The collapse of a popular arch formation on Lake Powell may well have been due to the 190-million-year-old sandstone being submerged by the 61-year-old manmade lake, two experts say.
Another factor could have been the rapid fall followed by an equally rapid rise of Powell’s water levels since 2020 due to wild weather swings, some attributed to human-caused climate change, the experts in geology and the Colorado River told the Arizona Daily Star.
The Double Arch prior to its collapse Aug. 8 in Lake Powell’s Rock Creek Bay in Glen Canyon National Recreation Area in southern Utah. The geologic feature has also been referred to as the “Toilet Bowl,” “Crescent Pool” and “Hole in the Roof”.
Known as the Double Arch, the geological formation collapsed last Thursday. When not submerged, it was a very popular place for boaters to visit and for swimmers to dive from, but no one was injured from the collapse. The arch has alternately protruded from water and been submerged since 1963.
National Park Service rangers have been monitoring the arch, which was in Glen Canyon National Recreation Area in southern Utah, “but we do not have a conclusive cause,” said Mary Plumb, a park service spokeswoman. “With the evidence that is now underwater, we are not able to evaluate it.”
The majestic, giant rock formation was made of porous Navajo sandstone of lush brown and reddish colors. It was formed from sandstone originating in the late Triassic to early Jurassic periods, the National Park Service has said.
The Double Arch stood in Rock Creek Bay, about 36 miles up the lake from Antelope Point Marina at the lake’s west end and nearly 60 miles down the lake from Bullfrog Marina at the lake’s east end.
Since formation, this fine-grained sand feature has been subject to spalling — the flaking, crumbling, chipping or breaking of rock and concrete — and erosion from weather, wind and rain, the park service has said.
But David Wegner, a retired U.S. Bureau of Reclamation engineer, and Jeffrey Moore, a University of Utah geology professor, agreed the submersion of the arch’s sandstone in the lake’s water could over time have helped dissolve some of the rock, making it more vulnerable to natural forces of wind and water erosion.
The lake began forming with Colorado River water in 1963 following the closing of Glen Canyon Dam’s gates, and has risen and fallen with changing weather conditions ever since.
The Double Arch prior to its collapse Aug. 8 in Lake Powell’s Rock Creek Bay in Glen Canyon National Recreation Area in southern Utah. The geologic feature has also been referred to as the “Toilet Bowl,” “Crescent Pool” and “Hole in the Roof”.
“The water inundating the sandstone of the arch can dissolve some of (the material) that holds it together,” said Moore, who runs a University of Utah research group that has studied other arch collapses and the growth and stability of arches for more than a decade.
Then, “when water dries out, it pulls some of the salts out of the rock. It evaporates,” which weakens the rock further, Moore said.
“Just like water will go up in a straw, it sucks water into the rock, especially porous rock like sandstone. It will draw water like a sponge,” said Wegner, who was program manager for the bureau’s Glen Canyon Environmental Studies program in the 1980s and ‘90s.
“It happens today, as water levels go up and down, you see more water being sucked into the sandstone and draining out of the sandstone. Once more water gets in there, it freezes and thaws, you’ll see more fractures and falls. But it doesn’t mean (this collapse) didn’t start months ahead of time,” Wegner said.
Soaking sandstone with water increases the probability of rock falls, he said. The more reservoir water levels fluctuate, the higher the probability of the erosion of rock.
“Water gets in there and lubricates it,” he said. “Over time it would have taken a lot longer for it to happen naturally. Because of the up and down of the reservoir, we may have accelerated the process.”
The lake levels are controlled by wetting and drying cycles, and the more you go through those cycles, “that’s adverse conditions for the arches,” Moore said. “Once it’s fully underwater, with the waves crashing on it, if the water level is at just the right level with waves, there’s really rapid wetting and drying.”
As recently as 2016, the lake was high enough under the arch that a paddle boat could barely squeeze through it. At the time, Powell stood at over 3,600 feet elevation, lower than its full elevation of 3,700 feet but significantly higher than its levels of more recent years.
“At lake levels over about 3630, the entire complex was underwater — no arches, no pool, nothing. The lake was generally above that level from about 1971-2003 or so,” said John Rickenbach, a longtime lake visitor.
The arch was also called Hole in the Roof when the lake was lower and Toilet Bowl when it was higher.
“When I was last at Hole in the Roof in August 2020, the lake was at 3604. At that time, it was impossible to boat underneath, and really was just a steep-sided pool that if you fell in, you’d have a very hard time getting out,” Rickenbach said. “On that summer day, there wasn’t a boat for a half mile around, since there was no reason to go.”
But in the past few years the water has been low enough that you could climb onto it or swim or boat underneath it.
Merril Campbell, who lives near St. George, Utah, told the Star he was on a friend’s houseboat at the arch early Thursday afternoon when another person on the boat saw a rock fall from it.
“When I looked, the water was all orange,” said Campbell, who photographed the arch early that afternoon and again late that afternoon after it collapsed. “Little pieces of rock were out there.”
Until then, at the lake’s current level, you could drive a boat underneath the arch and “see the hole in the roof” of it. It stood 30 to 40 feet above water last week and “was a longer drop” from the top of the arch to water two years ago, he said.
Every day, hundreds of people would climb via a short hiking trail to the top of the arch, he said, adding that he personally saw about 100 people there the afternoon before it collapsed.
From June 2020 to March 2023, the lake fell nearly 90 feet, from 3,610 to 3,521 feet. The latter elevation was only 31 feet above the level at which the dam would no longer be able to generate electricity.
But in the past 15 months, much wetter weather has raised the lake well over 80 feet, and it peaked at 3,585 feet last month.
In recent years, “We’ve seen a significant departure from the normal pattern of reservoir filling and seasonal drawdown, because of extreme drought levels,” said Wegner, now a National Academy of Sciences board member. “It’s the up and down that tends to accelerate erosional processes.”
The Park Service’s Plumb declined to comment on Wegner or Moore’s statements, since “We do not have conclusive evidence of what caused the failure.”
Many researchers who have studied the river and lake’s 21st century declines have fingered warming weather as part of the cause. That’s because hotter temperatures increase evaporation and partly dry out soils, meaning snowmelt that otherwise would have flowed into the river soaks into the soil, researchers have said.
Campbell said he wouldn’t be surprised if lake water seeping into the arch and its supporting rock contributed to its collapse.
“I know sandstone. I’m no geological expert. But I’m a rock climber. Wet sandstone breaks, man. It’s a fact,” said Campbell, who runs a restaurant equipment servicing business in Hurricane, Utah.
But on Facebook, several members of a group devoted to houseboating on Lake Powell noted that back in 2008, another arch in Arches National Park outside Moab fall, and the arch “was in the middle of the desert. No water by it,” said Todd Nu, a contributor to that group.
If anything, a full lake’s hydrostatic pressure would hold the arch in place and negate gravity issues, said Brison Cole, another member of the group.
“Gravity would have helped it fall earlier had it not been under water at some points in its lifetime. The weight against the arch would have been less underwater helping it to not fall,” said another contributor to the group, Audrey Cox.
Group member Rickenbach disagreed, saying, “If you leave Navajo sandstone in water long enough, it starts weakening, and the erosion forms sand —because that’s what it is! Add fluctuating lake levels, a little wave action from beneath, possibly spurred on by wind and storms, and you’ve got more erosion than might otherwise occur.”
As water in one of the nation's largest reservoirs recedes, geologic features hidden for nearly 50 years are revealed in Glen Canyon National Recreation Area in Northern Arizona. Video courtesy of Glen Canyon Institute, 2022
