Soon after I moved to Colorado from the humid Midwest 20 years ago, I learned that a reservoir is not a lake. My family and I were eager to test our new canoe on the local reservoir, which I'd driven by a month earlier. Its dark waters lapped against a thick conifer forest. I couldn't wait.

But by the time we got there in late summer, those waters had receded 20 yards from the shore. We had to slog through sucking layers of muck just to launch the boat. The canoeing wasn't great, either: no branches over the water, no quiet little coves with minnows below and birdsong above, just bathtub rings of barren rock and soil and numerous silt bars that grounded the boat. Every time we got out to push, we sank up to our thighs.

Eventually, I learned that this is how many Western reservoirs work: They fill up with runoff in the spring, only to be drained by late summer, when farmers downstream call on the water to irrigate late-season crops.

I also learned that reservoirs are filling up with silt. Surveys of 35 dams and reservoirs operated by the federal Bureau of Reclamation in the West reveal that they have lost some 4.6 million-acre-feet of their original storage capacity to the lowly dirt particle since they were built, most in the middle of the 20th century. That's about 8 percent of their storage capacity, or enough water to serve at least 9 million households. And these surveys vastly underestimate the problem because many were done two decades ago, and they include less than 10 percent of the dams managed by the Bureau of Reclamation.

At New Mexico's Elephant Butte Reservoir, built in 1915, sediment from the Rio Grande has already claimed a quarter of its capacity, while clogging canals, pipelines and farm fields. The irrigation district that manages the project maintains a fleet of dump trucks and dozers to remove the silt, but it is running out of places to put it.

Dredging reservoirs is always the last resort, the most expensive solution to removing sediment, says George Annandale, a South African-born dam expert who has consulted on projects around the world. The least expensive, he says, is to design dams so that sediment-laden waters can be easily diverted around them or flushed through. Many new dams incorporate these features, but unfortunately, silt was only an afterthought when the federal government went on its dam-building binge. Most of our dams were built with the expectation that silt would kill them in a century or two.

For environmentalists who want to see Glen Canyon on the Colorado River come down, sedimentation may seem like a gritty ally. Day by day it is reducing the useful life of Lake Powell for power production and water storage.

Meanwhile, Lake Powell holds a two-year supply of water for the states of Arizona, California and Nevada. Without Powell, the amount of water available for human use would depend entirely on the Colorado River's highly variable yearly flows -- flows that many scientists predict will diminish in the coming era of climate change. Though we may find alternative energy sources to replace Glen Canyon's hydropower, we won't be so lucky when it comes to the water supply itself.

Though few of the West's reservoirs are on their deathbeds yet -- water analysts predict Lake Powell could survive anywhere from 100 to 1,000 more years  -- no one denies that a day of reckoning is coming. Annandale believes society has a moral imperative to plan now for its future water supply. At the least, he says, each generation should be investing in a fund for decommissioning silted-up dams, so that future generations aren't stuck with the bills. At the most, we should be installing sediment-reducing retrofits to existing dams, and in some places, even building more dams.

The small farmers that rely on my local, little water project in western Colorado -- Paonia Dam and Reservoir -- don't have much time. Since the dam was built just 50 years ago, sediment has reduced the reservoir's capacity by about 19 percent. In those ever-building deposits, future paleontologists might find the fossilized bones of the 29 elk that, in the winter of 2008, plunged to their deaths through a thin ice layer. When state wildlife officials walked the reservoir's dry bottom the following fall, they found no trace of the animals.

Paul Larmer is a contributor to Writers on the Range, a service of High Country News (hcn.org). He is the magazine's executive director in Paonia, Colorado.