What Caused Uttarkashi Flash Floods? ISRO Images, Expert Papers Reveal Clues

NDTV

4 days ago

The devastating mudslide that killed five people and left over 50 people missing as it swept through the Dharali area in Uttarkashi has left the region reeling.
As rescue and relief operations continue, a new debate has emerged among experts and glaciologists: what triggered this deadly natural disaster? The answer, it seems, lies in a combination of the region's unique geology and the growing impacts of climate change.
According to a detailed report, the village of Dharali is built on an 'alluvial fan,' a cone-shaped landform created by debris from past floods at the confluence of the Kheer Ganga stream and the Bhagirathi River.
Despite the village being 200 metres from the river, commercial development has encroached directly onto this unstable landform, increasing its vulnerability.
The Landslide Theory: A Reawakened Threat
One of the leading theories is that the mudslide was a reactivated landslide. A glaciologist has pointed to a previous landslide that occurred 2 kilometres uphill from Dharali, suggesting it may have been triggered again. This idea is backed by findings from a paper, Bhambri R et al, High Mountain Hazards in Uttarakhand, which provides a historical context. The paper notes that following heavy rainfall in the Bhagirathi Basin in June 2013, a staggering 533 existing landslides were reactivated. The areas closest to Bhatwari, Pilang, Dharali, and the Tehri dam sites were hit the hardest. The paper explains that this region is prone to such events due to its geology, steep terrain, and unpredictable weather.
Glaciers and a "Breached" Lake: A New Threat?
Another powerful hypothesis points to a potential glacial lake outburst. Using satellite imagery and terrain analysis, glaciologists found two glaciers with huge crevasses approximately 7 kilometres above Dharali. The presence of these crevasses, they say, could be speeding up the melting process. Below these glaciers lie seven smaller lakes. Experts believe that rising temperatures could have caused one of these lakes to breach, sending a torrent of water, rock, and debris-known as moraines-downstream. The sheer destructive power of this debris-filled flow could be the reason for the extensive damage.
ISRO Images Capture the Aftermath
Adding weight to the analysis, ISRO has released satellite images that show the scale of the destruction, which show Dharali Village both before and after the event. The image from August 7, 2025, specifically reveals a significant deposition of sediments and mudflows near the Bhagirathi and Khera gad Rivers, painting a clear picture of the aftermath of the cloudburst.
The Orographic Twist: Rain, Not Snow
A third, less-widely accepted theory, introduces the concept of an "orographic barrier." A scientist suggested that a mountain peak nearly 5,700 metres high could have blocked moisture-laden clouds. This would have caused the clouds to condense and release heavy rain in the upper areas, rather than snow. The scientist argued that this phenomenon could have caused floods in Kheer Ganga, Harsil, and Sukhi top. This theory challenges the glacial breach hypothesis by suggesting that a breach would have only affected the Kheer Ganga catchment.
While the debate continues, what is clear is that the tragedy in Uttarkashi is a stark reminder of the fragile ecosystem of the Himalayas and the urgent need to understand and address the risks posed by climate change and unsustainable development.