Alpine hillslope failure in the western US: Insights from the Chaos Canyon landslide, Rocky Mountain National Park USA

Abstract. The Chaos Canyon landslide, which collapsed on the afternoon of June 28th, 2022 in Rocky Mountain National Park presents an opportunity to evaluate instabilities within alpine regions faced with a warming and dynamic climate. Video documentation of the landslide was captured by several eyewitnesses and motivated a rapid field campaign. Initial estimates put the failure area at 66,630 m², with an average elevation of 3,555 m above sea level. We undertook an investigation of previous movement of this landslide, measured the volume of material involved, evaluated the potential presence of interstitial ice/snow within the failed deposit, and examined potential climatological forcings at work in causing the collapse of the slope. Satellite radar and optical measurements were used to measure deformation of the landslide in the years leading up to collapse. From 2017 to 2019, the landslide moved ∼5 m yr^-1, accelerating to 17 m yr^-1 in 2019. Movement took place through both internal deformation and basal sliding. Climate analysis reveals the collapse took place during peak snowmelt, and 2022 followed 10 years of higher than average positive degree day sums. We also made use of slope stability modeling to test what factors controlled the stability of the area. Models indicate even a small increase in the water table reduces the Factor of Safety to <1, leading to failure. Material volumes were estimated using Structure from Motion (SfM) models incorporating photographs from two field expeditions on July 8th, 2022 – 10 days after the slide. Detailed mapping and SfM models indicate ∼ 1,258,000 ± 150,000 m³ of material was deposited at the slide toe and ∼1,340,000 ± 133,000 m³ of material was evacuated from the source area. Our holistic approach to the collapse of the Chaos Canyon landslide provided an opportunity to examine a landslide that may be representative of future dynamic alpine topography, wherein failures becomes more common in a warming climate.