The federal government is expected to announce on Monday that it will keep some water in one of the Colorado River's major reservoirs to help with water and electricity supplies.

The decision to keep more water in Lake Powell on the Arizona-Utah border, rather than releasing it downstream to the other major reservoir, Lake Mead near Las Vegas, comes as both are at record-low levels after 20 years of drought. After the Glen Canyon Dam was built in the 1960s, Powell held less than one-fourth of the amount it currently holds.

In a letter last month to officials of the seven states that receive water from the river, an assistant Interior Department secretary said there were concerns that Powell could decline further over the next 24 months. She wrote that it would cause problems for the water and electricity supply.

The proposal to keep more water in the lake was agreed to by the state officials in a letter two weeks later.

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The decision would keep half a million acres of water in Lake Powell. It is equivalent to an annual amount used by a million or more households. The bureau agreed last month to release half a million additional acre-feet into Powell from the Flaming Gorge upstream.

Without the additional water, the bureau estimated there was a one in four chance that the lake level would fall below 3,490 feet above sea level, at which point power could no longer be produced.

The expected decision was only a short-term fix, and work must be done quickly on ways to make the river sustainable over the long term, according to water policy experts.

Bart Miller, a program director with Western Resource Advocates, said his organization supported the bureau's decision.

Haley Paul, policy director at Audubon Arizona, said that the region needed to figure out how to share a river made smaller by climate change.

ImageA closed boat ramp extending into desert sand at Lake Powell.
A closed boat ramp extending into desert sand at Lake Powell.Credit...Caitlin Ochs/Reuters
A closed boat ramp extending into desert sand at Lake Powell.

The pending decision will cause the lake to fall even lower because it gets most of its water from Powell.

The Bureau of Reclamation declared a water shortage in August of last year because of the status of Lake Mead. The lake's supply was mostly cut to Arizona.

The bureau is evaluating the situation and could decide this summer if more cuts are needed. In response to a request from the officials in the seven states, the bureau is expected to say that it won't take into account the water held back from Mead in making its decision.

Global warming has made the decline of the two largest in the United States worse. The last two decades were the driest in 1,200 years.

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Over the past two years, soils have become so dry that much of the precipitation was absorbed before reaching the river, which has made the situation worse. The upper Colorado basin, which reaches Lake Powell, is projected to have only two-thirds of average precipitation this year.

The level of Lake Powell is 177 feet below capacity. The dam's water intakes are at 3,490 feet.

Hydropower is useful in maintaining the stability of grids because it can be quickly changed to match demand. The western electrical grid would be at risk if Powell reached 3,490 feet.

She wrote that water supplies to Western and Southwestern states would be at risk due to increased operational uncertainty.

The dam would face unprecedented reliability challenges because the water would have to be routed through tunnels that were not designed for continuous use.

Brad Udall, a senior climate scientist at Colorado State University, said that the concerns about the reliability of the power grid and the dam had not been raised in all the contingency planning over the past few decades.

We spent a lot of effort in producing plans for what will happen when the reservoirs fall to critical levels, Mr. Udall said. These new issues have not been considered before and are really important.