Twice this past week, big orange trucks commandeered Glenwood Canyon in western Colorado during rain, shutting down Interstate 70, one of the West’s key routes, a highway long celebrated as a national engineering wonder.

These disruptions resulted from a recent decision by state and federal authorities to extend a policy of closing I-70 in the canyon whenever the National Weather Service issues flash flood warnings. Precautionary closures must continue, authorities say, until geologists determine rock and mud is less likely to catapult downward.

All canyons naturally produce slumps and slides. But in 2020 the Grizzly Creek Fire scorched 51 square miles around Glenwood Canyon and burned away trees and shrubs that once stabilized super-steep cliffs, leaving chutes and gullies like loaded guns above the millions of cars and trucks rolling below by the Colorado River.

Now even relatively light rain — a quarter-inch over 15 minutes — can create havoc, federal geologists and meteorologists warn. CDOT records show agency crews have closed I-70 from Glenwood Springs to Dotsero 25 times since the fire, prohibiting travel for one to 12 hours each time.

The economic cost to the nation of an I-70 closure is estimated at $1 million an hour.

This emerging new norm for managing hazards on an essential highway through Glenwood Canyon reflects difficult decision-making that prioritized public safety. Deliberations like this are expected to widen as cascading impacts of climate warming include increased avalanches in addition to flooding and landslides. Beyond Glenwood Canyon, more land around the West is becoming more unstable as fires burn bigger and hotter — especially in Colorado where wildfires intensifying over several decades amid rising temperatures, drought and bug infestations have left 34 scars covering more than 700 square miles.

Summer rain this year in Glenwood Canyon didn’t trigger slides that caused damage along I-70 like those in 2021 that forced an economically devastating two-week closure. But debris flows off burn scars elsewhere around Colorado have hammered roadways.  Rainfall over the last month unleashed mudslides and boulders that hit Highway 14 west of Fort Collins near Drake; Highway 50 west of Canon City; roadways north of La Veta Pass in southern Colorado; Highway 125 north of Granby; and Highway 139 north of Grand Junction, according to CDOT and federal bulletins.

There’s growing scientific evidence that climate warming favors intense rain over shorter periods — all-at-once bursts from air that holds more moisture — which officials say may require increased travel restrictions.

The National Weather Service has forecast more rain in the coming week.

“It takes very little rainfall to create major issues on those burn scars,” said Paul Schlatter, a federal meteorologist who specializes in use of radar to anticipate flooding on the altered terrain.

“Those burn scars are going to be a major headache for at least two or three more years. You might as well have paved a parking lot over that mountain landscape, poured concrete over it,” Schlatter said. “No water can infiltrate the soil. It just flows right on down the slopes and picks up whatever rocks, trees and soil are in the way.”

Re-vegetation would be nature’s way of creating a new equilibrium. This traditionally took five years after major fires, though drought, heat waves and extreme scorching have raised questions about whether plants will return in line with old patterns.

Nobody anticipated this sort of trouble when federal crews built I-70 through Glenwood Canyon in the middle of the Rocky Mountains — 40 viaducts and bridges, tunnels, and steel-reinforced concrete to better connect the nation. This $490 million highway, completed in 1992, was celebrated as a resilient and environmentally-sensitive masterpiece symbolizing the success of the original U.S. interstate system. It won scores of American Society of Civil Engineers and other awards for outstanding planning and design.

But risk analysis at the time didn’t contemplate wide de-stabilization of steep canyon walls. The final impact statement concluded: “Neither the base flood used for this project nor the ‘worst case’ condition will result in the potential loss of private property or introduce conditions hazardous to health or life.”

Now deadly hazards around Colorado are multiplying, and state officials — scrambling to boost flash flood resilience in burn scar areas — seek federal funding. Much depends on whether natural revegetation re-stabilizes rapidly eroding slopes, said Kevin Houck, chief of watershed and flood protection for the Colorado Department of Natural Resources.

