Mapping the Hazards Along Denali's Only Road

U.S. Geological Survey Fact Sheet 2020–3016, April 2020. By Adam M. Hudson and Cal A. Ruleman (USGS) and Denny M. Capps (National Park Service).

One road, a region's lifeline

Denali National Park in Alaska surrounds the tallest mountain range in North America, with about 6 million acres of wild land used for recreation, subsistence hunting and gathering, cultural preservation and research. In 2017 it drew more than 600,000 visitors, bringing $632 million in visitor spending to the park and nearby communities, 8,150 jobs and $924 million in economic output — a major economic engine for Alaska.

Yet it has one road: the 92-mile Denali National Park Road, mostly gravel, with only 16 percent paved. It is the only way to most of the park's visitor centres, staff facilities, campgrounds and businesses, so the park's success — as a destination, an economy and a safe place for visitors, residents and staff — depends on the road holding up.

Denali National Park, the course of the Park Road and the boundary of the new geologic mapping. Inset: the park in Alaska. USGS.

A landscape on the move

The road crosses steep, constantly changing ground, shaped by the rapid uplift of the central Alaska Range, by glaciers and meltwater, and by permafrost heave.

• Rockfall, floods and landslides damage the road every year, forcing closures and costing millions of dollars in repairs and maintenance.
• The park lies on a major fault system and has many earthquakes a year, most too weak to feel. A magnitude 7.9 earthquake shook the region in 2002, and shaking in 1953 set off at least one major landslide, which formed Bergh Lake beside the road.

Left: the Pretty Rocks landslide (dashed outline) and where the scarp photo was taken. Right: a 2-metre landslide scarp across the road, formed in winter 2018 by a slump at the head of the slide. Photos: Denny Capps, National Park Service.

A new map of the ground

In response, the USGS and the NPS have made a new, high-resolution surficial geologic map of the road corridor, to help infrastructure planners find unstable slopes, active faults and hazardous ground. It adds detail to earlier, broad-scale maps of bedrock, using detailed radar elevation data (Interferometric Synthetic Aperture Radar) for Alaska. It maps the history of the Holocene and Pleistocene sediments left by glaciers, braided rivers, alluvial fans and landslides — and the active faults that cut across these young deposits.

Planning a reroute near Polychrome Overlook

The road's most urgent problem is the steep country around Polychrome Overlook, where it crosses several active landslides. Keeping the road on its current (2020) course is costly and risky for staff and visitors, but closing it to vehicles would badly restrict access and hurt the tour businesses operating in the park.

So the NPS is weighing a reroute, and the new mapping will help identify hazards and the kind of ground along the options, so engineers can build a road that is safe, cost-efficient and resilient for the long term.

Proposed reroutes around the landslides near Polychrome Overlook. USGS.

The new geologic map along Polychrome Overlook: large known landslides (Qls) on the current road, compared with the river and glacial deposits, at lower landslide risk, along the southern option. USGS.

Dating glaciers, measuring faults

The USGS and NPS are working with Purdue University to date glacial sediments using cosmogenic nuclide exposure methods. Several generations of Pleistocene glaciers crossed the Sanctuary River valley south of the road, and active faults cut across all of the sediments they left. The oldest deposits are offset most along the faults, and modern streams least. Dating the deposits therefore tells both the history of glaciation in the central Alaska Range and how fast the faults are slipping — the earthquake hazard to park infrastructure.

Pleistocene glacial deposits, older sediments and fault traces near the Sanctuary River, along the road corridor. USGS.

Sources

• Hudson, A.M., Ruleman, C.A., and Capps, D.M., 2020, Assessing geohazards to the Denali National Park Road with geologic mapping: U.S. Geological Survey Fact Sheet 2020–3016. https://doi.org/10.3133/fs20203016 · https://pubs.usgs.gov/publication/fs20203016
• Figures, their captions and the paragraph on reroute planning are recovered from the fact sheet's PDF.
• The fact sheet describes the reroute options as north or southeast of the current road in its text, and as north and south corridors on its map; this page does not choose between them.
• The fact sheet calls the Pleistocene "more than 2.6 million years old"; Pleistocene sediments are younger than about 2.6 million years, so that phrase is left out.
• Work supported by the USGS National Cooperative Geologic Mapping Program, the USGS Natural Resource Preservation Program and the National Park Service.
• Rewritten in hubnx's own words.