The Missionary Ridge rockfall is on the move.
In the past couple of days, several thousand tons of mud and debris have come down the mountainside to the Animas Valley floor, said Butch Knowlton, director of La Plata County’s Office of Emergency Management.
As a result, up to 10 feet of mud, silt and rock have buried East Animas Road (County Road 250), which has been closed since Friday. Three cars have tried to drive around the barricade on East Animas Road but immediately become stuck, he said.
Five to 10 homes sit below the rockfall area, but none were in immediate danger as of Tuesday.
“I’ve never seen it this big before,” said Knowlton, who has worked for the county more than 40 years. “We’ve seen a lot of movement come down that drainage, but this is far more substantial than what we’ve previously seen.”
On July 5, 1998, a massive rockfall carrying boulders the size of houses came tearing down Missionary Ridge. According to The Durango Herald archives, an estimated 50,000 cubic meters of rock broke away from the cliff, falling 600 feet and cutting a swath 200-feet wide.
The scar remains a prominent landmark on the eastern side of the Animas Valley north of Durango.
The bulk of debris settled in the upper part of the hillside, and didn’t come close to any homes. Around 2005, however, mudslides started oozing down the hillside, frequently closing East Animas Road, sometimes for days and weeks.
The debris pile is triggered in two ways: either by a sudden, heavy downpour that quickly carries rock and mud in a sort of flash flood, or through the slow process of snowmelt, which gets into rock formations and breaks apart weak layers, carrying larger debris with it.
It’s this latter process that is responsible for this week’s debris flow, which causes the mud to flow like lava, Knowlton said.
The consistent, heavy snowfall that hit Southwest Colorado this winter is slowly starting to melt as temperatures rise. Saturday night, several thousand tons of rock and mud came down the hillside, though it did not reach the road, Knowlton said.
“People need to be aware of the fact there’s all this material above their properties,” he said. “With the right conditions, there’s going to be thousands of tons of material that could come down.”
Homeowners below the rockfall have built a diversion ditch to guide mudslides to the Animas River. But the most recent runoff has filled the ditch, causing it to overflow.
“Debris is moving into areas we haven’t seen it in recent years,” Knowlton said.
Jon Harvey, a geology professor at Fort Lewis College, has been using innovative technology with a drone to create 3D maps that track how the debris pile has moved since October 2016.
The project surveys the conditions every six months in fall and spring to document movement that occurred during snowmelt in the winter and monsoons in the summer.
During the drought-stricken winter of 2017-18, Harvey recorded virtually no movement. But heavy rains in July 2018 brought down a fair amount of material, which closed the road for a time.
This year, Harvey expects a lot of activity.
“And if we get wet spring storms on top of the snow still up there, that’s about the worst-case scenario,” he said.
About 3 feet of snow remains at the top of Missionary Ridge, Knowlton said, and with temperatures expected to remain in the mid-60s this week, more mudslides could be on the way. The National Weather Service is not calling for rain until at least Saturday night.
Knowlton said the rockfall is most active at night, after the afternoon sun beats down on the west-facing hillside. Residents who live near the rockslide, including Knowlton, can hear the boulders as they trudge down the mountainside.
Tom McNamara, La Plata County emergency management coordinator, said it is far more difficult to try to clear the road when the mudslide is wet. It’s like moving concrete, he said.
“It’s moving about a foot a minute,” he said. “Just creeping like lava. But the more it dries out, the more settled it’ll become.”