Glacial Lake Outburst Floods and Their Long-Term Impacts on Himalayan Landscapes

Glacial lake outburst floods (GLOFs) are high-magnitude events that occur when the dam of a glacial lake fails, releasing huge volumes of water and entrained debris. The increasing frequency of GLOFs, driven by the ongoing effects of climate change, raises concerns about the long-term stability of high-mountain regions in Nepal and across the Himalayas. While the immediate impacts of catastrophic GLOFs are often devastating, the secondary hazards they trigger are frequently overlooked. These secondary hazards, including landslides, geomorphic instability, and stream channel destabilization, pose significant challenges to local communities. Although GLOF events typically last only minutes to hours, the geohazard cascades they initiate may affect communities for years or even decades. A recent GLOF event that caused extensive damage to infrastructure and farmland in Thame, a small mountain village in Nepal's Khumbu Himal region, demonstrated this chain of cascading hazards. Following the catastrophic outburst of two glacial lakes on August 16, 2024, the village now faces increased landslide risk due to significant stream-channel incision below the settlement. The geologic layers beneath the town are susceptible to slow-moving, deep-seated rotational landslides, particularly when lateral support is reduced by stream incision. As a result, the fluvial terrace on which the village is built is becoming increasingly likely to fail from landsliding. Field investigations in fall 2024 collected drone imagery and ground-based photographs of the flood deposits and affected downstream areas. These data were used to develop high-resolution photogrammetric topographic models, enabling reconstruction of the flood dynamics and the evolution of similar past events. Analysis of sediment deposits further reveals how GLOFs interact with ongoing geomorphic processes, contributing to landscape transformation over time. By integrating field observations with photogrammetric modeling, this study highlights the cascading nature of hazards following GLOFs and their role in shaping mountain landscapes.