Gary Esslinger, manager of Elephant Butte Irrigation District in southern New Mexico, spends as much time moving silt as he does water. Elephant Butte Reservoir, built in 1915, is fed by the naturally muddy Rio Grande, which drains 28,000 square miles of easily eroded desert in two states. Sediment has claimed 600,000 acre-feet of its 2.6 million acre-foot capacity. "If I could create a bumper sticker," Esslinger says wryly, "it would say, 'Silt Happens.'"

The sediment clogs canals, pipelines and farm fields. It has filled 33 small flood control dams below Elephant Butte, effectively rendering them useless. The district -- which supplies some 7,900 farmers --  maintains a fleet of excavators, dozers and dumptrucks, but Esslinger is running out of places to move dirt. He encourages developers to haul it away for fill, but demand remains low. "It's just going to become a bigger and bigger problem as these (dams) get older," he says.

It's a global conundrum: Dams slow the natural run of water, and slow water drops sediment to the bottom of reservoirs -- eventually filling them. Yet the problem has received precious little attention over the years, and as a result, it's not well understood.

A High Country News analysis of U.S. Bureau of Reclamation data -- which offers the most recent, publicly accessible surveys for eight of 11 Western states -- reveals that 35 reservoirs have lost some 4.6 million acre-feet of storage capacity to sedimentation. That's about 8 percent of their total storage, or enough water to serve at least 9 million households.

The actual amount of storage lost is certainly greater. The surveys examined by HCN cover less than 10 percent of the dams managed by BuRec. Thousands more are overseen by other agencies, from small irrigation reservoirs to giant multipurpose reservoirs owned by state agencies. And many of BuRec's surveys are already two decades old.

"(This) will be, in the next 20 to 50 years, an extremely important topic," says Robert Baskin, a hydrologist at the U.S. Geological Survey's Utah Water Science Center. "The economic impact ... could be dramatic."

Engineers have been aware of the problem for centuries. A dam built in Spain in 1394 is still operating because it was built with a gate at its base so sediment can be flushed out. Some modern dams, including the giant Three Gorges Dam in China, incorporate similar systems. But American engineers, while ingenious at storing and moving water, essentially ignored sediment. They generally set aside a certain percentage of each reservoir's volume, usually below its outlet gates or hydroelectric works, for sediment storage -- a space often dubbed the reservoir's "dead pool." Once that space filled, the dam's life was over.

Utah is one of the few Western states that have even attempted to assess sedimentation. In a March 2010 report, the state's Department of Water Resources estimated that in 40 years, Utah's total storage capacity will have declined 25 percent. Its reservoirs lose about 12,340 acre-feet a year to sedimentation, yet the state needs about double that amount annually in additional supply to keep up with population growth. The reservoirs "cannot be considered renewable resources unless sedimentation is adequately addressed," the report states.

But Utah was able to find data for only 18 of its 133 reservoirs larger than 1,000 acre-feet. Nationally, the state of knowledge is equally poor. John Gray, a hydrologist at the U.S. Geological Survey in Reston, Va., manages the nation's only large database on reservoir sedimentation, which includes surveys of 1,824 large and small reservoirs across the country compiled by the Soil Conservation Service, now the Natural Resources Conservation Service.