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Landslide Lessons: Scientists Study Impacts, Assess Statewide Slide Hazards

State scientists climb carefully up on the edge of the Cotton Brook landslide in Waterbury.
John Dillon
State scientists climb carefully up on the edge of the Cotton Brook landslide in Waterbury.

One of the biggest landslides ever recorded in Vermont is now giving scientists a living laboratory to learn what happened and to assess other slide-prone areas statewide. 

On May 31, the Cotton Brook landslideswept away 12 acres of hillside in the Mt. Mansfield State Forest in Waterbury. No one was hurt as the slide severed a popular recreation trail and tons of debris leveled a section of forest and damaged a nearby stream. 

Geologists and biologists are now studying the event and its aftermath. But to learn more about what a major landslide does to the land, water and people around it, you first have to look downstream – way downstream.

Below the Waterbury dam, about five miles as the crow flies from the slide site, the Little River is cloudy with silt.

Aaron Moore, a state biologist who works out of the agricultural and environmental lab in Randolph, used a wide net to catch bugs that should be crawling around on the underwater rocks.

"You know, we don't really see streams with this kind of turbidity a lot," Moore said, "and it's even more pronounced when we go up to Cotton Brook."

Two state biologists stand in a stream
Credit John Dillon / VPR
State biologists Aaron Moore, left, and Jim Deshler look at downstream impacts of the landslide on the Little River in Waterbury.

Moore and his colleague Jim Deshler, who works for the Department of Environmental Conservation, were looking for macroinvertebrates — the tiny creatures that usually inhabit clean, clear water. They're not finding much.

"What you would normally see is a lot of large-bodied macroinvertebrates crawling around," Moore said. "Like stone flies and caddis flies are primarily the ones that you'd ... be able to see really well in the net, but you don't really see any of them at the moment."

These bugs, and the much smaller ones that Moore will look for later with a magnifier back in the lab, are indicators of stream health. They need water rich in oxygen and low in pollution. Silt can smother them, or drive them away — as what seems to have happened on the Little River.

"Based on what we saw today, I think ... the density and richness of this site, it's going to be pretty dramatically affected," Moore said.

WATCH: A drone video via Vermont Agency of Transportation of the landslide site [June 12]

The scientists can't really predict how long the stream will stay this way. But considering how much sediment the landslide released, the Little River may stay murky for a while.

Closer to the landslide site, on the other side of Waterbury Reservoir, Cotton Brook flows with the color of cappuccino. The silt in this stream is so thick you can’t see a quarter inch below the surface.

The bug life here is even more affected than on the Little River downstream. Deshler had a hard time even moving the rocks to look for the bugs, because the pebbles are stuck together with concrete-like sediment left in the slide's wake.

"It doesn't leave a whole lot of space for the bugs to live in the substrate," Deshler said. "You can't even get the rocks out of the substrate. ... There's no space for anything to live."

Deshler's net came up empty.

A biologist peers into a net standing in a brook
Credit John Dillon / VPR
Jim Deshler hunts for macroinvertebrates in silt-choked Cotton Brook.

About 250,000 cubic meters of material slid down the hill during the Cotton Brook landslide, with about 100,000 cubic meters ending up in the brook and the expanding delta downstream — and geologists want to know if more is coming.

I met State Geologist Marjorie Gale for the short drive up a closed road to the slide site. She pointed out the vast expanse of bare ground as we neared it on the road.

The forest is wiped clean 100 meters up the slope. Way up the slope, small sections of hill hang almost in midair. Trees are toppled at the base, and a huge mound of earth has dammed the stream and created a little pond.

Why the slope failed at that moment in May isn't known. But, Gale said, you can blame it all on ancient Glacial Lake Winooski.

Some 14,000 years ago, a large lake covered a vast area of central Vermont. When the lake and its streams and deltas drained, they left behind a mix of sediment up to 1,100 feet up the hillsides. Thick layers of sand and gravel overlay fine silty clay, often on steep slopes. And that clay is slippery.

