Progressive Development of a Paraglacial Rock Slope Failure at Portage Glacier, Alaska

Abstract Paraglacial landscapes are rapidly transforming as thinning and retreating glaciers expose adjacent slopes to new conditions. In Southcentral Alaska, a large slope instability at Portage Glacier has been deforming progressively up‐glacier over the past six decades. The instability comprises two deep‐seated rock slope segments, Portage A and Portage B, located above the thinning and retreating Portage Glacier and its proglacial lake. Portage B lies at the glacier terminus, while the down‐glacier margin of Portage A is about 300 m further up‐glacier. To understand the mechanisms driving slope deformation, we integrated field observations, historical imagery, structural and kinematic analysis, differential DEMs, InSAR, and coherence radar to capture both short‐ and long‐term deformation patterns. We identified three distinct domains of movement: two in Portage A and one in Portage B. Our findings reveal that Portage A experiences rapid and variable displacement rates, whereas Portage B shows slower motion. Structural analysis indicates translational sliding and toppling as primary failure mechanisms controlled by pre‐existing geological discontinuities. Glacier thinning is identified as a key factor, initiating movement and enabling the progressive spatial up‐glacier propagation of deformation from Portage B to Portage A. But this process is not solely driven by thinning; rather, it reflects how ice loss progressively alters mechanical boundary conditions, granting kinematic freedom for the rock mass to deform along pre‐existing structural discontinuities. Consequently, our results underscore the importance of considering both glacier thickness thresholds and structural geology to better understand and assess the onset and evolution of slope deformation in paraglacial environments.