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Know the West

Muddy Waters: Silt and the Slow Demise of Glen Canyon Dam


Updated 5/17/11

The Lower San Juan River courses through a rather forsaken landscape of clay hills and redrock plateaus in southeast Utah. At the end of a long, dusty road, there is a boat ramp at the water's edge where, at any warm time of year, vans and roof-racked Subarus bake in the sun while their owners are out on the river. This is the end of the line. A large sign stands along shore, smacked with big red letters:


The sign and the waterfall mark the transition from a fast, muddy river to a bizarre and almost forgotten landscape created by upper Lake Powell, backed up behind Glen Canyon Dam a hundred miles away. Everyone gets off the river here.

In the bright and cloudless sunlight of a desert afternoon, I pump up a ducky -- a bright yellow inflatable boat that looks like a swollen banana -- and drag it down to the water past a group of river runners hauling rafts and kayaks onto the boat ramp. They pause and watch as I toss in drinking water and a drybag for my gear, and push into the current. They seem puzzled: Didn't I notice the sign? Am I just crazy? Before I can offer an explanation, the river has taken me out of sight beyond a bend in the tamarisk-thick shoreline.

River-runners generally know the San Juan by the river that lies above here. Swift and wild, it meanders through canyons where hackberries, cottonwoods and massive red boulders congregate along the shore. But few know the river just downstream, where it becomes an enormous science experiment in silt and fluid mechanics.

Lake Powell, which now starts several miles below the waterfall, is the second-largest artificial reservoir in the United States. It is the seeming endpoint for four rivers: the Colorado, Dirty Devil, Escalante and San Juan. But the reservoir isn't really an endpoint at all. These rivers aren't dead, and their persistent dynamics are slowly, steadily driving Lake Powell toward its demise.

I had come to see the San Juan in action. This confluence of moving and still water changes dramatically year by year as reservoir levels fluctuate. Mud flats grow and shrink, exotic tamarisk trees explode to the horizon, then drown underneath high water. The river itself is unstable, prone to jumping its channel, whipping across the desert in search of a new course. The waterfall that the sign warns of appeared along the river in the '90s, then vanished. A new waterfall formed about 10 years ago, this time miles downstream of the first, as if it were migrating.

This bottom stretch is legendary among river runners, although few have actually seen it. The only one I know is a dirtbag guide out of Bluff, Utah, named Paul. He told me that he caught a glimpse of the waterfall after crashing for hours through sweltering tamarisk thickets, dragging his kayak behind him. Advising me before my trip, he emphasized, "You just do not want to go over the waterfall, that's all."

Certainly not in my yellow banana.

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."

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.

It isn't just psychological, though. The river corridor is different. Birdlife flourishes. Troops of Canada geese hold the ground at every bend. I catch up with a coyote swimming across the river, and she climbs to shore 10 feet away, bony with dripping fur. With a quick shake, the animal fluffs herself and trots downstream. I keep pace, paddling just off her shoulder. She squats to put a scent on the ground, with a candidness I am unaccustomed to among coyotes. I try not to stare, impolite at such close range. We continue together until she finds herself cut off by a small backwater bay. She stops and finally looks at me with dark, inquisitive eyes. Then she trots the other way, and the current carries me downstream.

That bay is the first sign of the reservoir. Water has begun backing up, slightly more penetrable to light as sediment settles out. In the next mile or so, the main current grows more elusive -- meandering through an expanding fringe of slow water. In evening light, I pull into a massive, tilted outcrop, the shore thick with soft, knee-high vegetation, weedy invasives, Russian thistle. Bats chitter around my head snatching up midges and mosquitoes. I lay my sleeping bag in the desert on top of red sand and sun-beached clamshells.

At dawn I pack up, unleash my boat from fresh spider webs, and move onto the water's gradual twist. Within a mile, I cannot count on much of a current. No more idle moments staring at the canyon: I need to pace myself with long strokes. I come upon random currents carrying sticks, seeds and little moons of rabbit pellets, but the reservoir grows. What was once a hill becomes an island. I navigate across floating cattail reeds, flower petals and Styrofoam, while grebes pop up and down, heads periscoping to watch me pass. Each time I think the river is gone, I find vestiges again. It rises and falls, as if it were tunneling deep below the surface, a submarine breaching, then descending through the murky vaults of Lake Powell.

