Recent dynamics of ice-dammed lakes in Alaska

In a period of accelerated glacier mass loss, the global number and size of glacial lakes increased in recent decades. Glacial lakes can suddenly fail, resulting in Glacial Lake Outburst Floods (GLOFs) that pose a hazard to downstream ecosystems and settlements. Because GLOFs can travel tens to hundreds of kilometers, they have caused fatalities and widespread damage to homes, infrastructure, and supply networks far downstream. Due to their high discharge and sediment transport capacity, resulting in intense erosion and deposition, GLOFs can strongly modify downstream landscapes. This thesis assesses recent changes of ice-dammed lakes and associated outburst floods in Alaska – a region that, aside from the ice sheets, has contributed most to recent global glacier mass loss and has been the site of numerous ice-dammed lake drainages. As ice-dammed lakes may drain repeatedly, their associated GLOF hazard may persist for decades and change in response to changes in glacier mass. Previous regional inventories of GLOF size lack high temporal resolution. However, continuous optical satellite time series have increasingly enabled systematic GLOF detection and mapping, and thus, data-driven estimates of changes in ice-dammed lake and GLOF characteristics. In four studies, this thesis reports on regional and local trends in the size of ice-dammed lakes and associated GLOFs in Alaska, in light of changing glacier mass budgets and downstream geomorphological impacts. First, this thesis synthesizes the current state of global GLOF and glacial lake research based on a literature review. This first study demonstrates that Alaska is a hotspot of historic GLOFs and ice-dammed lake activity, which have also been the source of most reported GLOFs worldwide. Furthermore, based on preexisting lake inventories, this study estimates that the global glacial lake area increased by 22% per decade between 1990 and 2020. Second, this thesis compares the evolution of glacial lake size and burst lake sizes. Drawing on a recent global glacial lake inventory, the second study reveals that the increase in glacial lake size is not reflected in the size change of Alaskan ice-dammed lakes, whose total area has decreased between 1990 and 2020. Based on an updated database of mapped GLOF lake areas, this study estimates that the regional-scale sizes of burst ice-dammed lakes have remained unchanged in the Alaskan mountain regions, except for the North Coast Ranges, where they have decreased. Overall, a third of the ice-dammed lakes that had burst at least five times between 1990 and 2023 shrunk in size. A small fraction (<10%) of active ice-dammed lakes in Alaska, however, grew during this period, potentially posing increased flood hazard. Third, this thesis investigates changes in the GLOF characteristics of a growing ice-dammed lake in southeastern Alaska, which became one of the largest GLOF lakes in this region in recent decades. The study reconstructs the drainage chronology, estimates GLOF lake levels and volumes, and changes in the ice-dam surface elevation of Desolation Lake, dammed by Lituya Glacier, using a time series of satellite and elevation data. Since its formation in the 1960s, Desolation Lake has grown to a maximum size of 12.7 km² in 2014, while partially draining almost 50 times on a near-annual basis since the 1970s. During its rapid expansion, the lake released increasingly larger volumes of water despite declining maximum annual lake levels. The decline in lake levels is attributed to the thinning of the Lituya Glacier, which limits the storage capacity of the lake. Estimated drainage volumes peaked in 2015, with ∼700 × 10⁶ m³ of water released from the lake, five times the average reported GLOF volume in Alaska. Lastly, this thesis assesses changes in proglacial sedimentation in Lituya Bay to investigate the geomorphological relevance of the GLOFs from Desolation Lake, which drains into the bay. Based on historic bathymetric data and a newly acquired multibeam survey, the fourth study shows that the spatial pattern of accumulation in Lituya Bay differs between 1926–1959 and 1959–2023. This is likely due to the leveling of the bay floor caused by erosion and reworking of sediments during a landslide-generated tsunami following an earthquake in 1958. Subsequently, sedimentation was concentrated at the head of the bay, supporting the rapid growth of two proglacial deltas in front of the former Lituya and North Crillon tidewater glaciers. During the recent period of undisturbed delta growth, the study estimates that the Lituya delta, which is flooded during the frequent GLOFs from Desolation Lake, grew ∼50% more in subaerial size (1985–2025) and twice as much in volume (1959–2023), compared to the neighboring Crillon delta, which has not been flooded by GLOFs. These differences are driven by a larger meltwater supply from Lituya Glacier, whose surface lowering exceeds that of North Crillon Glacier, a larger hydrologic catchment induced by Desolation Lake, and intense evacuation of subglacial sediments beneath Lituya Glacier during the GLOFs. The findings and compiled data presented in this thesis can assist in assessing future changes in GLOF magnitudes by offering insights into the evolution of GLOF lake sizes due to glacier thinning and retreat. Thereby, the results may help identify lakes with high growth potential, which may inform strategies to mitigate potential damage associated with future ice-dammed lake drainage events. This thesis further underscores the relevance of GLOFs in shaping the geomorphological characteristics and growth pace of the proglacial Lituya delta. This site might serve as a valuable case study for future research on sedimentation and delta growth in Alaskan fjords.