Sequence of events that led to the South Lhonak lake outburst flood in Sikkim, India.

On October 4, 2023, a proglacial lake named the South Lhonak lake was the source of a catastrophic Glacier Lake Outburst Flood (GLOF) in the Teesta river basin area, resulting in 24 fatalities and leaving over 70 persons missing. The GLOF also destroyed 13 bridges and a major hydropower plant in the Chungthang region. Over 60,000 individuals in four districts of Sikkim were impacted by this GLOF event. This study examines the factors that led to the GLOF event. Our study shows that the cause of this GLOF was initiated by a landslide, that dumped a substantial amount (~ 38.31 million m3) of debris into the South Lhonak Lake. Furthermore, the glacier that was connected to the lake, lost a big chunk of ice mass (~ 7 million m3) due to calving. The combination of these two processes led to the collapse of the left lateral moraine that consequently generated flood waves which breached the terminal moraine dam of the lake. We recommend monitoring land subsidence and calving events for large proglacial lakes to prevent the disastrous consequences of such GLOFs in the future.