Last spring the ground was extremely wet from rain and snowmelt. Gale said gravity and the lack of friction did the rest; the sediment on top of the clay began to slide.

"Once it happens in just one small place — the set of right conditions, the overlying weight, the right amount of water, the right angle — once that starts, it then just propagates," Gale said.

Landslide aftermath
Credit John Dillon / VPR
The Cotton Brook landslide sent about 100,000 cubic yards of material into the stream. The site is still active, and dangerous.

If geology is the study of the earth across billions of years, the Cotton Brook landslide is change in real time: Vermont's version of an active geologic event, like a volcano or an earthquake.

"It's always interesting when you can see a modern process forming. ... And this is a catastrophic event," Gale said. "It's not a slow change over time."

And it's still happening. Gale pointed to the spot where a couple of us are standing. She said this particular piece of hillside started to ooze mud and then move a few weeks ago.

"So you might want to come this way," she said with a laugh. With that gentle warning, we quickly moved.

The state is working on a statewide landslide hazard assessment, so Gale and her colleagues are keenly interested in what happened here and why.

"Something like this certainly provides an example of what could happen in some of these areas that we're identifying — and the importance of having a statewide landslide hazard map — because failure tends to happen where it's happened before," Gale said. "Those are the unstable areas and they tend to stay that way."

Glacial Lake Winooski stretched west from what's now Richmond to south and east to Williamstown, with lots of fjord-like fingers cutting through the hills and mountains. That mean there's the potential for other sites to have clay and sediment built up on steep slopes.

Gale said if future slides like the Cotton Brook landslide happen around people or houses, the results could be devastating.

"It's almost 10 times the size of the landslide that happened in Jeffersonville … [in] 1999, right near the elementary school. And we thought that one was impressive," Gale said. "But you know, this sort of thing in a populated area would have been pretty tricky."

WATCH: Another drone video via Vermont Agency of Transportation of the landslide site [June 12]

While Gale and her colleagues assess the hazards, state land managers are dealing with other, perhaps more immediate, concerns at Cotton Brook.

Foster's Trail, a popular hiking and mountain bike route, was severed by the slide and is now closed. Walter Opuszynski, a field recreation specialist for the Department of Forests, Parks, and Recreation, is worried that even though the area is closed, people may try to backcountry ski or hike the exposed steep slope.

"We're at the point where we're trying to think through all those management concerns, come up with different options that we can implement in the field," Opuszynski said. "We're making recommendations to [Forest, Parks, and Recreation] Commissioner Snyder based on data that we've been getting from the geologists primarily."

And to underscore that point, as we left the site and drove down the road that's closed to hikers, a couple ambled up the trail, directly below the slide.

Opuszynski asked them if they had seen the "road closed" sign. They had but said they thought it applied to a separate section of road. He asked them if the sign should be bigger, or placed right in the middle of the road.

Gale was a little more blunt: “And you're in one of the worst places to be standing by the way. At least get to one of the sides,” she said.

"We're at the point where we're trying to think through all those management concerns, come up with different options that we can implement in the field." — Walter Opuszynski, Vt. Department of Forests, Parks, and Recreation

Another concern is that as precipitation increases from climate change, other water-soaked hillsides could become unstable.

Gale hopes to tap the public's eyes and smart phones to learn more about landslides statewide. She said she'd like people to use a state website to help build the database.

"They can just take a picture, they can ... give us an approximate location. If they're on their cell phones, the app will locate them, and they can just put a point so that it helps us find the landslides," Gale said.

Meanwhile, this landslide is still moving. Gale said she expects the area to stay active for several years.

Correction 8:48 p.m. A previous version of the post referred to Walter Opuszynski as a land use specialist; he is a field recreation specialist. The post has been updated.

John worked for VPR in 2001-2021 as reporter and News Director. Previously, John was a staff writer for the Sunday Times Argus and the Sunday Rutland Herald, responsible for breaking stories and in-depth features on local issues. He has also served as Communications Director for the Vermont Health Care Authority and Bureau Chief for UPI in Montpelier.
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