When the San Juan River flows out of the San Juan Mountains in Southwestern Colorado, it contributes 15% of Lake Powell’s water.

But there’s a problem: The river carries a hefty 55% of the sediment entering the reservoir, and that mud is piling up.

The sediment-heavy river flows south into New Mexico before jogging into Utah, then it joins the Colorado River close to the Arizona border. The confluence is submerged under Lake Powell.

After decades of drought, the reservoir created by Glen Canyon Dam has dwindled to just a third full. Now, as the San Juan River flows toward Lake Powell, it rambles over a huge pancake of mud that’s 49 miles long, a mile wide in some places, and as much as 120 feet deep in the final reaches of the San Juan River.

Unique hydrology has contributed to this plug, A relatively wide canyon and multiple waterfalls slow down the river, allowing sediment to drop out. Though the San Juan is the muddiest tributary, all the Colorado’s tributaries drop a good deal of mud 100 miles or more upstream of Glen Canyon Dam.

It’s a Western phenomenon caused by damming swift rivers, said Jeff Geslin, a geologist at Fort Lewis College in Durango, Colorado. The result is that reservoirs in the West have become “temporary sediment storage facilities.”

If that mud could move through the Grand Canyon, like it did before the dam, biologists say that would help restore the canyon’s ecosystem, which depends on sediment-laden flushes in spring to scour riverbanks. Then, as the river slows, beaches form and vegetation returns.

Gary Gianniny, professor of Geosciences at Fort Lewis College in Durango, Colorado has been studying the San Juan River, along with river researchers who call their team, “The Returning Rapids Project.”

The group’s big worry is that without drastic action–draining Lake Powell to let the Colorado River run free—time may be running out for the languorous San Juan River.

Mike DeHoff, principal investigator of the Returning Rapids Project said the sediment layer on the San Juan has created new channels and new waterfalls. DeHoff added that no one knows whether the river’s sediment plug would dissipate even if Glen Canyon Dam were breached.

Researchers boating the San Juan River where it approaches Lake Powell say they’re forced to navigate an ever-moving pile of sediment that also involves portaging around rock waterfalls. When they finally arrive at Lake Powell, there’s dangerous liquefied clay and sand to navigate.

“I’ve seen people sink to their chests in the mud, saved only by their flotation devices and nearby boaters,” said DeHoff of Moab, Utah.

“We’ll need a drone to study that area,” added Gianniny.

Researchers with the Returning Rapids Project talk a lot about what to call these giant slabs of calving sediment. DeHoff suggests “mud bergs.”

Semi-solid mud walls along the river have already been dubbed “the Dominy Formation,” named after the avid federal dam-builder Floyd Dominy.

“Technically, Gianniny said, the giant mud plug is a “mass of uncompacted mud and sand that causes alluvial fanning.” And falling slabs of sediment, those “mud bergs,” act as semi-permanent river features.

BLM River Ranger Chad Niehaus uses a packraft to regularly visit what researchers are calling the Lowest San Juan. He floats over 30-plus miles of the muddy river, finishing with a four-mile backpack out to a four-wheel drive vehicle 48 miles from Page, Arizona as the crow flies.

Niehaus marvels at the deserted region. “Sediment is moving around, and you must be vigilant in a different way than you do on a ‘normal’ river.”

Drought, climate change, “whatever you call it, the Lowest San Juan has re-emerged,” Niehaus said about wildlife in the once-submerged canyon. “I’ve seen river otters, mountain lions, coyotes–even pelicans—but the most astounding aspect is how quickly nature is coming back.” In places, cottonwood trees are 20 feet high, he said.

“When I was a teenager there were places on maps that were considered forever gone,” he said, pointing to sections on the map entitled, “Glen Canyon National Recreation Area.”

Now, he said, “some forever-gone places are revealed.” He mentions Cathedral in the Desert, a wondrous site on the nearby Escalante River. Enough water has receded to make it visible, though some of this sacred place for Indigenous people is buried under 30-plus feet of sediment.

Meanwhile, the muddy end of the San Juan River is wild again: “I rarely see a footprint.”

Dave Marston is the publisher of the independent nonprofit, Writers on the Range, writersontherange.org, dedicated to spurring lively conversation about the West. He lives in Durango, Colorado.

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