The Influence of Glacial Lake Development on Glacier Flow Velocities and Thinning Rates in the Bhutanese Himalayas

Abstract

Mountain glaciers in High Mountain Asia have experienced rapid shrinking and thinning in recent decades through global warming. In addition, more and more glaciers are developing massive lakes at their front, mainly forming in overdeepenings as glaciers retreat. These proglacial lakes have the ability to rapidly accelerate glacier melt as glaciers actively calve into the lake. The formation and expansion of these proglacial lakes heavily affect downstream regions as the water supply is reduced, and the risk for glacial lake outburst floods (GLOFs) increases. It is therefore crucial to study the development of these glacial lakes and to increase our understanding of the influence that these lakes exert on glacier dynamics. This study shows how glacial lake development has affected glacier flow velocities and thinning rates in the Bhutanese Lunana region over the past two decades. Five glaciers were selected from this region, among which one land-terminating glacier and four lake-terminating glaciers. The development of the lakes in this region was mapped using Landsat, RapidEye and PlanetScope imagery to calculate the NDWI. By applying a newly developed feature tracking tool, GIV, the flow velocities were derived over five selected periods from the past two decades. The thinning rate and mass loss from 1975 to 2022 were estimated by combining existing data with data obtained from DEM differencing. The results showed that the glacial lakes have experienced rapid expansion over the past two decades, heavily affecting the flow velocity and thinning rates of glaciers. Some lake-terminating glaciers in the region were characterized by significantly higher flow velocities and thinning rates, mainly at the glacier front compared to land-terminating glaciers, which are mainly influenced by supraglacial lake development and rising air temperatures. The dominant processes at lake-terminating glaciers, causing these high velocities and thinning rates, are dynamic thinning and buoyant uplift which reduce basal stress, causing an acceleration of the terminus and consequently buoyancy-induced calving through a positive feedback. Other lake-terminating glaciers in the region were less affected by their lakes as their fronts were only subjected to subaqueous melt and thermal-notching. For these glaciers, the velocities were decreasing, caused by a decrease in driving stresses by increased glacier thinning. It is estimated that proglacial lakes are likely to expand in the future, and new lakes will form through increased global warming and the presence of large overdeepenings underneath the glaciers. However, many aspects of these lakes, including lake bathymetry and temperature, remain unknown, while playing an important role in glacier-lake interactions.