Lucknow: The flash flood that struck Nepal’s Rasuwa district on Wednesday followed a pattern similar to last year’s Dharali disaster in Uttarakhand and highlighted an emerging Himalayan hazard driven by unstable glaciers and terrain, said Prof Mahesh Thakkar, director of the Birbal Sahni Institute of Palaeosciences.Thakkar said both events involved more than heavy rainfall, with unstable ice, rocks, steep slopes and sudden water release combining to create highly destructive floods.The Dharali flash flood of Aug 5, 2025, was triggered by the sudden collapse of a massive ice patch on the Srikanta Glacier. The collapse mobilised huge volumes of debris that surged down the Kheer Ganga channel, killing six people and leaving many others missing.Describing the Nepal disaster, which has claimed more than 360 lives so far, Thakkar said: “It was a cascading cryosphere-geomorphic hazard.”The cryosphere refers to the frozen components of the Earth, including glaciers, snow, ice and permanently frozen ground.According to Thakkar, the most likely immediate trigger was an ice-rock avalanche in the high-altitude Lhende Khola catchment. Satellite images and videos suggest that a large mass of glacier ice, rock and debris entered the river system and temporarily blocked its flow.He explained that such blockages can create short-lived natural reservoirs. When the loose dam of ice, rock and sediment weakens or breaches, the impounded water is released suddenly. As it rushes through a narrow Himalayan valley, it entrains boulders, mud and loose sediment, transforming into a powerful debris-laden torrent.“This was not simply a flood generated by excessive rainfall,” Thakkar said, adding that the rapid rise in water levels pointed to an exceptionally energetic flood wave.Water levels reportedly rose by about nine metres in 30 minutes at Galchchi and seven metres at Malekhu. Such a sharp increase, he said, could not be explained by a gradually developing monsoon flood.However, he cautioned that water-level readings alone could not determine the exact volume of water released. Detailed discharge measurements, river-channel studies and hydraulic modelling would be required for an accurate assessment. The flood nevertheless travelled more than 100 km downstream, damaging river channels and infrastructure.The event also has implications for India because the Trishuli River enters the country as the Gandak. Thakkar said monitoring should therefore cover the entire Bhote Koshi-Trishuli-Gandak corridor.He said it would be misleading to describe the disaster simply as a cloudburst. While rainfall and saturated slopes may have contributed to instability, the available evidence suggests the immediate trigger was a geomorphic and cryospheric process.Thakkar also cautioned against attributing the flood to the earthquake recorded in the region. According to him, the seismic signal may itself have been generated by the glacier or ice-rock collapse.The recurrence of such disasters in the same corridor is scientifically significant, he said. The Lhende Khola-Bhote Koshi region witnessed another major cryosphere-related flood in July 2025, when a supraglacial lake in Tibet drained suddenly.Glacier retreat, thinning ice, melting permafrost, expanding glacial lakes and unstable slopes are reshaping hazard patterns across the Himalayas, Thakkar said. While climate change cannot automatically be identified as the direct cause of this flood, it is increasing the underlying risk.He recommended real-time monitoring using river gauges, cameras, satellites, weather radar and seismic sensors. Because temporary blockages can collapse within minutes or hours, rapid data sharing among China, Nepal and India is essential to provide timely warnings to downstream communities.
