If the federal government goes through with its proposal to cut Colorado River releases from Lake Powell, water users in Arizona, California and Nevada won’t feel it this year β€” but Lake Mead will.

Due to what some observers call an accounting trick, the reduced releases from Lake Powell wouldn’t translate into immediate cuts or deeper water shortages for the three Lower Basin states.

Instead, the Interior Department’s plan would lower the already depleted Lake Mead to prop up the even more depleted Lake Powell. That’s so Glen Canyon Dam can keep generating electricity and avoid possible collapse of its concrete walls or other damage to its operations that could be caused by Lake Powell’s water level dropping too low.

It will be a different story for Arizona and other Lower Basin water users, however, if conditions stay bad enough on the Colorado River so that this cut has to be repeated next year and the years afterward.

Then, if Lake Mead continues to fall faster than it otherwise would have fallen, it could speed the pace at which it drops to β€œdead pool” β€” where no water could be removed β€” or to a higher but still dismal level where Hoover Dam could no longer generate power, said Sarah Porter, a high-level ASU water researcher.

But many observers agree that with the river’s chronically declining flows, the Interior Department may have no choice but to take such drastic action at least for this year.

Specifically, the department’s proposal would hold at Lake Powell 480,000 acre-feet of water this year that it would have otherwise released to Lake Mead.

That wouldn’t translate into additional, immediate cuts in deliveries to the 336-mile-long Central Arizona Project canal that delivers drinking water to the Tucson and Phoenix areas. The CAP is already suffering its first involuntary cuts this year, under previously approved plans to respond to river water shortages caused by drought and climate change.

The goal is to make the immediate impact on water users here β€œoperationally neutral,” said Tanya Trujillo, an assistant interior secretary whose April 8 letter conveyed the proposal to water officials in all seven river basin states, including the Upper Basin states of Utah, New Mexico, Colorado and Wyoming.

The department’s purpose would be to keep Powell above 3,490 feet. If the lake level falls below that, Glen Canyon Dam’s eight turbines couldn’t generate electricity β€” power that serves 5 million customers in Arizona and five other Western states.

β€œWe’re faced with a situation where we want to take swift, responsive action with respect to Glen Canyon Dam protection (but) we don’t want to do that in a way that will adversely impact the lower basin contractors,” Trujillo said Tuesday in an interview with the Star.

But as many other water officials and experts acknowledged, if our taps and lettuce and alfalfa fields are spared from this year’s cut, Lake Mead won’t be so lucky. A loss of 480,000 acre-feet could push the lake down another 7 or 8 feet, based on past federal estimates. Mead is now 32% full while Powell is 24% full.

If the river’s condition remains poor and Interior’s Bureau of Reclamation makes a similar cut in releases to Lake Mead next year and beyond, Mead will drop lower and lower. Trujillo declined to speculate on whether that will happen, but just the prospect of it draws concerns if not shock from other water officials, outside experts and advocates.

β€œIf we make these kind of adjustments we are putting Mead into peril faster,” said Porter, director of Arizona State University’s Kyl Center for Water Policy. β€œWe are in a situation where we don’t have a choice this year, but we are worried about Lake Mead getting to a point where it may not be feasible to generate hydropower or get water out of it.”

Guidelines that basin states and the feds approved in 2007 to operate the reservoirs, and the 2019 drought contingency plan, were aimed at preventing that from happening, Porter said. This proposal would reduce and, if continued in the future, remove that protection against those two eventualities, she said.

Beyond that, if such delivery cuts from Powell are needed, β€œThe problem is that if it’s every year, there have to be substantial water use reductions throughout the basin,” said John Fleck, author of two books on the river and former head of the University of New Mexico’s Water Resources Research Program.

β€œYou can’t solve this problem in future years by moving this water from one reservoir to another. Eventually we run out of water to use that way,” Fleck said.

