As a geography, the Southwest is considered "heavily dissected." It looks like it's been ravaged by atomic bombs of erosion, leaving the landscape ripped and carved. The rivers that flow here are conveyor belts for sediment, moving out the constant decay of mountains, mesas, buttes and canyons. But with those rivers now dammed, things have changed. The Colorado River, centerpiece of the Southwest, is slowed by six high dams and numerous low-heads and diversions, while the incoming San Juan pauses higher up in northern New Mexico at Navajo Dam. The equation is simple: When rivers are slowed by dams, the water can no longer carry its sediment. So the sediment settles out. The Colorado used to carry about 90 million tons a year through the Grand Canyon; with Glen Canyon Dam in place since 1963, it now carries about 15 million tons. Where did the rest go? It sank to the bottom of reservoirs stacked along the river system.

Wayne Ranney, a northern Arizona geologist who has closely studied erosion and deposition in the Southwest, says that no dam in a place like this survives long, at least geologically speaking, without eventually holding back more mud than water. "We think the dam is controlling the river, the dam is mightier than the river," Ranney says, "but nothing could be farther from the truth."

The San Juan flows at about 10 percent sediment by volume. (Wet concrete, by comparison, is 30 percent sediment.) Though Glen Canyon Dam is 710 feet tall, its lowest outlet is 237 feet, which is how high silt must reach before the game is over. That silt is now creeping up the face of the dam at about four inches a year. Recent studies on Lake Powell show that the lower the water levels fall, the faster sediment moves toward the dam, turning into subaqueous mud flows -- gravity-rivers of sediment picking up speed.

Civilization in the Southwest is for the most part tied to the Colorado River and its tributaries. Glen Canyon Dam's hydropower output of 4.5 billion kilowatt-hours annually, the equivalent of 11 million barrels of oil or 2.5 million tons of coal, helps feed the region's sidesplitting growth, and an estimated 85 percent of the reservoir's water is earmarked for downstream agriculture. But the increasing sediment impoundment, combined with frequently falling water levels, makes it clear: This resource is eventually going to run out. "It is a problem that would be on the order of climate change for us in the Southwest," Ranney says. "As sediment accumulates, these reservoirs we depend upon will become ineffective and unusable, and then you're stuck with a lot of people in a place they have no business living. At some point there's going to be a price to pay."

Determining just when that point will be for Lake Powell and other big Southwestern reservoirs, though, is complicated, and estimates vary widely. That's because they depend on highly variable factors, namely how much sediment is coming down behind them -- something that changes by the season, the year, and the individual drainage. The Bureau of Reclamation originally put a 700-year functional lifespan on Glen Canyon Dam. But in his landmark book Cadillac Desert, the late Mark Reisner forecast a much earlier demise. And James Powell, executive director of the National Physical Science Consortium, has put the number as low as 55 years. Although most of Reisner's assertions about water in the West have stood up to scientific scrutiny, some recent studies show that his apocalyptic forecast of all major reservoirs becoming unusable soon, and at roughly the same time, may not be accurate.

One of the more definitive studies, led by Will Graf, distinguished professor and chair of the Department of Geography at the University of South Carolina, came out late last year. Long before Glen Canyon Dam was built, Graf says, original research prior to 1942 anticipated that a reservoir in that location would last 300 years at most. After 1942, the same sorts of studies came up with a lifespan of 700 years or longer. "That's what piqued my interest," he says. "How, in considering one place, do we come up with two different answers?"

The answer is that, after 1942, large water-storage projects started going in upstream. As these captured sediment, the lifespan of a dam in Glen Canyon -- no longer the sole catchment for the upper Colorado River drainage -- increased considerably. Conditions have not changed much since that time, so Graf's current estimates on longevity come out looking more like the Bureau of Reclamation's, going as high as 1,000 years for the Southwest's largest reservoirs. These dams appear, he writes, "to be sustainable parts of a regional water management system."

But Graf is careful to note that his numbers stand only as long as conditions remain stable. The fact that sedimentation shifted so much before and after the 1940s -- even if driven by human development -- shows how sensitive the river system can be. "It can change just as suddenly back in the other direction," he warns. "The surprise we would not like to see for Glen Canyon is extensive drought conditions and overuse that diminishes riparian conditions along the river, followed by large flood events where we would return to pre-1940s sediment conditions. In case you're following the box score, climatic projections in the Southwest are exactly that."