DEHRADUN: The flash flood in Dharali earlier this week reshaped the course of the Bhagirathi river, widening stream channels and altering river morphology , satellite images from the Indian Space Research Organisation (Isro) showed. The high-magnitude flood eroded a debris fan on Kheer Gad - a tributary of the Bhagirathi located just upstream of Dharali village - forcing it back to its earlier path and pushing the Bhagirathi towards the right bank.
Satellite imagery from Isro's Cartosat-2S, comparing data from June 2024 and Aug 7 this year, revealed a vast fan-shaped debris deposit roughly 20 hectares in size - measuring about 750m by 450m - at the confluence of Kheer Gad and the Bhagirathi, just above Dharali. These images showed extensively altered river channels, submerged or buried buildings, and major topographical shifts.
Piyoosh Rautela, senior geologist and former executive director of Uttarakhand State Disaster Management Authority, said pre-disaster images revealed a triangular debris fan on Kheer Gad's left bank, just upstream of its confluence with the Bhagirathi. "This deposit was formed during a previous catastrophic downslope movement that diverted Kheer Gad's course at the time. Traditionally, such deposits were only used for agriculture, with homes built on higher, stable ground to avoid landslide and flood risk," he said.
He added that rapid tourism growth and the pilgrim influx over the last decade, along with commercial activity near the road, had prompted settlement on the alluvial fan. "The flash flood eroded the entire fan deposit and Kheer Gad reclaimed its previous course. Currently, the debris has pushed the Bhagirathi's flow towards the right bank, but it will eventually erode this deposit in time," he said.
Hydrologists warned such sudden geomorphic changes could have cascading effects far downstream. Altered river channels can increase flow velocity, change sediment transport, and destabilise banks kilometres away from the flood site. Over time, this can create new erosion hotspots , threaten bridges, and change floodplains, forcing riverside communities to adapt to a new hydrological pattern.
Dean at the School of Environment and Sustainability in Bangalore-based Indian Institute for Human Settlement, Dr Jagdish Krishnaswamy, who is an ecohydrologist and landscape ecologist, said the Himalayas' geology and climate make them prone to such shifts. "These are the world's youngest mountains - tectonically active, geomorphologically dynamic, and generating some of the highest sediment loads globally. Glacier retreat, both natural and intensified by warming, releases vast amounts of debris, which intense rainfall can mobilise into avalanches and mudslides. This sediment can drastically alter river courses, especially where loose deposits exist on lower slopes or in narrow valleys," he said.
He cautioned that embankments and retaining walls often give a "false sense of security" when infrastructure and homes are built on unstable ground. "Given the Himalayas' dynamic geomorphology and the increasing intensity of rainfall, extreme caution is needed in designing and locating any infrastructure. Otherwise, we are exposing people and assets to unacceptably high risk," he said.
Satellite imagery from Isro's Cartosat-2S, comparing data from June 2024 and Aug 7 this year, revealed a vast fan-shaped debris deposit roughly 20 hectares in size - measuring about 750m by 450m - at the confluence of Kheer Gad and the Bhagirathi, just above Dharali. These images showed extensively altered river channels, submerged or buried buildings, and major topographical shifts.
Piyoosh Rautela, senior geologist and former executive director of Uttarakhand State Disaster Management Authority, said pre-disaster images revealed a triangular debris fan on Kheer Gad's left bank, just upstream of its confluence with the Bhagirathi. "This deposit was formed during a previous catastrophic downslope movement that diverted Kheer Gad's course at the time. Traditionally, such deposits were only used for agriculture, with homes built on higher, stable ground to avoid landslide and flood risk," he said.
He added that rapid tourism growth and the pilgrim influx over the last decade, along with commercial activity near the road, had prompted settlement on the alluvial fan. "The flash flood eroded the entire fan deposit and Kheer Gad reclaimed its previous course. Currently, the debris has pushed the Bhagirathi's flow towards the right bank, but it will eventually erode this deposit in time," he said.
Hydrologists warned such sudden geomorphic changes could have cascading effects far downstream. Altered river channels can increase flow velocity, change sediment transport, and destabilise banks kilometres away from the flood site. Over time, this can create new erosion hotspots , threaten bridges, and change floodplains, forcing riverside communities to adapt to a new hydrological pattern.
Dean at the School of Environment and Sustainability in Bangalore-based Indian Institute for Human Settlement, Dr Jagdish Krishnaswamy, who is an ecohydrologist and landscape ecologist, said the Himalayas' geology and climate make them prone to such shifts. "These are the world's youngest mountains - tectonically active, geomorphologically dynamic, and generating some of the highest sediment loads globally. Glacier retreat, both natural and intensified by warming, releases vast amounts of debris, which intense rainfall can mobilise into avalanches and mudslides. This sediment can drastically alter river courses, especially where loose deposits exist on lower slopes or in narrow valleys," he said.
He cautioned that embankments and retaining walls often give a "false sense of security" when infrastructure and homes are built on unstable ground. "Given the Himalayas' dynamic geomorphology and the increasing intensity of rainfall, extreme caution is needed in designing and locating any infrastructure. Otherwise, we are exposing people and assets to unacceptably high risk," he said.
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