The Colorado gets most of the water from the snowmelt, and the situation has worsened in the past two years as the soil has become so dry that much of the runoff was absorbed before it reached the river. This year, runoff in Colorado’s upper watershed, reaching Lake Powell, is expected to be only about two-thirds of the average.

Lake Powell’s level is currently at an elevation of 3,523 feet, 177 feet below capacity. The inlets that allow water through the dam to generate hydroelectric power are located at 3,490 feet.

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

In addition, she wrote, water supplies to western and southwestern states “would be subject to greater operational uncertainty.” Water supplies to Page, Ariz., near the dam, and a nearby native tribe, would be especially at risk, she wrote, because their intake is about the same as hydropower intake.

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

Brad Udall, a senior climate scientist at Colorado State University, said the letter’s concerns about grid and dam reliability hadn’t really surfaced in all of the drought emergency plans over the decades.

“We’ve put a lot of effort into making plans” for what will happen if the reservoirs fall to critical levels, Mr. udal. β€œAnd what we are unfortunately discovering is that these plans are proving to be totally inadequate. All of a sudden, these new issues come up and they haven’t been considered before and they’re really important.”

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