A new study by scientists from the Indian Space Research Organisation (ISRO), published in NPJ Natural Hazards, examines the August 5, 2025 flash flood that destroyed Dharali village in Uttarakhand and killed six people. It sheds light on how warming temperatures affect glaciers, especially exposed ice patches on retreating glaciers, and highlights the need to monitor glaciers using satellite images to provide early warnings of possible disasters.
The study, titled ‘Ice-patch collapse and early-warning implications from a Himalayan flash flood: emerging cryo-hydrological hazards under deglaciation’, concludes that the collapse of an ice patch on the glacier above Dharali is linked to deglaciation in the Himalaya.
The authors say the findings could help improve understanding of climate risk and disaster preparedness. The study shows that the flash flood was triggered by the collapse of an ice patch in the nivation area of the Srikanta glacier.
Nivation is defined as the erosion of the ground beneath and around a snow bank, primarily as a result of alternate freezing and thawing. This can form a nivation hollow, which gradually becomes deeper when snow repeatedly accumulates in the same place.
History of extreme events
The study area lies in the upper Bhagirathi river basin in Uttarkashi district of Uttarakhand. It covers the ridge-to-valley system from the Srikanta glacier to Dharali village, located at a height of 2,650-2,700 m along the Bhagirathi river. The village lies downstream of the glacier-fed Khir Gad stream, which originates from the Srikanta glacier, flows through Dharali, and then joins the Bhagirathi river. The Khir Gad divides Dharali into right- and left-hand bank settlements, increasing its risk of flash floods.
The region has a documented history of extreme events, including the large landslides that brought down massive boulders during the June 2013 Himalayan floods. The researchers used satellite observations, high-resolution topographic analysis, and visual records to reconstruct the sequence of events linking unstable glacier ice to the sudden flood.
The findings expand the range of recognised glacier-related hazards in the Himalaya and identify exposed ice patches as an under-recognised risk from glacier melt.
The Dharali event also shows how instability in the cryosphere can create hazards downstream in high-altitude regions.
Exposed ice patches
The study calls for closer monitoring of glaciers and argues that the focus should extend beyond glacial lake outburst floods (or GLOFs) to include smaller, often overlooked instabilities in the cryosphere. The exposure of ice patches in the Srikanta glacier before the flood represents a transitional state in the snow-ice regime.
“Pre-event imagery during the ablation period revealed exposed ice patches on steep north- to northeast-facing slopes, indicating thinning seasonal snow and firn cover consistent with ongoing deglaciation,” the authors wrote in the paper.
Such exposure indicates thinning seasonal snow and firn, which is ice that lies between snow and fully formed glacial ice.It usually occurs when warmer conditions reduce the insulating snow layer that stabilises the ice beneath.
Ice patches covered by firn and seasonal snow are relatively resistant to short-term temperature changes whereas exposed ice is more likely to shift or loosen. Because exposed ice responds more quickly to temperature changes or heavy rainfall, it can melt, fragment or collapse more easily, releasing ice, meltwater, and debris that can trigger flash floods.
Therefore, the study finds that the presence of exposed ice patches on the Srikanta Glacier immediately before the floods of August 5 was a signal of ongoing deglaciation and a direct indicator in the landscape that increased the likelihood of such floods.
Satellite observations
Srikanta Glacier is a small- to medium-sized valley glacier with a height of 6,133 m, around 9.8 km upstream of Dharali. The glacier has steep accumulation and ablation zones, seasonal snow cover, and extensive nivation areas. Mountaineering and expedition reports describe unstable ice surfaces, steep slopes, avalanche-prone terrain, and persistent nivation zones beneath the ridge below the Srikanta peak, the paper said.
A key implication of the study is that it shows the value of pre-event satellite observations for early warning. Satellite images showed exposed ice patches persisting in the nivation zone during the ablation period, when the glacier loses ice and snow. This indicated that the seasonal snow and firn cover had thinned.
Studies from other cold regions, including the Canadian Arctic and Greenland, also show that the collapse of ice patches can trigger hazards as glaciers lose more ice and snow due to regional warming.
Identifying nivation hollows
Although rarely reported from the Himalaya, events such as the February 2021 Chamoli rock-ice avalanche demonstrate the growing prominence of cryospheric hazards in deglaciating terrain, the study said. In steep nivation hollows, such instability can suddenly release ice, meltwater, and debris, triggering downslope mass movement and related cryo-hydrological hazards.
The Dharali flash flood illustrates how cryospheric instability can trigger geomorphic changes and create hazards downstream in high-altitude Himalayan regions. The study finds that the rapid disintegration of an exposed ice patch in the nivation zone of the Srikanta glacier was the main cause of the event.
The reconstructed sequence, from the exposure of the ice patch during the ablation period to its disappearance and the resulting downstream impacts, shows how earth-observation data can help identify and reconstruct such extreme events in remote mountain terrain.
“A broader implication of this analysis is the identification of nivation hollows as geomorphologically sensitive zones for cryospheric instability,” the authors concluded.
“Persistent snow and ice retention on north- to northeast-facing slopes below the Srikanta ridge created conditions conducive to ice-patch exposure and subsequent failure during the ablation season. Similar settings are widespread across the Himalaya, suggesting that systematic identification and monitoring of such zones should form part of regional hazard assess- ment and disaster-risk reduction strategies in the context of ongoing deglaciation
Meena Menon is an independent journalist, researcher and author. She has a PhD from the University of Leeds.
Published – March 16, 2026 07:15 am IST
