Fourteen years of drought in the West and a revised rule book on allocating water along the Colorado River have prompted the US Bureau of Reclamation to make the deepest cut in water released from Lake Powell in the reservoir's 46-year history.

Lake Powell is the second largest engineered reservoir in the United States by capacity, bested only by Lake Mead, more than 200 miles downstream.

The bureau formally announced the cut Friday. The amount of water that the bureau will release from the lake starting Oct. 1 will be some 10 percent less than the amount released in the prior 12 months – some 7.48 million acre-feet of water during the 2014 water year, compared with 8.23 million acre-feet during the current water year, which ends Sept. 30.

Retaining more water in Lake Powell reduces the amount of electricity that generators at the Glen Canyon Dam can produce. But it also means that water managers upstream can keep more water on hand for cities and farms than they otherwise might.

While the move also cuts the amount of water flowing into Lake Mead, a crucial source of supply for Nevada, California, and Arizona, the lake will still be able to supply the water that users have ordered for 2014, the bureau notes. And Mexico will still get its treaty-based share.

What happens beyond 2014 is less clear. The bureau forecasts little likelihood that water-delivery shortages will hit Lake Mead users in 2015. But it also forecasts a significant chance of delivery shortages there in 2016, depending on what the snowpack is like in the mountains during the preceding winters. Winter snows, and the length of time they linger into summer, are the West's ultimate reservoir.

While the move will be hard, if not impossible, for people to detect at the faucet, it still carries symbolic importance, some water-policy specialists say.

"We're in an unparalleled – at least in recorded history – period of droughts on the Colorado River system," says Michael Cohen, a senior research associate who specializes in Colorado River water issues at the Pacific Institute in Oakland, Calif.

"It seems that this is the new normal," he says, noting that climate scientists studying the potential effects of global warming have been predicting these conditions in the region for more than a decade.

Indeed, last December, the Bureau of Reclamation released a study looking at the future of supply and demand for Colorado River water. Between the effect of climate change on the region's hydrology as well as projected population trends, the study estimated that demand would outstrip supply by about 3.2 million acre-feet of water by 2060.

The bureau's move "is truly historic," adds Taylor Hawes, director of the Nature Conservancy's Colorado River Program. "The bureau has never released less than 7.5 million acre-feet" of water since the Glen Canyon Dam, which formed Lake Powell, was built.

The bureau took the action under an interim allocation agreement hammered out in 2007 between states in the upper Colorado River Basin and the lower basin. The upper-basin states include Wyoming, Colorado, Utah, and New Mexico. The lower-basin states include Arizona, Nevada, and California.

The decision to trim the flow from Lake Powell is the bureau's response to low flows into the lake during the past two years – "the worst two years on record," says Terry Fulp, the Bureau of Reclamation's regional director for the lower Colorado River Basin.

Flows for the past two months give a sense of the magnitude of the problem. Data from the National Oceanic and Atmospheric Administration's Colorado Basin River Forecast Center in Salt Lake City show that flows in June were 35 percent below average, while July's flows were 13 percent of normal. August looks a bit better, with stream flows projected at 32 percent below normal.

Under the 2007 allocation agreement, the bureau tries to strike a balance in water levels between Lake Mead and Lake Powell, Dr. Fulp explains.

"We try to get a better balance during high-flow years between the two basins," he says. "We're trying to balance the pain of low-flow years between the two basins."

Although Lake Mead is drawing the short straw this time around, it received about 800,000 additional acre-feet of water from Lake Powell last year, Mr. Cohen adds. This was preceded by a bigger release from Powell to Mead in 2008, he adds.

"So in some ways, it's a wash," he says.

Even so, the cost of Friday's announcement to Lake Mead is a waterline eight feet lower than it is today – a falling level that Las Vegas is desperately trying to beat as it installs a new intake system at the lake to provide water for the city.

If current precipitation trends continue, falling levels at Lake Mead could begin to trigger a stepwise series of reduced deliveries.

For instance, when the lake level drops to 1,075 feet above sea level, Arizona faces an 11 percent cut, Nevada a 4 percent cut, and Mexico a 3.8 percent cut. California would still be entitled to its full allocation – a subject of intense interest at a water-resources meeting that ends Friday in Boulder, Colo.

In July, the surface was 1,106 feet. Based on that level, an eight-foot drop would bring the lake roughly 25 percent closer to the 1,075-foot level, the first of three triggers.

The region has been bracing for these changes for several years, Cohen notes. Arizona and Nevada, for instance, have been taking the biblical approach – taking water surpluses in good years and banking them in aquifers underground against the lean years.

Also, cities have been adopting ever-stricter landscaping regulations aimed at reducing the amount of water lost to evaporation.

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The supply-demand study that the bureau released in December was a call to action for more-aggressive efforts to improve water-use efficiency and for other approaches to adapting to a future in which climate and populations continue to put the squeeze on water resources, Ms. Hawes says.

But looking at the bureau's historic reduction in water to be released from Lake Powell, she adds, "Nobody expected it to happen so quickly."