The power of the lowly dirt particle

 

Soon after I moved to western 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 beckoned to me, lapping against a thick conifer forest. I couldn't wait.

But by the time we got there, those waters had receded 20 yards from the shore. We had to slog through sucking layers of muck just to launch the boat. The paddle 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 were swallowed 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.

Our particular water project -- Paonia Dam and Reservoir, completed by the federal Bureau of Reclamation in 1962 -- is part of the great World War II-era dam buildup in the West.  Such projects have shaped the region, providing water storage vital to agriculture and urbanization in the desert. It seems odd, then, that -- as dam-expert George Annandale notes in our interview with him on page 18 -- most were built to last only a century or two, as if we'd only planned to stick around for a short time.

Blame the lowly dirt particle. The inexorable accumulation of silt was only an afterthought when the federal government went on its dam-building binge, but today no one denies that a day of reckoning is coming. As Wayne Ranney, a northern Arizona geologist, tells writer Craig Childs in this issue's cover story, "We think the dam ... is mightier than the river, but nothing could be farther from the truth."

Childs' thoughtful float down the lower San Juan River into the West's second-largest reservoir, Lake Powell, reminds us that rivers and the sediments they carry work ceaselessly against the impediments we erect. And unless we ceaselessly -- and expensively -- maintain and manage these structures, the rivers will one day triumph.

Paonia Reservoir, which holds back the aptly named Muddy Creek, may be one of the first to go; in 50 years, silt has reduced its 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.