Unforeseen risks of flash floods emerged from minuscule glacial reservoirs in the Himalayan mountain range

In the high-altitude regions of the Himalayas, glacial lake outburst floods (GLOFs) from smaller lakes have become more hazardous than previously perceived. This was tragically demonstrated in July 2023, when a flood surge from a small glacial lake in Nepal's Rasuwagadhi area swept through the Bhote Koshi River, causing the death of at least 11 people and leaving 17 missing.

The impacting GLOF may have been sediment-laden, which can cause more destruction to infrastructure and potentially change the shape of downstream river channels. In the same month, a tiny ice-dammed lake in central Nepal's Upper Mustang, of just 0.016 sq km, burst and sent floods rushing downstream, destroying four bridges.

Monitoring these small glacial lakes is challenging due to their remote locations and the logistical expenses involved. However, regular, year-round mapping of glacial lakes through high-resolution satellite images is crucial for mitigating GLOFs. For instance, the formation and draining of a glacial lake at the Purepu Glacier, about 35km upstream from Rasuwagadhi, was observed by the International Centre for Integrated Mountain Development (ICIMOD) over a week in July 2023.

Artificial-intelligence-based technologies can help detect and map glacial lakes on an annual or even seasonal basis, track their dynamic changes over time, and potentially identify lakes that may pose a future threat. However, smaller supraglacial lakes can be hard to identify due to their speed of draining and rapid changes in size and merging patterns. Satellite imagery can be limited in detecting smaller lakes, especially in the presence of snow or cloud cover, and during the monsoon season.

Awareness among local communities regarding newly forming or small supraglacial lakes is still lacking, and many residents remain unaware of changing conditions upstream and lack access to timely or accessible information about lake formation and expansion. This was evident in the Rasuwagadhi incident, where the trigger for the flood was a newly formed small glacial lake on top of a glacier that was not visible before March this year.

Transboundary cooperation is key for monitoring and reducing potential threats from unstable glacial lakes in upstream areas. However, regional cooperation in data sharing and early warning is still insufficient. There are at least 47 potentially dangerous glacial lakes in Nepal, China, and India. In May 2025, two glacial lakes burst above Tilgaon, a village in Nepal's Humla district, triggering a destructive flood and subsequent landslide. A major bridge connecting Nepal to China was swept away due to the flood.

Hari Krishna Joshi, a researcher at ICIMOD, emphasised the importance of continuous monitoring and early warning systems for mitigating the risks associated with GLOFs. He also highlighted the need for improved transboundary cooperation to effectively manage these threats.

In August 2024, a flash flood struck Thame village in Nepal's Solukhumbu district, following a GLOF from Thyanbo Lake measuring only 0.05 sq km. The flood from the July 2023 draining only travelled about 10km downstream, but just two years later, flooding from the expanded lake would impact areas over 100km downstream.

These incidents underscore the urgency for increased monitoring, improved data sharing, and early warning systems to protect communities living in the path of GLOFs. The potential for devastating impacts from these events continues to grow, making it crucial for all stakeholders to work together to mitigate these risks.

Unforeseen risks of flash floods emerged from minuscule glacial reservoirs in the Himalayan mountain range