Flash Flood Alters Bhagirathi’s Route, River Shifts Back to Old Path

A sudden flash flood in Dharali earlier this week altered the course of the Bhagirathi river, expanding its stream channels and shifting its flow direction. According to satellite visuals from the Indian Space Research Organisation (ISRO), the strong flood forced the Bhagirathi back to its natural path and shifted it to the right bank by removing a debris fan on Kheer Gad, a tributary just upstream of Dharali.

ISRO’s Cartosat-2S satellite images, comparing data from June 2024 with August 7 this year, show a massive fan-shaped debris deposit covering nearly 20 hectares, measuring around 750 metres by 450 metres. This formation lies at the meeting point of Kheer Gad and the Bhagirathi, above Dharali. The pictures reveal altered river channels, flooded or buried houses, and major changes to the landscape.

Settlements on Risky Land

Previous photos showed a triangular debris fan on the left bank of Kheer Gad, upstream of where it meets the Bhagirathi, according to Piyoosh Rautela, a senior geologist and former head of the Uttarakhand State Disaster Management Authority. According to him, this feature was formed during a previous landslip that had changed the course of Kheer Gad at the time. Traditionally, these deposits were only used for farming, while homes were built on higher and safer ground to avoid floods and landslides,” he explained.

However, over the past decade, the rise in tourism and pilgrim visits, along with growing commercial activity near the road, led to new settlements on this risky alluvial fan. “The flash flood swept away the entire fan deposit, allowing Kheer Gad to return to its old route. The debris is currently driving the Bhagirathi towards its right bank, but the river will eventually destroy this deposit as well," he continued.

Downstream Dangers

Experts warn that such sudden changes in river paths can trigger chain reactions far downstream. Many kilometres away from the flood site, altered channels can undermine riverbanks, change the way sediments are carried, and increase water speed. Over time, these changes can create new erosion zones, damage bridges, alter floodplains, and force riverside communities to adjust to a new water flow pattern.

Dr Jagdish Krishnaswamy, Dean of the School of Environment and Sustainability at the Indian Institute for Human Settlement in Bengaluru, noted that the geological structure and climate of the Himalayan region make it highly susceptible to such incidents. The Himalayas are the youngest mountains on Earth—still rising, constantly changing, and carrying huge amounts of sediment. Melting glaciers, both from natural causes and climate warming, release massive debris, which heavy rains can turn into mudslides and avalanches. This loose material can dramatically change a river’s course, especially in narrow valleys or where deposits sit on lower slopes,” he explained.

Caution Against Unsafe Construction

Dr Krishnaswamy also warned that embankments and retaining walls often create a “false sense of security” when buildings and infrastructure are placed on unstable land. “With the Himalayas’ constantly changing terrain and the rising intensity of rainfall, extreme care is needed in planning and placing infrastructure. Otherwise, we are putting both people and property at a very high risk,” he cautioned.