Abstract: Expanding glacial lakes and associated potential for catastrophic Glacial Lake Outburst Floods (GLOFs) are growing concerns globally. With above-average warming rates (0.3 ± 0.2 °C per decade), the Hindu Kush Himalayan (HKH) region is witnessing significant glacier mass loss, reduced snow cover, and increased permafrost thawing, all of which increase meltwater availability, leading to formation and expansion of glacial lakes. Glacial lakes, in turn, further accelerate glacier mass loss through calving and subaqueous melt, creating a vicious feedback loop of glacial melting and expansion of glacial lakes. Lake with larger areas has a higher probability of interception of mass movements. Additionally, it exerts greater hydrostatic pressure on moraine dams and therefore higher magnitude of GLOF which can be extremely destructive, arriving unnoticed and destroying property, infrastructure, and agricultural land, often resulting in extensive loss of life and property. Sikkim Himalayas hosts a large number of glacial lakes and has received considerable scientific attention due to the potential GLOF hazards. This study focuses on 13 vulnerable glacial lakes (VGLs) in the Sikkim Himalayas, analysing their decadal dynamics from 1962 to 2020 using declassified Corona and Hexagon KH-9 imagery and Landsat satellite data. Our analysis revealed a significant increase in the total area of the VGLs, expanding from 3.594 km2 in 1962 to 8.664 km2 in 2020. However, individual lakes exhibited highly variable dynamics and growth histories, falling into either of the two categories, quasi-stable and dynamic lakes. Most VGLs were quasi-stable, with minimal growth over a decade, while dynamic lakes, connected to parent glaciers, showed continuous growth. Based on modelled future lake expansion, South Lhonak and KhangchungTsho were found to have future expansion potential of 0.472 km2 and 0.333 km2, respectively. With growing infrastructure and population downstream, the risk of future GLOF events is exceptionally high. Understanding glacial lake expansion is crucial for effective risk assessment and policy development in these vulnerable areas.
Keywords: Glacial lakes, Lake expansion, Glacial lake outburst floods, Sikkim Himalayas
