. The releases have been 9 million acre-feet, compared to normal annual releases of 8.23 million acre-feet.

These releases, welcomed by some Arizona users as “bonus water,” have worked with conservation efforts to prevent shortages. Otherwise, Central Arizona farmers would almost certainly have already suffered CAP cutbacks. Another 9 million acre-foot release is likely for 2019, federal officials say.

In their new report, the scientists studying the Colorado warn that the continued extra water releases threaten to lower Powell to the point where its operations will be jeopardized.

“This is not all good news, and is not evidence of successful crisis management,” the report says of Lake Mead’s continued narrow escapes from shortages. “The reality of the situation is that the dominoes have already begun to tumble, and the proof lies upstream in Lake Powell.”

Lake Powell cannot rescue Lake Mead forever, said Karl Flessa, a University of Arizona geosciences professor who worked on the report.

“Those of us here in the Lower Basin are so focused on Lake Mead, but what’s propping up Mead is Lake Powell, and Lake Powell is going down, too,” Flessa said.

A total of about 11 million acre-feet of the extra water has been sent from Powell to Mead since 2000, the report says. That’s more than seven years’ worth of CAP water. Powell has dropped 94 feet since 2000. Had all that water stayed in Powell, that lake wouldn’t have dropped at all since 2000, the report says.

“The math suggests that, without these extra releases, we could today have a full Lake Powell and an empty Lake Mead. We are certainly not saying that would be a ‘better’ outcome; that would be chaos,” said another researcher involved in the report, Douglas Kenney, director of the University of Colorado’s Western Water Policy Program.

“What we are saying is that it’s important to understand that the actions taken to keep Lake Mead out of shortage have had real impacts upstream at Lake Powell, and all users dependent upon Lake Powell now face risks associated with looming Lake Powell shortages.”

The scientists say “the status quo (of reservoir management) is untenable,” and that a crisis on Lake Powell may already be at hand.

“It is impossible to keep a bathtub full while the drain is left open,” the report says.

Electricity generation at risk

If the river’s operations continue like this, Lake Powell will drain further, eroding the lake’s ability to generate electricity, the report says.

Also threatened is “the delicate interbasin truce” of the river’s seven states that was made possible by the two massive reservoirs, the researchers say.

Besides generating power, Lake Powell serves as a water bank repository, storing water so it can be released when needed for the Upper Basin to meet its legal requirements for delivering adequate supplies to the Lower Basin, as required under the 1922 Colorado River Compact. The river’s Lower Basin states are Arizona, California and Nevada; the Upper Basin’s are Colorado, Utah, Wyoming and New Mexico.

The report comes from a team of 10 researchers, including Flessa, that calls itself the Colorado River Research Group. Its stated goal is to “provide a nonpartisan, basin-wide perspective on matters pertaining to the Colorado River.”

Three of the 10 researchers, including Flessa and UA economics professor Bonnie Colby, work in the Lower Basin. Six, including Kenney, work in the Upper Basin. The 10th, former UA climate scientist Jonathan Overpeck, is now dean of the University of Michigan’s School for Environment and Sustainability.

In part, their new report blames Powell’s problems on federal guidelines, approved by the seven basin states, under which the river has been managed since 2007.

The guidelines seek to balance water levels in the two reservoirs to provide maximum benefit for people living in both basins. But they are triggering the extra water releases that the report says threaten Lake Powell.

The river’s continuing structural deficit “is the true villain in this story,” says the report. That deficit is caused by the three Lower Basin states taking more water out of the river than nature provides each year. The deficit is estimated at about 1.2 million acre-feet a year.

The deficit is seen as the key cause of Lake Mead’s continuing declines. At the end of this year, Mead is expected to be 1,080 feet above sea level, five feet above where a shortage is declared. There’s a 57 percent chance of a 2020 shortage at Mead, the Bureau of Reclamation predicted.

But, the report says, “To view the structural deficit as a Lower Basin and/or a Lake Mead problem is ... much too simplistic; it is central to all the basin’s water supply woes.”

Powell is expected to drop to 3,587 feet by the end of 2018, the report says. At 3,525 feet, the lake’s power deliveries can be jeopardized.

The Upper Basin states take 4.5 million acre-feet, or less than two-thirds of the 7.5 million they’re legally entitled to take from the river each year, the report says. The Lower Basin states have diverted a far greater slice of their annual 7.5 million acre-foot share, although last year’s diversions were the lowest in 25 years. Another 1.5 million acre-feet of river water goes to Mexico annually.

The Bureau of Reclamation, which operates the two reservoirs, defended the management system, saying the two dams are operated jointly to meet the water needs of both basins.

The Central Arizona Project, whose water supply has benefited from the extra Powell releases, said through a spokeswoman, “CAP is not going to comment on the report at this time.”

The Arizona Department of Water Resources, which seeks to protect Arizona’s Colorado River supply, also would not comment on the report.

The past four years of 9 million acre-foot releases from Powell helped to balance the contents of the two reservoirs, the Bureau of Reclamation told the Star.

“Without the storage in both Lake Powell and Lake Mead, the basin would not have been able to withstand this long into the ongoing drought,” the bureau said.

“Maintaining their operation, coupled with basin-wide efforts like a completed drought contingency plan, is crucial,” the bureau said, “to continued reliable and consistent water for the 40 million people who rely on the Colorado River.”

Contact reporter Tony Davis at tdavis@tucson.com or 806-7746. On Twitter@tonydavis987