Slides on mesa frequent, smaller
Although geologists are marveling at the size of the landslide that came off Grand Mesa Sunday, it’s far from the only one the flat-top mountain has seen.
“The flanks of Grand Mesa are really just a whole complex of landslides,” said Jonathan White, a senior engineering geologist with the Colorado Geological Survey, who toured the weekend slide site by helicopter Monday to help assess it for authorities.
“Landslides commonly occur along the flanks of Grand Mesa. It’s just the size of this — that’s what was kind of eye-opening,” he said.
Rex Cole, a Colorado Mesa University geology professor, said landslides, earthflows and mudflows are “really common all the way around Grand Mesa” as evidenced by the visible scars they leave behind. But they’re typically much smaller.
“Having one this big at this time is, really I don’t know if it’s shocking, it’s just unexpected I guess,” he said.
The geologists say clay-based geological formations that become weakened by moisture contribute to the instability on the Grand Mesa. The reddish-colored Wasatch geological formation was part of what gave way in this weekend’s slide.
“Clay is very susceptible when it gets wet — it gets slick,” Cole said.
“… The Wasatch is rock. It’s just that when you get it wet, it gets really slimy.”
Aldis Strautins, a hydrologist for the National Weather Service in Grand Junction, said areas around Collbran got anywhere from a half-inch to an inch of rain just prior to the slide. And that followed a wet snowstorm a few weeks back that delivered anywhere from 1.75 inches to 2.8 inches of water, coming on the heels of a close-to-average winter snowpack and above-average rain last fall.
“Which one helped it (slide), I don’t know that we can say yet, but they may all be contributing factors,” said Strautins, who also never has seen a slide this big before.
White said the area that slid has been mapped as having previous slide activity, but the mapping didn’t show that activity extending as far down the valley as the current, 2.5-mile-long slide has traveled.
“I’ve been studying landslides in Colorado for what, 27 years, and that’s the largest one I’ve been involved in,” he said.
Cole said there have been other larger landslides in the state.
“Most of them were before white men settled this area,” he said.
Landslides were likely more prevalent on the Grand Mesa and elsewhere in the state during the wetter, colder Ice Age, he said.
An “enormous” landslide came off Mount Lincoln in De Beque Canyon 10,000 or more years ago, probably blocking the Colorado River, Cole said.
White said the Slumgullion slide in the Lake City area may offer a comparable example to this week’s event. It occurred some 700 years ago and created a dam that formed Lake San Cristobal.
In 1986, a large landslide coming off Ragged Mountain between McClure Pass and Paonia Reservoir crossed Colorado Highway 133 and dammed East Muddy Creek, forcing emergency actions by the state to avert a potential flood and restore travel on the road. White said this week’s Grand Mesa incident is more of an earthflow following a drainage rather than a broader landslide like the one in the East Muddy Creek area.
With spring runoff comes the concern for other slides occurring around the region, White said. But they wouldn’t be likely to be anywhere near the size of the one that just struck.
Cole said while such large-scale events have been happening for millions of years, trying to forecast any one of them is like trying to predict an earthquake.
“Eventually something like this is going to happen again, but is it going to be a thousand years from now or next spring?” Cole asked.
He said he wouldn’t dare hazard a guess.