Added Sharon Megdal, director of the University of Arizona’s Water Resources Research Center, β€œI think it’s a shock to the system to hear that so much water would be held back that’s expected to flow into Lake Mead. It’s another indication of the sorry state of the basin.”

Trujillo’s letter is obviously concerning and eye-opening about how serious the river’s situation is, said Mark Taylor, a CAP board member from Tucson.

β€œEvery time we have some poor hydrology, it has a greater and greater effect on the system as a whole,” Taylor said. β€œAs they get lower and lower water levels, every time this happens it has more serious consequences and they have to take drastic action like this.”

β€œTrying to be responsible”

As of Friday, Lake Mead stood at about 1,058 feet above sea level. That’s roughly halfway between where the lake would be if it were full β€” 1,220 feet β€” and where it would be at β€œdead pool” β€” 895 feet. At the latter point, most of its water could not be removed for human use.

Interior’s midyear cutback proposal would, first, keep Mead from getting 350,000 acre-feet that Interior’s Bureau of Reclamation had already held in Powell from January through April in hopes of keeping Powell above 3,525 feet. That’s the target the bureau set to maintain a buffer to keep the lake above 3,490. The idea was to release that water over the summer, after melting snows brought the lake back up.

But Powell fell farther than expected, to about 3,522 feet as of Friday. Now, under Interior’s current plan, the feds would hold back that water plus another 130,000 acre-feet it would have released by Sept. 30. Trujillo told the Star she hoped to make a decision on it β€œwithin a month or so.”

As for next year, she said, β€œThe only operations we are discussing right now are for this current year. We don’t know what the circumstances will be for the next operational year.

β€œWe’re really trying to be proactive. We’re trying to be responsible. We are trying to be reactive to what we’re seeing hydrologically and trying to have an inclusive discussion regarding these issues because so many people are affected by the resources in the Colorado River Basin,” she said.

About 40 million people use the river’s water for drinking, irrigation and recreation.

The prospect of Powell falling below 3,490 feet has triggered concerns about how the Western power grid would respond if the dam’s turbines shut down, and about whether the dam’s β€œjet” tubes that would pass water onto the Lower Basin could carry as much water as now passes through the turbines.

The major concern is that there’s uncertainty about how the dam’s infrastructure will respond if Powell falls below 3,490, Trujillo said.

β€œOur operators have not seen these conditions before. They haven’t operated the system using those jet tubes for any extended period,” she said. β€œThey don’t know exactly how the infrastructure will respond.”

A commonly expressed fear is that as the lake approaches and then drops below 3,490, cavitation will occur, in which air bubbles or vapor pockets will form and collapse, leading to possible tearing apart of some of the dam’s concrete walls.

Speaking Friday at a Denver University water law conference, Trujillo observed: β€œWhen engineers and dam managers with 40 to 45 years of experience say we’ve never seen these conditions before, and we’re concerned about cavitation at the dam, when we don’t know what will happen if we try this or that operation, it’s daunting.”

β€œA jolt to the system”

This year, the three Lower Colorado River Basin states are in what’s known as a Tier 1 shortage, reducing deliveries to the CAP by more than 500,000 acre-feet.

This shortage affects mainly Central Arizona farmers and some suburban deliveries to replenish suburban aquifers depleted by pumping. It was triggered by continued declines in river flows since 2000.

Now, even without Interior’s proposed new cut, Lake Mead is forecast by the bureau to fall to 1,047 feet by the end of 2022. That puts it below 1,050 feet, the threshold that typically requires a second-tier shortage, leading to CAP cuts to numerous Phoenix-area cities and to Indian tribes. It’s officially known as a Tier 2A shortage.

Trujillo’s proposal would almost certainly lower the lake below 1,045 feet. Under the 2019 drought contingency plan, that would normally set off a deeper shortage still, one called a Tier 2B shortage. That tier would bring deeper cuts to those Phoenix-area cities and the tribes than a Tier 2A shortage. It also, for the first time, would have required cuts in deliveries to California.

