“We’re trying to manage a changing climate, its impact on water supplies and our ability to generate power, all at once,” said Michael L. Connor, commissioner of the Bureau of Reclamation, the Interior Department’s water-management agency. Producing electricity accounts for at least 40 percent of water use in the United States.

Warmer and drier summers mean less water is available to cool nuclear and fossil-fuel power plants. The Millstone nuclear plant in Waterford, Conn., had to shut down one of its reactors in mid-August because the water it drew from the Long Island Sound was too warm to cool critical equipment outside the core. A twin-unit nuclear plant in Braidwood, Ill., needed to get special permission to continue operating this summer because the temperature in its cooling-water pond rose to 102 degrees, four degrees above its normal limit; another Midwestern plant stopped operating temporarily because its water-intake pipes ended up on dry ground from the prolonged drought.

Scott Burnell, a spokesman for the Nuclear Regulatory Commission, said the safety of America’s nuclear plants “is not in jeopardy,” because the sources of water cooling the core are self-contained and might have to shut down in some instances if water is either too warm or unavailable.

“If water levels dropped to the point where you can’t draw water into the condenser, you’d have to shut down the plant,” he said.The commission’s new chairman, Allison Macfarlane, has asked her staff to look at “a broad array of natural events that could affect nuclear plant operations” in the future, such as climate change, Burnell added.

For more than three-quarters of a century, the Hoover Dam has represented an engineering triumph, harnessing the power of the mighty Colorado River to generate electricity for customers in not just nearby Las Vegas but as far away as Southern California and Mexico.

But the bleached volcanic rock ringing Black Canyon above Lake Mead, the reservoir created by the dam, speaks to the limits of human engineering. Higher temperatures and less snowpack have reduced the river’s flow and left the reservoir 103 feet below elevation for its full targeted storage capacity, which it last came close to reaching in 1999.

In the Colorado River’s 100-year recorded history, 1999 through 2010 ranks as the second-driest 12-year period, yielding an average of 16 percent less energy.

Scientists have just begun to study some key questions, such as the rate of evaporation off dams’ storage facilities. Predicting river flows — which can flood one year and dry up the next — is even harder.