A stretch of the Denali Park Road sits atop a creeping landslide. And it’s picking up speed.

These photos show the same area of the Denali Park Road in September and March. Over the winter, a slow-moving landslide pushed a 100-yard stretch of road about six feet downhill, leaving this drop-off behind. (Photos by National Park Service)

Officials at Denali National Park and Preserve are studying whether the existing path of the park’s 92-mile road can be spared from a creeping landslide, in what scientists say could be a preview of Denali’s future as its permafrost thaws.

The creeping pile of earth, at Polychrome Pass halfway along the road, is not just rocks falling down a hillside — it’s whole section of mountain, including the road, that’s slowly sliding downhill.

Just between September and March, the landslide pushed a 100-yard stretch of the gravel road six feet, leaving a head-high cliff where it once connected to to the rest of the route. Maintenance staff had to fill the gap with rock and gravel to make it passable.

An image showing the site of the Pretty Rocks landslide. (Courtesy National Park Service)

And the pace of the slide is accelerating. In 2016, it pushed the road three feet down the mountain. In 2017, that pace doubled. And in 2018, it doubled again, moving a full 12 feet over the course of the year.

Since most tourists access the park using the road, the “Pretty Rocks” landslide poses an “existential threat” to Denali’s current visitor model, according to a new National Park Service report. And Congress is paying attention: With support from U.S. Sen. Lisa Murkowski, lawmakers earlier this year asked the Park Service to report on the potential for a re-route and to deliver quarterly updates on risks.

Park managers are fixing problems at the site as they crop up and hope to maintain the road’s existing path. But they are also exploring other options, in case the slide keeps accelerating.

Ideas include bridging over or tunneling under the slope. Structures could be built to block the landslide, or rock could be removed from above to reduce the risk.

Or, managers could move an estimated five miles of road. But that idea faces logistical and financial obstacles. Congress would likely have to approve any new alignment through Denali’s designated wilderness, according to the report. And while park officials declined to give a cost estimate, Murkowski said it would be more than tens of millions of dollars.

“It’s basically taking all the funding that we would see as a state, for what would go to parks in the first place,” Murkowski said in an interview last week, after a speech at an Anchorage conference. “To rebuild this, or to do an alternate route, is going to be exceptionally expensive. It is just not easily done.”

The landslide is the park road’s most imminent threat, but it’s not an isolated one: Geologists have identified more than 100 other unstable areas along the road corridor.

Experts say Denali’s broader landslide problem is a natural result of putting the park in a dynamic, scenic environment. But they also say global warming is likely to worsen the risk by thawing permafrost that helps stabilize some of the area’s rugged terrain.

Temperatures have been far above average at the park headquarters every year since 2014, said Rick Thoman, a specialist at the Alaska Center for Climate Assessment and Policy. In 2016, the average temperature was the highest in a record dating back to the 1920s, at four degrees Fahrenheit above normal, he added.

One study predicts that the portion of the park underlaid with permafrost will shrink from 50 percent in the 2000s to 6 percent by the 2050s.

“If we want to understand how to maintain infrastructure in areas where we have permafrost like this, we need to understand how that’s going to respond to continued warming,” said Denny Capps, the Denali park geologist.

It took 15 years for the Alaska Road Commission to build the park road, which was finished in 1938.

Planners originally eyed a lower route near Polychrome Pass. But park service officials pressed for the higher path along a mountainside because of its “expansive views,” after a site visit by their chief landscape architect, according to a 2011 history of the road.

Building at that spot required extensive blasting, and the dropoffs on the south side are so precipitous that some nervous early tourists insisted on walking the stretch of road, rather than driving. Today, the majority of Denali’s 600,000 annual visitors access the park through the road as passengers on tour buses; during the summer, the road is generally closed to private vehicles after 15 miles.

A tour bus drives past the site of the Pretty Rocks landslide at Polychrome Pass, in Denali National Park and Preserve. The old road bed can be seen below. (Photo by Joey Mendolia / Alaska’s Energy Desk)

Landslides aren’t a new challenge for park managers. Two decades ago, maintenance staff were fixing small cracks in the road caused by the Pretty Rocks slide. And periodic mudslides and landslides have covered the park road and interfered with traffic.

But at Pretty Rocks, the problem is quickly worsening and threatening to become unmanageable.

The process causing the slide is a basic one. Rockfall and debris flows deposit loose material on the slope above the road, and it can only get so deep before it starts sliding downhill — Capps compares it to dumping sand on a pile.

A group of cyclists pedals on the Denali Park Road through the site of a creeping landslide at Polychrome Pass. (Photo by Nathaniel Herz / Alaska’s Energy Desk)

It’s been hard for park staff to determine how much global warming and permafrost thaw are contributing to the landslide, in part because the landslide is moving so quickly that it breaks their monitoring equipment. But they have confirmed that the permafrost beneath the site is near thawing, and they know that when it thaws, it does less to stabilize the slope.

“When it’s frozen, it helps hold things together,” Capps said. “When it thaws, it releases.”

Experts expect that as permafrost thaws across Denali, more landslides are likely to develop at other places along the road. And the park is working with University of Alaska Fairbanks researchers to build a model to show where the risks are highest, by mapping data on past landslides with risk factors like slope angle, vegetation and permafrost.

One of the researchers, Louise Farquharson, said the focus on the Pretty Rocks landslide threatens to distract from what is likely to be a broader problem.

“It’s like we should be triaging, and we’re just focusing on one patient,” she said.

The businesses that give tours along the park road and use it to ferry visitors to Denali’s lodges are concerned about the park’s landslide problem, and Alaska’s tourism industry has pushed Murkowski and the Energy and Natural Resources Committee she chairs for more federal spending on park maintenance.

A 2016 landslide that limited traffic along the Denali road cost one company $20,000, according to federal testimony last year from Sarah Leonard, president of the Alaska Travel Industry Association. The company chartered flights to move guests to and from its lodge and lost customers who were planning day trips, Leonard said.

A co-owner of one lodge at the end of the road said he wants to see Denali’s managers continue drafting response plans, so that an individual landslide doesn’t force a hasty reaction.

“I think now is the time to start recognizing: Maybe it’s not a question of if, it’s a question of when. So let’s not wait,” Simon Hamm, co-owner of Camp Denali, said in a phone interview from the lodge. “Let’s start doing something thoughtful and artful now, so that in the heat of the moment we’re not dropping bulldozer blades and plowing up wilderness.”

In the past year, managers have become increasingly focused on the problem, Hamm said. He added: “Good people are making good efforts to try to come up with contingencies.”