Landslides & Debris Flows

Pretty Rocks Landslide

The Pretty Rocks landslide intersects the Denali Park Road near its midpoint at Mile 45.4 and displaces 100 yards (90 m) of the full width of the road. In recent years this landslide has evolved from a minor maintenance concern to one that has caused substantial road restrictions.

The Pretty Rocks landslide has been active since at least the 1960s, and probably since well before the Denali Park Road was built through this area in 1930. Before 2014, the landslide only caused small cracks in the road surface and required moderate maintenance every 2–3 years. However, in 2014, road maintenance crews began to notice a change. By 2016, the movement had increased further, a slump had developed in the road, and a monitoring program was begun. The rate of road movement within the landslide evolved from inches per year prior to 2014, to inches per month in 2017, inches per week in 2018, inches per day in 2019, and up to 0.65 inches per hour in 2021. By late August, 2021, maintaining a drivable surface was no longer possible, and the road was closed at Pretty Rocks. The Polychrome Area Plan was developed to restore reliable road access across the landslide, and bridge construction began in 2023.

Pretty Rocks is just one of several known landslides in the Polychrome area and one of the more than 140 mapped unstable slopes along the entire Park Road. The National Park Service is working with the Federal Highway Administration to manage the park road proactively and holistically in order to restore and maintain access along the entire length of the road.

Read more about the Pretty Rocks Landslide.

Igloo Debris Slide

2018

On September 30^th and October 18^th 2018, the Igloo Debris Slide partially blocked the Denali Park Road. In each event, the Denali road crew cleared approximately 500 cubic yards (450 m³) from the roadway within a few hours. Like the 2013 event, 12’ (4 m) blocks of ice-rich, unconsolidated debris slid on the same or similar unfrozen clay layer. 2013 and 2018 both experienced unusually mild fall weather. Such weather further thaws permafrost, which decreases cohesion and increases pore water pressure. Therefore, this combination of processes can cause and trigger landslides. As more permafrost has thawed and the slide has continued to move, the area of the slide has increased. Several smaller events have filled the road ditch since September 30^th.

Geology and engineering experts from the NPS and Federal Highways Administration expect activity at this site to continue in subsequent years. Therefore, we are currently developing designs to reduce risk at this site and others as part of the Unstable Slope Management Plan for Federal Land Management Agencies.

2013

In late October 2013, road maintenance staff discovered that a 600’ (180 m)-long, 110’ (35 m)-wide debris slide had blocked the park road near Mile 38.

Blocks of ice-rich, unconsolidated debris as thick as 15’ (5 m) 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. 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 trend at the site 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.