The 2023 Island Lake Landslide in the Uinta Mountains, Utah as an Example of an Emerging Climate Hazard in Mountain Regions

A landslide in July, 2023 mobilized loosely indurated clastic sediments at 3780 m elevation on the steep glacial headwall near Island Lake in the Uinta Mountains (Utah).  Sediment mobilized by the failure was conveyed ~10 km downvalley by streams, turning a chain of connected lakes a striking orange color.  This coloration persisted until the lakes froze at the end of October, and was still visible in satellite imagery when the lake ice cover melted the following June.  The longevity of this effect testifies to the involvement of particularly fine-grained material that remained suspended in the water.  Previous work at a similar elevation 7 km to the east documented the presence of orange-colored soils rich in clay-sized (<2 µm) material shown by XRD to be smectite.  Grab samples (n=32) of sediment collected from the landslide area in August, 2024 were assessed for color (by spectrophotometry) and mineralogy (by XRD), and compared with selected samples (n=14) from a previously collected sediment core spanning the Holocene from a lake impacted by the landslide.  This analysis revealed that the grab samples with the most orange colors contained the largest component of smectite.  In contrast, none of the lake sediment samples displayed such high orange values, and all of the lake sediment samples were dominated by illite, with no detectable smectite in the XRD patterns.  These results support the interpretation that the orange color in the lakes was produced by smectite mobilized by the landslide, and that the 2023 slope failure was unusual in the context of the Holocene.  

Analysis of local meteorological data (1-hr resolution) revealed that the 2023 melt season (April 1st-July 31st) was anomalously cold relative to melt seasons in the previous decade (2013-22).  By July 31st, 2023, 21,125 thawing degree-hours had accumulated over the melt season representing a 17% decrease from the 2013-2022 average.  In addition, 2023 was characterized by an above average snowpack, with nearby SNOTEL stations recording >155% of the median April 1st snow water equivalent (SWE). Snow covered area was quantified using a machine learning approach in Landsat-8 and Sentinel-2 imagery, which revealed that snow persisted on the landscape substantially later in the 2023 melt season compared to the preceding decade.  Particularly notably, during week 9 of the melt season (May 27-June 3) in 2023 the landscape was ~89% snow covered compared to the 2013-2022 average of only ~52%.  Ultimate snowpack ablation occurred more rapidly in 2023, with a 27% greater daily average melt rate compared to the long term median from peak SWE to zero.  This combination of persistent and greater snow cover, with delayed and accelerated snowmelt, likely triggered the July 2023 landslide.