PAGE, Ariz. — For Tyler Knudsen with the Utah Geological Survey, watching a video of a cliff wall collapsing into Lake Powell is a fantastic example of a normal event.
"We've got these large cliffs throughout the Colorado Plateau, and rockfalls like this happen," the geologist said. "Actually, quite frequently — it's just not often we happen to get it on video."
He added that "quite frequently" in geologic terms is once every 1-10 years.
Knudsen says the rock responds to the pressure caused by changing lake levels.
"We know the presence of the lake itself has probably accelerated rockfall, and particularly when you have fluctuations in lake levels, it can at times trigger accelerated rockfalls," he said.
The water forces itself into the rock's open spaces when the lake fills, affecting the rock with a force called positive pore pressure.
When the lake level drops, the water pulls out, creating a suction-like effect called negative pore pressure.
Both forms of pressure can destabilize the rock.
Along with pore pressures, natural waves and boat wakes add to the erosive impact of the lake.
Knudsen and three fellow geologists authored a study about rock hazards in the Glen Canyon National Recreation Area available online here.