The level of Lake Powell is currently at an elevation of 3,523 feet, 177 feet below capacity. The intakes that allow water through the dam to generate hydropower are at 3,490 feet.

Hydropower is useful in maintaining the stability of electrical grids in part because the amount of electricity generated can quickly be changed to help the grid match demand. In her letter, Ms. Trujillo said that if Powell reached 3,490 feet, “the western electrical grid would experience uncertain risk and instability.”

In addition, she wrote, water supplies to Western and Southwestern states “would be subject to increased operational uncertainty.” Water supplies to Page, Ariz., near the dam, and a nearby Native tribe, would especially be at risk, she wrote, because their intake is at about the same elevation as the hydropower intakes.

The dam itself would face “unprecedented reliability challenges,” Ms. Trujillo wrote, because with the hydropower intakes above the water level, the lake water would have to be routed through the dam using lower tunnels that were not designed for continuous use. “We are approaching operating conditions for which we have only very limited actual operating experience — and which occurred nearly 60 years ago,” she wrote.

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

“We’ve expended a lot of effort in producing plans” for what happens when the reservoirs fall to critical levels, Mr. Udall said. “And what we’re finding out, unfortunately, is that these plans are turning out to be completely inadequate. All of a sudden these new issues arise and haven’t having previously been considered and are really important.”

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