Quantifying the Rapid Expansion and Outburst Susceptibility of Cascading Moraine-dammed lakes in the Sikkim Himalaya: The Case of Gurudongmar Lake Complex

The ongoing retreat of glaciers in the Sikkim Himalaya as a result of climate change (Racoviteanu et al., 2015; Chowdhury et al., 2021) has far-reaching implications for the development and expansion of glacial lakes. Gurudongmar lake complex (GLC) represents a typical paternoster lake system, resembling a series of glacial lakes connected by single or braided streams with a surface or subsurface drainage system. A detailed study of the GLC evolution and outburst susceptibility assessment is required. Glacial lake volume estimation and lake outburst susceptibility assessment were carried out to reveal different characteristics for all four lakes (GL-1, GL-2, GL-3, and GL-4) from the lake complex. Each of these lakes has a moderate to very high potential to outburst. As the dam of GL-1 provides no retention capacity, there is a very high potential of a combined effect with the sudden failure of the moraine-dams of GL-2 or GL-3 located upstream. Temporal analysis of GLC using optical remote sensing data and in-field investigations revealed a rapidly increasing total lake area by ~74 ± 3%, with an expansion rate of +0.03 ± 0.002 km² a^–1 between 1962 and 2018 due to climate change and ongoing glacier retreat. The overall lake area expansion rates are dependent on climate-driven factors, and constantly increasing average air temperature is responsible for the enlargement of the lake areas. Simultaneously, changes in GLC expansion velocity are driven by changes in the total amount of precipitation. The deficit in precipitation probably triggered the initial higher rate from 1962 to 1988 during the winter and spring seasons. The post-1990s positive anomaly in precipitation might have reduced the rate of the glacial lake area expansion considerably.