Most of the surveys are much older than BuRec's and haven't been updated in at least two decades; many were completed using crude techniques -- measuring reservoir depth with weighted rope, or noting how much piano wire sank to the bottom from a boat crossing a known transect. As a result, it's hard to know for sure how rapidly the nation's reservoirs are filling, or how full they are. "We're just not even the tip of the iceberg here," Gray says. "We should be ... getting a lot more data on this, and where there are problems, start alerting those locations. There's time to address this."

The U.S. Army Corps of Engineers provided funding last year to help move the data to a more modern software platform. The data are available for public download, but are not searchable online. Gray hopes to obtain $1 million over four years to fix that, and to allow new surveys to be easily uploaded. But it's unclear where the money will come from, given the current budget crisis.

Meanwhile, BuRec has been plodding away making new surveys, says Ronald Ferrari, a Denver-based BuRec hydraulic engineer who oversees the agency's sediment survey program, though many have not been added to the USGS database. Ferrari spends two to three months every year visiting reservoirs with a specially equipped boat, using GPS, multibeam sonar and lidar to render detailed profiles of their bottoms. It once took a team of six people five or six months to survey a massive reservoir like Lake Powell, on the Colorado River. Now, two people can do it in two or three weeks, and they get far better data. Even so, the agency tackles just four or five a year. "We'd like to have more of them completed," Ferrari says, "but it's definitely dictated by the allowable budgets."

Southwestern reservoirs are generally surveyed more often because of known higher sedimentation rates in that region. The last full survey at Powell, however, was in 1986. No new survey is planned, Ferrari says, because its sedimentation rate is considered low and because of budget constraints.

How quickly reservoirs fill, and where, varies widely, but all are shrinking every day. Soil type and local geography and land use all play roles. And unexpected events, such as wildfire in the watershed, can have huge consequences.

Elephant Butte in New Mexico is an extreme example, with a 23 percent decline in capacity. Most others surveyed by HCN are filling more slowly; McKay Reservoir near Pendleton, Ore., for example, has lost 2.6 percent of its capacity over 90 years. Yet even at slow rates, problems can emerge early, especially when exacerbated by drought or other factors.

Southern Montana's Bighorn Lake has lost 3.9 percent of its capacity over 50 years. The long, narrow reservoir stretches across the state line into Wyoming. For half of every year, cliffs and plateaus of sediment maroon a popular boat launch and marina in Lovell, Wyo. "We no longer have a lake," says Bob Croft, president of Friends of Bighorn Lake. "We have a river channel that has 18-foot mud walls on it when it goes empty."

The problem is aggravated by water releases at Yellow Tail Dam -- 30 miles north -- which serve a trout fishery below the dam. This creates dramatic water-level changes around Lovell, which in turn cause sediment to settle out of the water there, rather than dispersing widely on the reservoir floor.

Most large reservoirs have centuries to go, but small ones may have only decades, and many have already filled completely. On the Stanford University campus in California, sediment has reached the brim of Searsville Dam. Meanwhile, the Matilija Dam on California's Ventura River, which was built in 1948 for water supply and flood control, now provides neither. A single storm in 1969 filled 27 percent of the reservoir with sand and mud; it's currently 90 percent filled with sediment. There is agreement to remove the dam, but finding a place to move the accumulated muck remains an obstacle.

"Every reservoir is headed towards that condition, just at different rates," says Baskin. A variety of solutions are possible; none are cheap. Sedimentation can be reduced by improved land management upstream to minimize erosion. Mechanical fixes are also available, from dredging to adding diversion structures upstream to separate and transport sediment elsewhere. Dams can also be retrofitted with new gates so sediment can be flushed out. At small reservoirs, dams are often raised, recovering lost capacity, but only for a short time.

At Otter Creek Reservoir on Utah's Sevier River, officials are attempting a multifaceted fix. Built in 1897, the reservoir had lost about 22 percent of capacity after a century. In 1999, the state helped Otter Creek Reservoir Company pay for $4 million in dam improvements, including new spillway gates to raise the water elevation 2 feet to compensate for lost capacity. At the same time, state and federal agencies are working to restore the watershed, which was degraded by livestock grazing. A 2005 fire, followed by rainstorms, delivered a fresh shot of sediment that may have reversed some gains. But the effort continues.

"We've been trying to get people to realize there may be things they could do now to prevent a major issue later down the road," says Ferrari. "Eventually, somebody's going to have to deal with it."

Matt Weiser writes about water and the environment for the Sacramento Bee.