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On July 5, 1998, a massive rockfall carrying boulders the size of houses came tearing down Missionary Ridge in the Animas Valley, the scar on the forested hillside stands out to this day.

The rare collapse caused quite a stir. Many thought there had been an earthquake or massive lightning storm, and several people mistakenly reported that the cloud of dust was smoke from a forest fire.

No one was hurt, and no property was damaged. But the event served as a reminder of the inherent risks of living in an area prone to landslides.

Twenty years later, the rockfall is still on the move.

For the past two years, Fort Lewis College professor Jon Harvey and his geology class have used innovative technology with a drone to track how the debris pile is moving down the ridgeline north of Durango.

The goal is to predict the rockfall’s future movements and, as a result, protect homeowners below.

“I think it’s more active than most people would recognize,” Harvey said. “This is a living, moving thing that’s gotten so much bigger and seems to be evolving over time.”

A major eventButch Knowlton, who has been director of emergency management for La Plata County since 1976, lives in the Animas Valley, not too far from the ridgeline that collapsed 20 years ago. When he received the call about the landslide, he knew the spot.

“There had been a huge crack there formed over thousands of years,” Knowlton said. “So it was a natural process, but it was a major event.”

According to The Durango Herald archives, a massive slab of Dakota sandstone fell 600 feet down the hillside, cutting a swath 200-feet wide and impacting an area of about 7 to 10 acres.

An estimated 50,000 cubic meters of rock broke away from the cliff, previous reports said.

In the rockfall’s path, all vegetation was destroyed. Knowlton said he saw a 10-inch-wide spruce tree snapped in half.

“It’s amazing the force of nature that took place that day,” he said.

The rockfall, however, didn’t come close to any homes. Instead, the bulk of the debris settled in the upper part of the hillside. It’s there that the real danger to people down below started.

Over the years, the rock pile has slowly oozed its way down the hillside. In 2005, mudslides started to occur with some frequency, closing East Animas Road (County Road 250), sometimes for days and weeks.

The last closure on East Animas Road was in February 2017, when snowpack levels near record highs began to melt. The heavily used road was closed for an entire month.

This became such an issue that Knowlton asked the National Weather Service in Grand Junction to put the spot on its watch list. This way, when a storm is approaching carrying heavy rain, the county will be notified.

“It’s a problem and something we watch very closely,” he said.

In 2008, La Plata County commissioned a study that verified these concerns. Although the rockfall at that point had occurred a decade before, the area was unstable and prone to landslides.

The debris pile is triggered in two ways: either by a sudden, heavy downpour that carries material down the slope or, perhaps even more destructive, the slow melt of snowpack that gets into rock formations and breaks apart weak layers.

About five homes are in an area considered at risk from potential landslides, Knowlton said. Since these concerns gained traction, homeowners have conducted a number of mitigation measures, including creating diversion banks.

“So far, it’s worked adequately,” Knowlton said. “But anytime we get rain, it could bring down a lot of material. There’s rocks up there as big as your house.”

Mapping debris movementFLC’s Harvey moved to Durango about three years ago. As a geologist who studies rocks on the surface, the scar of the Missionary Ridge landslide immediately caught his attention.

With the last major study of the slide nearly a decade old, Harvey wanted to bring a new approach to an old problem by monitoring debris movement with new technologies.

The first field study was conducted in October 2016. The class used a drone to create a 3D map by taking hundreds of pictures from different angles. They have since created the same maps every six months, in spring and fall.

The maps show the slow movement of debris on the slope, as well as the larger events that are less frequent but more visible. From October 2016 to April 2017, they could see movement that led to the closure of East Animas Road in February of that year.

Something else changed in that time period that has raised alarm. A huge boulder, the size of a house, became wedged in a narrow shoot with weak layers of sandstone on either side.

Knowlton, too, noticed this change. He estimated that the wedged boulder has backed up 40 to 50 vertical feet of rock, boulders and other debris. Essentially, the boulder has acted like a dam, holding back tons of material.

“That one definitely has us concerned,” he said.

Harvey would like to see his study help with direct risks like this.

His geology class is considering reaching out to FLC’s engineering department to talk about the possibility of creating a detection monitor that could alert residents in the rockslide’s path should a major event occur.

“I think there’s a credible concern that, as this thing evolves, what happened in the past doesn’t fully describe what could happen in the future,” he said. “So I would like for what we’re learning to translate somehow to better public information.”

Knowlton said all along the Animas Valley, as well as in other parts of the county, there are spots ready to break loose. It’s part of living where we live.

“Whether that happens today or in 500 years, that’s something we can’t predict,” he said. “Mother Nature has a process, and sometimes man gets in the way. We never know when it’s going to occur, but we have to be aware of it.”

jromeo@durangoherald.com