How powerful Dharali flood altered Bhagirathi river's course - ISRO images show massive changes

Source: ISRO

The flash flood in Dharali, Uttarakhand, last week has drastically changed the course of the Bhagirathi river, according to new satellite images from the Indian Space Research Organisation (ISRO), which shows that the powerful flood widened the river’s channels, and has shifted its path., The Times of India reported.

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The disaster was triggered when the high-intensity flood eroded a large debris fan on Kheer Gad, a tributary of the Bhagirathi located just above Dharali village. This erosion forced Kheer Gad to return to its earlier path, which in turn pushed the Bhagirathi’s flow towards its right bank.

Satellite Insights Aiding Rescue & Relief Ops

ISRO/NRSC used Cartosat-2S data to assess the devastating Aug 5 flash flood in Dharali & Harsil, Uttarakhand.

High-res imagery reveals submerged buildings, debris spread (~20ha), & altered river paths, vital for rescue teams on… pic.twitter.com/ZK0u50NnYF

— ISRO (@isro) August 7, 2025

Satellite pictures taken by ISRO’s Cartosat-2S on August 7 and compared with earlier images from June 2024 reveal the full extent of the change. A massive fan-shaped deposit, roughly 20 hectares in size (about 750 metres by 450 metres), now sits at the confluence of Kheer Gad and the Bhagirathi. These new deposits have buried or submerged buildings, reshaped river channels, and caused major changes in the local terrain, the satellite image showed.

Piyoosh Rautela, a senior geologist and former executive director of the Uttarakhand State Disaster Management Authority, explained that older images had shown a triangular debris fan on the left bank of Kheer Gad, just before it met 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 was quoted by TOI as saying.

However, in recent years, rapid growth in tourism and pilgrim visits, along with commercial activity near the main road, has led to settlements being built on these unstable deposits. “The flash flood removed the entire debris fan, allowing Kheer Gad to return to its old path. The debris has temporarily pushed the Bhagirathi towards the right bank, but over time, the river will erode this material,” Rautela added.

Hydrologists warned such sudden geomorphic changes could have cascading effects far downstream. Alterations in the channel can increase water speed, shift how sediment is carried, and weaken riverbanks many kilometres away. These changes could threaten bridges, alter floodplains, and create new erosion-prone areas, forcing riverside communities to adapt.

Dr Jagdish Krishnaswamy, Dean at the School of Environment and Sustainability, Indian Institute for Human Settlements, explained that the Himalayan region is particularly vulnerable. "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 was quoted by TOI as saying.

He also cautioned that embankments and retaining walls may give only an illusion of safety. “With the Himalayas’ unstable terrain and increasingly intense rainfall, building on fragile land is extremely risky. We must be very careful when planning infrastructure in these areas, or we risk exposing people and property to disasters.”