Glacier calving and moraine collapse triggered the glacial lake outburst flood in South Lhonak Lake, Indian Himalaya

Abstract Glacial lake outburst floods (GLOFs) are destructive and threaten downstream communities in the Himalaya. Through satellite image analysis, we investigate the 2023 GLOF event at South Lhonak Lake, Sikkim, India, focusing on the lake’s historical evolution and the geomorphic controls that caused the GLOF. Multi-temporal data from 10 satellite missions revealed a significant increase in glacier surface lowering from −0.19 m year −1 (1970–1983) to −0.87 m year −1 (2015–2023). Initially a supraglacial lake in 1962, it evolved into a moraine-dammed lake by 1983 and expanded 12-fold from 0.11 km 2 (1962) to 1.4 km 2 (2023). Between 27 September and 6 October 2023, satellite imagery revealed an unusually strong retreat of 49.6 ± 7.1 m, indicating glacier calving and presence of massive icebergs visible on the lake. Our analysis shows 7 large glacier retreat and calving events between 2017 and 2023, further weakening the lateral moraines. This, combined with intermittent rainfall triggered the moraine dam collapse, leading to the GLOF. These findings emphasize the need for long-term monitoring of Himalayan glacial lakes.