Click here to download the report from the Colorado Geological Survey. From the website:
On May 25th, 2014 the longest landslide in Colorado’s historical record occurred in west-central Colorado, 6 mi southeast of the small town of Collbran in Mesa County. Three local men perished during the catastrophic event. The landslide was 2.8 miles long, covered almost a square mile of the West Salt Creek valley and the net volume displacement was 38 million yd3. The fast-moving (40-85 MPH), high-mobility landslide was caused by an initial rotational slide of a half-mile-wide block of Eocene Green River Formation. The resultant rock failures, rockmass disaggregation, and mostly valley-constrained rock avalanche, dropped approximately 2,100 ft in elevation as a rapid series of cascading surges of chaotic rubble composed of fragments of pulverized rock, vegetation, topsoil, and mud. Local seismometers recorded a magnitude 2.8 earthquake from the event with a seismic wave train duration of approximately 3 minutes. The toe of the landslide came within 200 ft of active gas-production wellheads and loss of irrigation ditches and water impacted local ranches and residents.
From The Grand Junction Daily Sentinel (Gary Harmon):
The West Salt Creek landslide changed over the summer, as a water course formed to drain much of a pond formed by the slide and as the slide itself compacted and settled into place, according to scientists who are studying the event.
“Continuing threats to nearby residents remain,” warns the Colorado Geological Survey in a new report on the landslide, which took the lives of three men on May 25, 2014.
Clancy and Dan Nichols — father and son — and Wes Hawkins died in the slide while they worked to clear an irrigation ditch. The Colorado Geological Survey report is dedicated to their memory.
Collbran, six miles below the slide, is in no danger, but ranchers and others in the area should be aware of the threat looming high above, especially in the spring, when the “sag pond” is most likely to refill, said Colorado State Geologist Karen Berry.
“The biggest unknown is what will happen when the sag pond fills and spills” into the West Salt Creek Valley, most likely with the spring thaw, Berry said.
The sag pond was formed when 35 million cubic yards of the Green River Formation faltered and slid down, the report said.
The slide covers nearly a square mile atop the West Salt Creek valley and in some cases is as much as 123 feet [deep], the report says.
The slide left a 65 million cubic-yard slump block tipped 15 degrees toward the mesa, creating a “V,” the bottom of which is the pond. according to the report.
The sag-pond water percolated through the slump block forming what geologists call a “pipe,” or conduit that lowered the level of the pond by about eight feet.
“At some point, the stream (West Salt Creek) is going to try to re-establish itself,” Berry said.
How that will work is still an unknown, said Jeff Coe, a U.S. Geological Survey geologist who has studied the slide.
“We don’t know where it will stop and we don’t know what the response will be” as snows melt and feed into the West Salt Creek drainage, Coe said. “That slump block continues to evolve. I’d like to get through a few more springs before I say we’re off the hook.”
Mesa County has built a sophisticated monitoring system intended to detect changes in the slide.
The most recent study cleared up a lingering question for Tim Hayashi, senior engineer for Mesa County, who has led the county’s monitoring of the landslide.
It wasn’t clear why the slide traveled three miles down West Salt Creek Valley, Hayashi said, until the study made it clear that more was going on than just a single slide.
“It wasn’t one enormous slide,” Hayashi said. “It was several large slides” that pulsed down the mountain, one after the other.
The Jet Propulsion Laboratory in Pasadena, California, also has been monitoring the north side of the Mesa with special radar, Coe said.
“Those data are still showing shifts, not real large ones,” Coe said. “The whole West Salt Creek slide is compacting and small areas on the slump block are changing.”
There is no evidence that a catastrophic change is imminent, Coe noted.
The USGS is preparing its own report on the landslide, he said.
The 2014 landslide took place at the same place as a slide that occurred in the Holocene period as it transitioned into the Eocene, at least 11,700 years ago, the report noted.
Studies of the slide could shed light on similar looming threats, Berry said.
“What we hope to do is, take what we’ve learned and look at landslide susceptibility for Grand Mesa and Mesa County in general,” Berry said.
From The Denver Post (Jesse Paul):
The 55-page Colorado Geological Survey study says risks remain specifically because of water that has collected in a depression near the slide’s head, creating a “sag pond.”
“A failure of the sag pond or the remaining rock slide could create a similar event to what happened,” Karen Berry, the state geologist, told The Denver Post on Thursday…
The report also highlights how there are areas throughout the Grand Mesa and Colorado that are also at risk for breaking away.
“Mesa County, and especially Grand Mesa, contain hundreds of active and inactive landslides,” the report says. “Fortunately, many of these features are in remote or underdeveloped areas and have caused little damage.”