Inventory, Classification, Evolution, and Potential Outburst Flood Assessment of Glacial Lakes in the Chhombo Chhu Watershed (Sikkim Himalaya, India)

The Sikkim Himalaya, similar to other mountain regions, has lost considerable ice cover over the years owing to the changing climatic factors leading to enlargement of glacier-fed lakes, and thus posing a potential threat to downstream communities in the mountain and Tarai (foothills) region in case of breach anytime in the future. The Chhombo Chhu Watershed (CCW) of the Tista Basin in the Sikkim Himalaya, located between the Greater Himalayan Range and the Tethyan Sedimentary Sequence, is the storehouse of number of glacial lakes with large areas and volumes. In this study, we mapped the glacial lakes' changes between 1975–2018 and assessed their dynamics based on manual analysis of optical satellite images using KeyHole-9 Hexagon (∼4 m), Landsat Series (∼15-30 m), and Sentinel 2A-MSI (∼10-20 m) imagery and verified during field surveys. The results show that the number of lakes has increased from 62 to 98, and its total area expanded significantly by 34.6 ± 5.4%, i.e., from 8.5 ± 0.2 km² in 1975 to 11.4 ± 0.6 km² by 2018, at an expansion rate of 0.8 ± 0.1% a^–1. Lake outburst susceptibility result reveals that a total of twenty-seven potentially dangerous glacial lakes exist in the watershed; 5 have a status of ‘high’ outburst probability, 17 ‘medium’ and 5 ‘low’. The majority of the proglacial lakes in the watershed have significantly enlarged due to the faster melting and calving processes as a result of accelerating increasing long term average annual trend of temperature (+0.283° Ca^–1; 95% confidence level) and homogeneous or slightly declining precipitation.