FOLSOM, Calif. — Drew Lessard stood on top of Folsom Dam and gazed at the Sierra Nevada, which in late spring usually gushes enough melting snow into the reservoir to provide water for a million people. But the mountains were bare, and the snowpack to date remains the lowest on measured record.

“If there’s no snowpack, there’s no water,” said Mr. Lessard, a regional manager for the Bureau of Reclamation, the federal agency that built and operates a vast network of 476 dams, 348 reservoirs and 8,116 miles of aqueducts across the Western United States.

For nearly a century, that network has captured water as it flows down from the region’s snowcapped mountains and moves to the farms, cities and suburbs that were built in the desert. But as the snow disappears, experts say the Bureau of Reclamation — created in 1902 by President Theodore Roosevelt to wrest control of water in the arid West — must completely rebuild a 20th-century infrastructure so that it can efficiently conserve and distribute water in a 21st-century warming world.

“The bureau is headed into a frightening new world, an uncertain new world,” said Jeffrey Mount, an expert on water resource management with the Public Policy Institute of California.