Kathmandu: In the high Himalayas, floods can originate from a variety of sources such as a glacial lake burst, a mountainside collapse, or an avalanche of ice and rock crashing into a river. These events can rapidly trigger subsequent hazards, ultimately resulting in destructive floods downstream. Scientists are currently investigating a major flash flood that occurred on Wednesday at the Nepal-Tibet border, affecting Nepal's Rasuwa district and Kyirong in Tibet. The flood's swift rise saw water levels on the Trishuli River at Galchhi increase by as much as nine meters in only 30 minutes, as reported by the International Centre for Integrated Mountain Development (ICIMOD).
According to Anadolu Agency, initial investigations suggest an ice-rock avalanche from a glacier on Langtang-Lirung mountain's north side plunged into the Lhende Khola river near the Nepal-China border. This collapse was so powerful it registered a seismic signal originally reported as a magnitude 4.4 earthquake, later reclassified as a magnitude 5.2 landslide. Mohan Bahadur Chand, a Himalayan glaciologist at Kathmandu University, highlighted how similar floods can have diverse origins high in the mountains.
Rasuwa district, about 160 kilometers north of Kathmandu, has experienced significant floods twice in just over a year. Scientists believe these events had different causes. In July 2025, a sudden flood swept down the Lhende River, killing at least 11 people and damaging infrastructure over 100 kilometers downstream. That disaster was attributed to the rapid drainage of a supraglacial lake on China's Purepu Glacier. However, Wednesday's flood appears to have originated from an ice-rock avalanche, as Chand's preliminary assessment indicates.
ICIMOD scientists are collaborating with partners in Nepal and China to analyze seismic, hydrological, and remote-sensing data to establish the flood's cause. Determining the origin is crucial for disaster preparedness, as systems monitoring rainfall or river levels may not detect sudden collapses of ice and rock. This challenge extends across the Himalayas, where major floods can be triggered by various processes including glacial lakes, landslides, and avalanches.
The difficulty in prediction is compounded by how one hazard can rapidly transform into another. For instance, an ice-rock avalanche can become a debris flow or flood far larger than the initial collapse. The 2021 Chamoli disaster in India's Uttarakhand state is a significant example of this transformation. Early reports misidentified it as a glacial lake outburst flood, highlighting the need for improved detection and response.
Chand emphasized the importance of understanding mountain conditions, calling for increased research and monitoring of glaciers and permafrost. Although resource-intensive, such efforts are crucial for preventing disasters. However, replicating intensive monitoring across the vast Himalayas is challenging due to the region's scale and terrain. Rasuwa's hazards, which can cross international borders, highlight the need for cross-border early warning systems.
Climate change is also a factor, accelerating glacier melt and the formation of glacial lakes. While Chand cautioned against directly linking climate change to Wednesday's flood before confirming its cause, he noted its impact on glacier melting patterns. A study co-authored by Chand in July 2026 found rapid growth in glacial lakes, particularly in the Koshi basin shared by Nepal and China.
The immediate threat in Rasuwa is ongoing. Debris from the recent disaster formed a barrier lake near the Nepal-China border, which grew significantly by Friday. Chinese authorities have deployed an engineering team for aerial surveys and 3D modeling of the lake while monitoring the risk of another sudden water release. This threat has already affected rescue operations downstream, with temporary withdrawals in place.
Chand warned of the potential for continued water accumulation to lead to another breach, though not necessarily matching the scale of Wednesday's disaster. The broader challenge remains in gaining sufficient knowledge of the rapidly changing mountains to identify potential dangers before they become disasters.
Source: Anadolu Agency