Landslides & Debris Flows

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heavy equipment clear a debris flow cause by flooding

NPS Photo / Riley Tingue


A Dynamic Landscape

"Wilderness" is a word that conjures ideas of ruggedness and timelessness for many people - but the reality is that the wilderness of the far north is surprisingly fragile. The massive landscape of this park - an area half the size of Switzerland - can witness dramatic alterations, such as landslides.

aerial view of a mountainside with two circled areas, one high up labelled "current landslide," the other circling a debris pile at the base of the mountain, labelled "older landslide deposits"
Oblique view of the Pretty Rocks Landslide. Background is a composite image from June 15, 2015. Approximate outlines of current and older landslides indicated in red and yellow dotted outlines, respectively.

NPS Photo

Pretty Rocks Landslide

Section last updated: April 16, 2018
The Pretty Rocks Landslide at Mile 45.4 of the Denali Park Road has the potential to disrupt transportation, impact visitor experience, cause resource damage and affect public safety. Over the last two decades, the landslide has evolved from a minor maintenance concern to a threat. During the 1990s, the landslide caused small cracks in the road surface. Between 2016 and 2017, the full width of a 100-yard section of road was dropping up to six inches per month, creating a swale that steepens the road gradient and limits sight lines. If the landslide continues to increase in velocity, we will soon reach a threshold where the National Park Service can no longer maintain the current road alignment.

The management of Denali National Park and Preserve has made substantial gain in its knowledge of the Pretty Rocks Landslide and associated risk reduction techniques; however, ongoing and proposed work will provide higher-value decision-making tools. Long-term solutions are likely very costly, so park managers must make well-informed decisions early in the process. Changing environmental conditions such as permafrost thaw and rainfall that is increasing in amount and severity will aggravate the issue. When the efforts listed are completed, managers will have the tools to prioritize risk-reduction actions. This process also offers a transition from reactive to proactive management of this and other geohazards.

  • Federal Highway Administration and NPS staff installed monitoring equipment in two bore holes below the road surface in 2003. The bore holes confirmed the presence of permafrost and substrate type; however, landslide motion sheared the monitoring equipment before staff obtained measurements.
  • Landslide velocity increases immediately after material is added to the swale created by the slide. This was particularly noticeable in 2014 and is consistent with landslide science -- more material increases the weight and translates into additional movement.
  • NPS maintenance staff adds minimal material to the swale as needed to provide a safe road surface. However, road gradient is steepening once again and sight distance is becoming limited. Soon, more material must be added for safe passage and this may increase movement.
  • The road hazard was quantified in 2016 through the Unstable Slope Management Program for Federal Land Management Agencies (USMP) and ranked highest of 141 unstable slopes analyzed along the road.
  • Also in 2016, park staff created a probability-based landslide susceptibility model and associated map.
  • Federal Highway Administration and park staff collaborated with the Army Corps of Engineers in 2016 to determine subsurface characteristics of bedrock and permafrost affecting the site. Mapping showed permafrost containing massive ice below the road. Permafrost is known to be thawing regionally.
  • At least once a year between 2015 and 2017, aerial photogrammetry of the site was taken. The photos and associated digital elevation models validate past observations of landslide velocity and show displacement patterns.
  • Denali hosted a multidisciplinary risk assessment workshop in 2017 that identified unstable slopes as the greatest risk to the road with permafrost degradation also rating high as a causative agent.
  • Three highly-rated unstable slopes along the road were prioritized in 2017 for risk reduction work: Pretty Rocks (Mile 45); a recurrent landslide at Bugstuffer Creek (Mile 52); and a recurrent landslide at Toklat Bluffs (Mile 53).
  • The USMP rating and susceptibility model are updated as necessary. USMP ratings change as site conditions evolve. While the first susceptibility model is now complete, new information or techniques will improve the model and subsequent outputs.
  • A multidisciplinary work group is monitoring and responding to geohazard threats along the road. The team will develop short- and long-term plans. A report summarizing our existing knowledge of the landslide should be complete this summer.
  • The landslide will be monitored with aerial photogrammetry surveys and analyses annually.
  • USGS and park staff are mapping the surficial geology of the entire road corridor. The data will help management of unstable slopes identified in the USMP and craft potential reroutes.
  • Federal Highway Administration and park staffs are investigating the feasibility of rerouting the road if stabilization efforts are impractical or fail. The reroutes are complex because of their length (five miles), terrain and location in designated wilderness.
  • In the summer of 2018, five new boreholes will be drilled for instrumentation. The drilling and instrumentation will provide data on substrate composition, ground temperature (permafrost), water level, and ground displacement location and magnitude.
(contingent on funding)
  • Researchers from University of Alaska Fairbanks will create a permafrost model to predict ice-rich permafrost thaw through time. In turn, the researchers will use the model, borehole data, and other information to create a displacement model of the landslide to inform risk reduction techniques.
  • In coordination with U.S. Geological Survey, park staff will conduct a geohazard vulnerability assessment of the road, with emphasis on the landslide. Managers must understand the geohazards and associated risks, as well the ways in which visitors, staff and assets are vulnerable to the hazard.
  • Park staff, along with Federal Highway Administration, will conduct a benefit/cost analysis for the most significant unstable slopes in the USMP. We will then use the benefit/cost analysis to develop a prioritized list for programming based on the greatest benefit to stakeholders.


Igloo Debris Slide (2013)

In late October, road maintenance staff discovered that a 600’-long, 110’-wide debris slide had blocked the park road near Mile 38.

Blocks of permafrost-frozen, unconsolidated debris as thick as 15’ and the size of a small cabin had slid on a slippery, unfrozen clay that acted as the failure plane. With winter snows held off by unseasonably warm weather, the Denali road crew managed to clear the road of debris after considerable effort.

The trigger for the slide remains unknown. Ground, aerial, and satellite imagery of the site in the years and months before to the event indicate that a small slide had previously occurred here, groundwater seeped from the area, and the ground was beginning to move slightly (see imagery). In the days preceding discovery of the slide, the area was experiencing temperatures that fluctuated near the freezing point. Therefore, the forces associated with the expansion of ice during the repeated freezing and thawing of water near the surface may have triggered the slide. Alternatively, we also know that a thick layer of permafrost slid on an unfrozen layer of clay.

Regionally, permafrost is thawing; while the local trend is presently unknown, thaw in the area would be consistent with regional trends. Therefore, it is possible that the permafrost thinned through the clay layer, which triggered the slide. Many other triggers are also possible and are being examined.

The exact timing of the slide also remains under investigation. The last confirmed passage through the area before discovery of the slide occurred on October 12th. Anyone that traveled in the area of the slide between the 12th and the 23rd is encouraged to contact our park geologist. The Alaska Earthquake Information Center also attempted to determine the timing of the slide; however, due to its relatively small size and apparent slow movement, it is possible that the slide did not create a distinctive seismic signal like larger, faster events.

The future hazard to the Denali Park Road to debris slides and other mass movements is under active investigation. In the spring, when the ground thaws, additional activity at the Igloo Debris Slide can be expected. A ditch to capture these materials has been excavated. If deemed necessary, Denali National Park staff will institute additional protective measures with the assistance of the Federal Highways Administration. Denali staff has already begun the process of analyzing other sections of the Park Road for similar problems so that potential hazards can be mitigated.


Landslide & Debris Flow Science Stories

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    Last updated: June 25, 2018

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