With long-term severe drought continuing to take a toll on the Colorado River, the federal government announced on Monday that it will retain some water in one of the river’s major reservoirs, describing it as an extraordinary action to temporarily stave off increased uncertainty in water and electricity supplies in the West.

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 made worse by climate change. Powell, behind Glen Canyon Dam, currently holds less than one-fourth of the amount it held when it filled after the dam was built in the 1960s.

“We have never taken this step before in the Colorado River basin,” said Tanya Trujillo, an Interior Department assistant secretary. “But the conditions we see today, and the potential risks we see on the horizon, demand that we take prompt action.”

Together with the release of more water into Lake Powell from an upstream dam, the decision will keep the lake at a level at which it can continue generating hydropower for the next 12 months. Beyond that, Ms. Trujillo said, the situation will be re-evaluated.

Loss of hydropower production would create a number of problems for the water and electricity supply, and for the dam itself.

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All told, the actions will result in about a million acre-feet of additional water in Lake Powell. That is about 15 percent of the lake’s current volume and is equivalent to the annual amount used by 2 million or more households.

Ms. Trujillo acknowledged that the actions were a short-term fix, and that work was needed on solutions to make the river, which supplies water to 40 million people, sustainable over the long term.

Conservation groups and water policy experts agreed.

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