I once spoke with a Park Service ranger who worked to recover the body of someone who drowned on this part of the river. She described floating particulate matter obliterating sunlight to a depth of 20 feet. Down deeper, the water became textured, viscous, until it felt like she was swimming through chocolate pudding.  Below 50 or 60 feet, the abyss of half-suspended mud felt like liquid concrete and it became hard to move, and for a moment she panicked, her fins trapped in the lightless tomb of mud where she had to unbuckle and extract them.

Here, the river's current may more or less disappear from the reservoir's surface, but the sediment it delivers keeps moving in the form of gravity flows, streamers of mud and silt slowly but inexorably advancing along the reservoir's floor toward the dam. In this fluctuating underworld, side-canyons dump their own sediment-deltas into the mainstem, forming subaqueous dams through which both San Juan River current and sediment carve their own channels. In the dark water far below, new canyons form within the sediment itself, and in places, walls of the original bedrock canyon have collapsed, forming dams where fine mudfalls spill over one after the next. "There are real small currents heavier than the rest of the water," explains Mark Anderson, head river ecologist for Glen Canyon National Recreation Area, who has been involved in mapping the underwater terrain of Lake Powell. "They are turbidity flows, not actually water, but they can roll down through the basin over a long period of time."

Until recently, most believed that silt was collecting mainly where rivers enter the reservoir. (Currently, the Colorado River has deposited material 150 feet deep where it slows around Hite Marina, rendering it inoperable.) But underwater mapping reveals that the majority of infilling is happening in the reservoir's center. As a result, Lake Powell might fill more quickly and thoroughly than expected, another variable to figure into the many equations.

Back on the surface, the water takes on an emerald tone. White-washed boulders alongshore reflect light off the water while mouth-sucking carp splash on the surface. Sometimes, the only evidence of the river is the yawning shape of a canyon the San Juan left behind. But then I notice a downstream-trend to floating debris, plastic water bottles and oil cans all heading the same direction. Methane bubbles rise from organic decomposition in the sediment below, and the slow current moves the bubbles into trains.

As I paddle into the afternoon, a stiff wind strikes -- 40 mile-per-hour gusts kicking up chop as I fight my way across the reservoir. Eventually, I come into the remnants of the old Glen Canyon, big domes of Navajo sandstone melting into each other, dark springs and seeps sheeting from seams in the rock. Side canyons lean back into enormous alcoves, places that once towered high over the river and are now at eye-level.

In the open expanse, shelters become few for miles at a time. Wind creates standing waves and sheets of mist, giving the reservoir a last illusion of movement.

The next day I hunt diligently for a current, zigzagging across flat water. When afternoon winds come up, I put my head down and paddle straight, forgetting about the river.

By evening, the wind settles a bit. I search for the river a last time, but cannot find it. It is now a ghost sliding along the bottom of Lake Powell. I pull the banana onto a sandstone bench; water clear enough to see a rock face descending sharply. It is a cliff dropping off, but I can only see the top several feet before it falls into greenish shadow.

Here, I make camp, setting my bag among big linen blooms of datura. The water gulps against cliffs, and canyon tree-frogs call loudly from a spring across the way. The channel is hardly wide enough to call a lake. It still looks like a river. In fact, all of Lake Powell looks more riverine than the typical pool-shaped bodies we know as reservoirs. Its 2,000-mile shoreline (more waterfront than the entire Eastern Seaboard) consists of narrows and meanders, each the shape of the river and canyons it possesses.

I have a friend named Katie Lee, now 91, a longtime defender of free rivers and a fist-pounding enemy of Glen Canyon Dam. She knew these rivers before they were dammed, and she still cries over the loss. She once spread maps and photos across her living room floor and told me stories of wonderful places now drowned. She is the one who first told me that you cannot kill a river. She felt as if her rivers were gone when the dams went in, but she knew that they would not truly be stopped, that they would still obey the most intrinsic of laws, continuing to run downhill dammed or not. What looks static is actually in motion, still water deceptively reworking itself, and the landscape it passes through. There is a heartbeat down there at the bottom.

That night, sitting in the crease of the San Juan's canyon quarter-filled with lake water, I pull out my journal. The top of each preceding page had been titled, "Lower San Juan." Starting the next page under a headlamp, I hesitate, go to the top, and finally pen, "Lake Powell."

Related: an animation of dropping water levels in Lake Powell, from NASA.

This story was made possible with support from the Kenney Brothers Foundation.

Craig Childs writes from near Crawford, Colorado, where he adds to sediment coming down the Gunnison River. He is this year’s winner of the Ellen Meloy Desert Writers Award, and he has authored several books. His work has appeared in the New York Times, Men’s Journal, Orion and The Sun.