Glacier Velocities and Ice Dynamics in the St. Elias Mountains, Yukon-Alaska

Abstract

Despite their relatively small ice volume, mountain glaciers contributed nearly one third of global sea level rise since 2000, with one of the largest total mass loss rates (73 ± 17 Gt a-1) occurring in the Yukon-Alaska region. However, there is uncertainty surrounding how ice dynamics are being affected by such losses and whether glacier flow instabilities, such as surges, are changing in a warming climate. The St. Elias Mountains contain a major cluster of surge-type glaciers, yet a detailed analysis of their characteristics, including surge frequency, morphology, magnitude, and propensity over time has not been undertaken on a regional basis. This thesis presents a review of surging behaviour and an updated surge event inventory in the St. Elias Mountains, and quantifies the processes influencing both surging and non-surging glacier velocity variability using a variety of remote sensing and field measurements. An updated inventory of surge-type glaciers and observed surge events (1874-2023), compiled from existing inventories, recently published articles, and velocity analysis, is used to analyze the characteristics of surge-type glaciers and velocity patterns during surge events. The modern (1985-2023) trends in annual, winter and summer velocities of selected surge-type glaciers is then used to classify dynamic instability events into 4 categories. While 231 glaciers were classified as surge-type, only 42 were observed to have experienced rapid velocity events over the period 1985-2023, through either direct measurements or remote sensing observations. For glaciers with observed rapid velocity events, these predominantly fall into two categories: Alaskan-style surges with short active and quiescent phases, and glacier pulses, which are velocity accelerations that are limited in both magnitude and extent. An unnamed former tributary to Kluane Glacier underwent a dramatic surge from 2013-18. Using a combination of air photos, remote sensing and field observations, the characteristics and changes of ‘Little Kluane Glacier’ were reconstructed from the 1940s until 2021. While only the single full surge of 2013-18 was identified, it is likely that a partial surge of just the upper north arm occurred between 1963 and 1972. Repeat Digital Elevation Models (DEMs) and velocity profiles show that the recent surge initiated from the upper north arm accumulation area in 2013, which developed into a full surge of the main trunk from 2017-18. Terminus positions show long-term retreat from 1949-2017, followed by rapid advance of >2 km from May to September 2018, with surface velocities reaching a peak of ~3600 m a-1 in summer 2018 over the lower ablation area. This was likely enhanced by the drainage of supraglacial lakes and streams to the glacier bed through crevassing as the surge progressed. Changes in surface topography caused by initial mass movement, the resulting reorganization of the supraglacial hydrological system, and ponding of surface water, may drive a partial surge into a full surge, and therefore exert a direct control on glacier dynamics. In May 2016, Kaskawulsh Glacier underwent a dramatic proglacial hydrologic reorganization instigated by the rapid drainage of proglacial Slims Lake: as a result, water which previously drained north into Ä’äy Chú, (Slims River) toward Lhú’áán Män (Kluane Lake), was redirected south into Kaskawulsh River, eventually flowing into the Gulf of Alaska. A long-term (up to ∼120 year) record of terminus retreat, thinning and surface velocities from in-situ and remote sensing observations is used to determine the impact of this reorganization on glacier dynamics. After an initial deceleration during the late 1990s, terminus velocities increased at a rate of 3 m a-2 from 2000-12, while the area of proglacial Slims Lake increased simultaneously. The rapid drainage of the lake substantially altered the velocity profile of the adjacent glacier, decreasing annual velocities by 48% within 3 km of the terminus between 2015 and 2021, at an average rate of ∼12.5 m a-2. A key cause of the rapid drop in glacier motion was a reduction in flotation of the lower part of the terminus after lake drainage. This has important implications for glacier dynamics and provides one of the first assessments of the impacts of a rapid proglacial lake drainage event on local terminus velocities. The results of this study provide an examination of factors controlling glacier dynamics, as well as the characteristics of rapid glacier velocity events, in the St. Elias Mountains. This provides insights into the behaviour of mountain glaciers, how they are changing in a warming climate, controls on glacier surging, and the hazards they may pose for downstream communities, which are particularly vulnerable to disturbances.