Daily to annual controls on paraglacial slope stability at Portage Glacier, Alaska

As glaciers worldwide rapidly lose mass due to human-induced warming, the adjacent mountain slopes are undergoing important changes. In this work, we sought to understand the processes controlling the motion of paraglacial instabilities during glacier retreat. To do so, we deployed a time-lapse camera and seismic and geodetic monitoring devices at Portage Glacier, Alaska, between 2022 and 2024. Two instabilities, which are connected but make up distinct kinematic units, border the retreating glacier and its expanding proglacial lake. We also leveraged multitemporal Digital Elevation Models and existing datasets to analyze air temperature, water availability, and glacier velocity. At annual scales, expanding subsidence at the instability toe, paired with high down-slope velocities, appears to be mainly caused by glacier thinning. At seasonal to daily time scales, we show that water availability strongly controls slope velocities and that seven of eight distinct slope acceleration events were preceded by storms with daily rainfall totals of 60 mm or more. We also detect a strong and rapid response of several seismic variables to intense precipitation and freezing events, suggesting the highly fractured subsurface leads to short reaction times. Furthermore, seismic amplification results indicate the potential for the studied instabilities to expand up-slope and up-glacier in the future. This work provides a valuable opportunity to study the impact of glacier mass loss, in addition to other environmental factors, on slope stability over multiple years. Given the scarcity of such in-situ data, these results have broader implications by improving understanding of paraglacial instabilities.