This 2015 image shows low water levels in Lake Mead behind Hoover Dam with a “bathtub ring” marking once-higher levels.

Irfan Khan / Los Angeles Times 2015

The agency that runs the $4 billion Central Arizona Project is being accused of manipulating Colorado River reservoirs’ operations to suck out more water for its Tucson, Phoenix and Pinal County customers.

The accusation came in two letters in the past few days from representatives of four Upper Colorado River Basin states, the federal government and the Denver Water Dept. They say CAP’s approach threatens a Western water supply serving nearly 40 million people. It also threatens the harmony that has marked relations among the seven basin states since they approved guidelines to run the Colorado River’s reservoirs in 2007, they say.

Under criticism is CAP’s practice of limiting how much river water it conserves each year, in order to prop up Lake Mead’s declining reservoir levels. The CAP has resisted pressure from other water agencies in Arizona to boost its conservation beyond about 200,000 acre-feet a year, enough to cover that many football fields a foot deep.

CAP says that’s because as Lakes Mead and Powell are managed under the 2007 guidelines, conserving too much, or “overconserving” as CAP officials put it in the past, could reduce water releases from Powell to Mead. That would trigger shortages and cutbacks in water deliveries to Arizona users. CAP brings drinking water to Tucson and Phoenix and irrigation water to Pinal County via a 336-mile-long canal.

That stance irks the Upper Colorado commission, representing the Upper Basin states of Colorado, Utah, New Mexico and Wyoming and the U.S. government.

Last Friday, commissioners wrote that the Central Arizona Water Conservation District — a three-county water district running CAP — “intends to disregard the basin’s dire situation at the expense of Lake Powell and the other basin states.” CAP is trying to “maximize demands” to get larger water releases from Powell, said the letter to Arizona Department of Water Resources Director Tom Buschatzke.

Officials of the CAP water district responded in a statement, “We are surprised and disappointed to have received a letter from the Upper Colorado River Commission questioning CAWCD’s intentions in leaving water in Lake Mead.”

"We have been reaching out to our partners in the Upper Basin, hoping to clarify apparent misunderstandings, and to facilitate in-person, collaborative discussions aimed at finding solutions" to help communities and the environment that the river serves, the statement said.

On Twitter, CAP general manager Ted Cooke recently said the agency places its water order wisely, following federal guidelines.

“Not my definition of ‘manipulation,’” Cooke tweeted.

Since 2014, CAP and its partners reduced water use enough to be able to leave more than 850,000 acre-feet in Mead, the statement said.

“These contributions (to conservation) have come at a significant cost to CAP water users in terms of water and water rates,” the statement said.

Officials of the two basins have always had a potentially tense relationship because the river is over-allocated, and because when one basin uses too much water, the other gets less. The lower basin is made up of Arizona, Nevada and California.

The problem is aggravated by the fact that Lower Basin users have run up a major structural deficit in their use versus their supply, while the Upper Basin still isn’t making use of all its water rights. Because of the 2007 guidelines and continued efforts at collaboration, however, the tensions haven’t erupted into open conflict.

But that harmony may not last, thanks to drought and climate change.

In its letter, the Upper Colorado commission noted that because of a high water release expected from Powell this year and continued low snowpack and poor river runoff, Powell is expected to drop 30 feet in the next year. If these conditions persist, CAP’s efforts to boost water releases from Powell could make future reservoir conditions worse and trigger more severe shortages in the long term, the letter said.

“CAWCD’s statements run contrary to the spirit of interstate comity and goodwill” that led to the 2007 guidelines, the commission letter said.

The letters were triggered by a graphic recently posted on CAP’s website, saying the agency has maintained a “sweet spot” for Lake Mead’s water levels.

By that, it means conservation has kept Lake Mead high enough to avoid a shortage, but not so high as to cause the federal government to release only 8.23 million acre-feet of water each year — the customary average annual delivery from Powell to Mead. Instead, the feds have released 9 million acre-feet each of the past four years.

The graphic, which the agency took down after it generated controversy, made Lake Mead’s level appear to be a bigger factor in determining water releases than the weather, which others disagree with.

The Upper Colorado commission and Denver Water are also concerned that this conflict threatens an interstate program in which the feds, Lower Basin water agencies and Denver Water pay farmers and other users to use less water, with the savings held in Mead.

This program has saved about 139,000 acre-feet of river water. But Denver Water is prepared to end its support of the conservation program unless, among other things, CAP can show “it has ceased all actions to manipulate demands and is fully participating in aggressive conservation measures,” Denver Water chief Jim Lochhead wrote to the CAP.

"CAP's manipulation of demands in order to take advantage of the supposed 'sweet spot' in Lake Powell water releases undermines this purpose," Lochhead wrote Monday.

Arizona water director Buschatzke said he’s extremely concerned about these two letters. They show the Upper Basin sees CAP’s actions as potentially “blowing up” collaboration among the states, said Buschatzke, who has gone toe-to-toe with CAP in the past year over the conservation issue.

He noted the commissioners’ statement that this controversy may require consultations among all seven basin states’ governors or their representatives. Such a move could lead to the federal government stepping in to solve the problem, or to lawsuits, he said.

Paul Orme, an attorney for four irrigation districts in Central Arizona, said he continues to support CAP. Farmers will be the first to lose water during a shortage and they’re more interested in year-to-year releases, Orme said.

“What they are doing is permitted under the (2007) guidelines,” Orme said, referring to the CAP. “I know the Upper Basin says they’re not in the spirit of the guidelines, but they’re in the letter of the guidelines.”

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