September 16, 2021, photo of eastern side of the Pretty Rocks landslide. The displacement of approximately 14 vertical feet seen below the person standing on the stable road surface (for scale) occurred over two weeks following the cessation of maintenance and road use on September 2, 2021. The rate of the slide is causing the Denali Park Road to remain closed in 2022. NPS photo.
About half of Denali Park Road will remain closed during the 2022 visitor season due to a continuous and unstoppable landslide that forced managers to close the road in late August 2021. The Pretty Rocks landslide is at mile 45.4 of the 92-mile park road. Buses will turn around at mile 43 while construction crews work to build a bridge over the landslide.
“We look forward to getting started on the long-term solution at Pretty Rocks. Meanwhile, Denali is ready for visitors in 2022. We will continue to have access to great wildlife viewing, views of Denali, front-country trails, and backcountry hiking and camping,” Brooke Merrell, deputy superintendent of Denali, said in a press release. “Denali National Park and Preserve will continue to work with inholders, neighbors, and partners to ensure an unforgettable Denali experience during construction.”
The slide
Park managers have known about the Pretty Rocks landslide since the 1960s. A roughly 100-yard swath of the mountainside has been slipping for decades, causing the road that crosses that section of land to sink beneath the level of the road on either side. But the slide was slow enough it was never dangerous or a cause for concern. By the 1990s the slide started causing small cracks in the road’s surface and maintenance was required every few years. In 2014, park workers noticed things were speeding up. The park started monitoring the rate of the landslide in 2016, and measured a consistent and rapid increase the slide’s movement. The road was only moving a few inches per year prior to 2014. By 2017 the road over Pretty Rocks was sinking a few inches each month. By the time Denali managers closed the park road late in the 2021 season, the road was slide downhill more than half an inch every hour.
Road crews worked diligently over the last several years to maintain the road so it could stay open for bus traffic. Prior to the road opening at the beginning of visitor season, crews would dump gravel over the slide area and rebuild the road’s height to a passable level. By August 2021, the park says road crews were spreading up to 100 truckloads of gravel over the road every week. Park managers then decided the slide’s rapid movement made it unsafe and unsustainable to maintain a passable road.
What’s causing the slide to speed up? Scientists say climate change is a major culprit. The layer of earth that’s sliding is a slurry of volcanic ash that was deposited about 55 million years ago, and as the earth shifted it tilted to its current slope.
The swath of mountain is now a clay slurry of rock and ice that remains frozen year round. Denali has warmed over the last several decades to the point where the mean annual temperature in the Pretty Rocks area is close to 32 degrees Fahrenheit. The warmer weather and the increase of heavy rains has caused the ice in the permafrost to melt, which in turn allows Pretty Rocks to slide faster and faster.
The solution
Denali National Park and the Federal Highways Administration have explored several options for long-term solutions. The options included rerouting the road permanently south or north of the slide area. Either would likely take years of construction and add more human impact across a natural landscape. The most viable option, the park says, is to build a bridge over the Pretty Rocks slide. The bridge will be about 400 feet long and will be anchored into solid rock on either side of the melting permafrost.
The project, dubbed the Polychrome Improvement Plan, includes multiple phases. The first phase is to build the bridge, which the park is planning to start in summer 2022. Completing the bridge will take at least a year, so the road will remain closed past mile 43 for the entire season. Once the bridge is complete, the road will be reopened to visitor tours.
The next phase of the plan includes building a retaining wall at the nearby Bear Cave landslide and making improvements that would reduce the hazard of potential rockfalls in other areas. The road will remain open during this phase.
The public comment period for the plan closes on February 13.
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