Fast-forming and difficult to detect, small glacial lakes are proving to be more hazardous than previously thought, causing devastating and unpredictable floods across the Hindu Kush Himalayan region. Recent disasters, including a deadly flood in July 2025 on the Nepal-China border, have highlighted that the impact of a glacial lake outburst flood (GLOF) cannot be predicted based on lake size alone. These smaller supraglacial lakes, which form on the surface of glaciers, are dynamic and often go unnoticed, posing a significant risk to downstream communities.
The New Hazard of Small Lakes
Traditionally, large, well-established glacial lakes were considered the primary flood risk. However, recent events demonstrate a shift. In July 2025, a newly formed, small glacial lake on the Purepu Glacier burst, sending floodwaters over 100 kilometers downstream into Nepal’s Rasuwagadhi village. This GLOF killed at least nine people, left dozens missing, and destroyed a major bridge. Similar incidents have occurred in Nepal’s Solukhumbu and Humla districts, where small lakes triggered destructive floods that destroyed infrastructure and displaced residents.
Finu Shrestha and Sudan Bikash Maharjan, remote sensing analysts at the International Centre for Integrated Mountain Development (ICIMOD), explain that these smaller lakes are dangerous because their internal changes are hard to monitor. When they drain, even a small release of water can be amplified by downstream factors like sediment from previous landslides, debris, and the shape of river channels. The presence of sediment can make the flood more destructive to infrastructure and can even change the course of a river.
Challenges in Monitoring and Prevention
Detecting and mitigating the risks from these small supraglacial lakes is a significant challenge due to several factors:
• Speed of Formation: They can form and drain within days or weeks, making them hard to track with conventional methods.
• Satellite Limitations: Small lakes may be too tiny for medium-resolution satellite imagery to distinguish, especially with snow or persistent cloud cover, which is common during the summer monsoon season when these lakes are most active.
• Difficult Terrain: Their high-altitude and remote locations make in-person monitoring and the installation of early-warning systems logistically challenging and expensive.
A Call for Cooperation
To address this growing threat, experts are urging for a more comprehensive approach. Shrestha emphasizes the need for regular, year-round mapping of glacial lakes using high-resolution satellite imagery and AI-based technologies to track their dynamic changes over time.
Most critically, both experts highlighted the need for transboundary cooperation among Himalayan countries. GLOFs do not respect borders, and a lack of data sharing due to political sensitivities and trust issues between nations hinders the ability to provide timely warnings to downstream communities. Without better monitoring, regional collaboration, and local education, these small, fast-forming lakes will continue to be a growing threat to the lives and livelihoods of those living below them.