“We need to do whatever we can to try to help that out,” Houck said. “But there are definitely limitations to what these projects are able to accomplish. The public has to be reasonable. We cannot just go out and solve the problems. It’s going to take time.”

CDOT’s chief engineer Steve Harelson said computer analysis tools developed after the 2013 floods along headwaters of the South Platte River have helped weigh benefits of repeated short-term fixes versus overhauls of flood-prone highways around the state.

Where avalanche control by shooting explosives from helicopters at snow cornices is becoming too costly, CDOT officials are preparing to install fixed blasters like those west of Denver above Berthoud Pass. They’re looking at Loveland Pass, the canyon along I-70 between Frisco and Copper Mountain, and Red Mountain Pass between Silverton and Ouray, Harelson said. And, CDOT teams this summer began working with Garfield and Eagle county officials on plans for improving Cottonwood Pass, an unpaved road linking Carbondale and Gypsum, as an emergency alternative to I-70 for when Glenwood Canyon is closed.

County officials and residents, including ranchers, oppose construction of a high-traffic route that tourists might use to reach Aspen.

“Everything we do is a balancing act,” Harelson said. “We’ve got to balance the resources we have with the needs of the highway system and the citizens of Colorado. We cannot harden everything perfectly so that nothing bad ever happens again. But we can try to play the percentages and improve our most vulnerable roads.”

This week, CDOT engineer Andrew Knapp, who has led efforts to protect I-70 against landslides in Glenwood Canyon, hiked up a goat path toward the rim 1,300 feet above the Colorado River to assess stability. He found evidence of re-vegetation — oak brush three feet high on the canyon walls shooting out roots similar to aspen trees that help hold soil and absorb water. But pine trees may not return.

State and federal officials plan periodic check-ins to evaluate their extended I-70 closures.

Knapp looked down on contractors working with a yellow loader to strengthen canyon slopes using rocks and makeshift walls built using more than 100 white sacks of gravel weighing 1.5 tons each. CDOT deployed helicopters this month to place barriers high in chutes as part of their continuing Glenwood Canyon Emergency Project.  Knapp has asked U.S. Forest Service rangers to help out atop the canyon by sawing down dead trees and laying trunks horizontally across gullies — “beaver dam analogs” to slow potential slides.

CDOT contractors also plan to install large rectangular wire cages filled with rocks adjacent to I-70 to contain mud and rock and at least reduce damage to I-70 after slides.

“The weather patterns of the past aren’t going to be the weather patterns of our future. Landscapes, as they stand today, aren’t going to be the same,” Knapp said.

“Nothing’s a given in the future. We have to be flexible and understand that we have to take what Mother Nature gives us,” he said.

“We understand how impactful those closures are to the state, to communities, to people who implement the closures. It’s not taken lightly. It’s a lot of work. We don’t want to continue to do these if they don’t make sense.”

He’s working with a U.S. Geological Survey team equipped with detailed topographical maps drawn from aerial imagery. Immediately after the August 2020 Grizzly Creek fire, USGS officials conducted stability studies and warned that landslides threatened I-70 — a year before they happened.

On one hand, hundreds of thousands of tons of rocks, trees and mud catapulting off the burned terrain above I-70 means less material is left to threaten vehicles in the future, said Jason Kean, a Denver-based USGS research hydrologist leading a team to assess rising risks across the West.

“There’s not as much ammunition in the gun as there used to be. That’s not to say you cannot have more debris flows. But there’s been a lot scoured out that is now in the Colorado River. The volume coming down on the road would not be as big.”

Yet “the stakes are high. We want to play it conservatively,” he said, citing the closures.

The latest wildfire burn scars in New Mexico and California destabilized broader areas, creating more havoc as rain hits. “We’re simply exposing more steep terrain, making more terrain vulnerable. So we’re going to see more of these impacts,” Kean said.

Climate research on how warming temperatures lead to more rain falling at once — intense storms – is closely watched. “We want to know where we are headed. If we’re still in this trend of increasing burn acreage across the West with increasingly severe fires, and we’re also going to have heavier rain, then we’re going to have more of these problems.”