Sediment is not the only issue, however. Water being drawn down for human use can play just as a large a role in reservoirs' longevity. Another 2010 study that projected relatively long lifespans for dams, this one led by John Louis Sabo of Arizona State University, concludes that the capacity for water in the West to support cities, industry, agriculture and ecosystems is already "near its limit under current management practices," and that a projected doubling of human population would use more than 99 percent of available streamflow, making storage practically obsolete. Downstream of Lake Powell, Lake Mead, the largest reservoir in the U.S., is already facing a crisis. With water levels expected to drop below one of its old intakes in the next few years, the Southern Nevada Water Authority anticipates spending at least $740 million tunneling under the reservoir to put in a new intake. Meanwhile, the opposite has been proposed for Lake Powell, where intakes might be elevated to rise above the silt, although this is still an evolving idea.

No matter what the estimates say, though, everyone agrees that reservoirs have finite lifespans. And the Southwestern landscape, with its ancient history exposed like bones, suggests a different sense of time, one where reservoirs rise and fall, as evanescent as clouds.

Over the last 700,000 years -- a geological blink -- a number of dams formed naturally in the Grand Canyon when volcanoes erupted on the rim and  poured lava down inside, blocking the river with solid, basalt impoundments much taller than Glen Canyon Dam, and miles thicker. (Some flows moved 75 miles down canyon.) Today, hardly any evidence remains of these ancient dams: basalt on the edges of the canyon, and driftwood stranded up high from old reservoirs much deeper and longer than Lake Powell. Even these enormous lava dams and their reservoirs are long gone, scrubbed out by erosion or possibly exploded all at once by outburst floods from the impounded river. In comparison, Glen Canyon Dam is a mere concrete wafer -- a momentary feature on the land.

The river runs dark and gritty as unfiltered coffee, silt hissing softly against the rubber of my boat. It is a fair downstream current, the kind that does not require much paddling, lulling me into a groove as I pass below red-ledged outcrops. Alongshore there are signs of what this river carries -- a car tire stuck in the bank, a bloated cow carcass spattered with raven droppings.

Soon, I pick up white noise mixed with a familiar arrhythmic pounding. The booming sound becomes more articulate as I approach: It is the waterfall. I brush along the left shoreline, feathering under tamarisk trees. Where the river flat-lines into a single horizon, I turn the banana upstream and, at the nearest clearing, punch into the brush, finding the eddy Paul told me about.

Five feet away, the river falls over a bedrock arm of red sandstone into a tongue as smooth as wet marble. A thousand cubic feet per second speed into a maelstrom below. The drop is only 15 feet, but it extends out much farther, hitting a bedrock bench before descending into a violent recirculation hole, a thumping rapid best not attempted. The muddy river is flushed with oxygen; it looks silvery in the sun.

The last time the reservoir was nearly full, in the late '90s, it reached this far upstream. When the San Juan plunged into that still water, it dropped most of its sediment. The reservoir then receded in drought in the early 2000s, rolling back miles downstream and revealing a new landscape of silt. The old river channel had been buried, so the San Juan simply formed a new one, seeking the nearest low point, which was Paiute Farms on the Navajo Reservation. There, it hit a piece of previously uneroded topography and poured over, forming this waterfall.

The waterfall is more than a local impediment. It acts like a low dam, changing the way the river behaves as far as 30 miles upstream. If you have ever run the San Juan just above the Clay Hills boat ramp, chances are you had to dodge sandbars until you finally got out and had to drag your raft across shallows, maybe for miles, while being devoured by mosquitoes bred in slack waters. It wasn't always this way. There were once rapids along that stretch, with names like Slickhorn and Boxcar, but they silted up in the slowed-down current and no longer exist.

Below the falls, I slip back into the fast-moving river, which runs with renewed vigor. Crackled layers of silt stack in strata along the edges, growing taller downstream. Cliff-rows of Wingate sandstone beam in the sun. Even my paddle sounds different, gliding through the water -- the psychological effect of a place that people rarely see. This is nowhere, a nonentity, a river's limbo: too far up the lake for speed boats, too far down for river-runners. Lake Powell's bathtub ring grows around the shoreline among dry whips of tamarisk and willow drowned by the lake. Boulders above shore are weirdly eroded, turned to polygons by wave action the last time the lake was up here.