But now under Trujillo’s proposal, the Lower Basin wouldn’t sink into a shortage any worse than would already happen without the new, 480,000-acre-foot cut, according to a statement to the Star from the CAP and the Arizona Department of Water Resources. So there would be no Tier 2B shortage and no water cuts to California.

In another statement, Southern California’s Metropolitan Water District, which sells water wholesale to agencies in six counties, said it supports Interior’s proposal, is working to minimize its Southern California impacts, and is committed to working collaboratively with other states and the federal government to address the river’s short- and long-term problems. Its comments came in a statement from Adel Hagekhalil, the district's general manager.

The CAP and ADWR said in their statement that all basin states are working on a β€œcoordinated response” to Trujillo’s letter.

But to ASU’s Porter, Interior’s β€œaccounting trick” will also end up costing Arizona cities more in the long run.

That’s because in the earlier, less drastic shortage tiers, more money will have to be paid to local governments and tribes to compensate for the cuts they have to take.

As the shortages escalate, the amount of money paid in compensation diminishes and ultimately goes away. That money will come at least in part from property taxes and water rates paid by CAP customers, Porter said.

Also, if the Lower Basin were to go more quickly to the worst possible shortage category, known as Tier 3, β€œthe cities would arguably be in better shape, water supply wise,” because that would leave more water in Mead, she said. A Tier 3 shortage is called if Mead falls below 1,025 feet.

β€œInstead, it’s like an emergency measure we’re doing. It’s done to get the states to agree to it,” Porter said.

The UA’s Megdal asked how Interior’s proposal will mesh with ongoing efforts by Lower Basin states, started last fall, to save 500,000 acre-feet of water this year by paying farmers and cities to use less water as a way of propping up Mead.

β€œPeople are trying to keep the level of Lake Mead from falling below 1,030 or 1,025 feet. Then, something else is coming and dropping it down,” Megdal said. β€œHow will people feel about renewing commitments to those agreements? This is a very interrelated system. People are making decisions on what they think is going to happen.

β€œNow, this is a jolt to the system. And do we know if next year will be any better?” Megdal asked.

At a crossroads

Looking ahead, β€œI think the (entire) basin is in a very serious and critical crossroads,” due to low reservoirs, said Colby Pellegrino, deputy general manager of the Southern Nevada Water Authority.

β€œCollectively, we’re going to have to figure out how to manage the system. That’s going to take us needing some operational certainty, tracking the river’s hydrology really closely and really throwing everything we can at using less water.”

The Nevada authority is a wholesale agency that delivers water to seven Las Vegas-area agencies serving 2.2 million people. Pellegrino said that since Nevada gets 1.8% of the river’s entire supply, the best thing the state can do is β€œtrailblaze and show there’s a lot we can achieve through conservation.”

β€œBut the math is daunting. When you look at current flows and climate change projections the reason we’re declining is that we’re using more than Mother Nature is providing us. That is not going to change any time soon. We have to do more and we have to do it now.”

With the Interior Department taking unprecedented action to protect Glen Canyon Dam’s infrastructure, β€œI think it’s clear that continued drought and climate change impacts on the Colorado River are pushing operations to the brink,” said Jennifer Pitt, director of the National Audubon Society’s Colorado River program.

β€œIt’s clearly important what they’re doing, but I guess I would pause there and say that it’s clear those actions are not sufficient for the long run. The way we understand climate change is that the conditions we are experiencing now may be some of the best conditions we experience in the coming decades,” Pitt said.

β€œWe need to have everyone take care of the problem or we’re at risk of dire consequences,” Pitt said. β€œWho wants to tell water users there is going to be a shortage? Will that political will be caused ... when they finally can appreciate that not making the decisions is worse than actually making decisions to manage the water supply?”

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Contact Tony Davis at 520-349-0350 or tdavis@tucson.com. Follow Davis on Twitter@tonydavis987.