It’s still early days for analysis, although, for the victims of Dharali in Uttarakhand and Chisoti in Jammu and Kashmir, it’s the end times.
On August 5, a debris flood washed off people and homes at Dharali. At least four people were killed and more than 100 people are missing. On August 14, a debris flood at Chisoti left least 60 dead and more than 70 missing.
For the independent mountain geographer Vaibhav Kaul, “the two geohazard chains and their geomorphic settings were remarkably similar.” Kaul was awarded his PhD from the University of Sheffield for his thesis on “Holistically understanding and enhancing the adaptation of remote high-mountain communities to hydrometeorological extremes and associated geohazards in a changing climate.”
The Himalayan region is experiencing two significant and simultaneous changes whose signatures are written all over Dharali and Chisoti. As per Kaul’s thesis, there is a significant increase in extreme rainfall along with a trend towards wetter summer monsoons since the 1960s, even despite a slight regional-scale weakening of the summer monsoon over the past century. This is in tandem with the Himalayas having warmed up at a regionally averaged rate of about 0.2C per decade since the mid-20th century, which is roughly equivalent to the average rate of global land surface warming.
“Notably, climate warming across the Himalayan region has accelerated sharply since the 1980s, with annual mean temperatures increasing at a regionally averaged rate of 0.6C per decade in the period 1982-2006, which is considerably higher than the global rate of land surface warming.”
As Kaul details the geography, Dharali, in the Garhwal Himalayas, is located on the banks (alluvial fan) of a steep glacier-and-snow-fed mountain torrent known as Kheer Ganga, very near its confluence with the River Bhagirathi. The Bhagirathi originates at Gaumukh (Gomukh), the famed snout of the Gangotri Glacier, and unites with the River Alaknanda to form the Ganga at Devprayag. The Kheer Ganga (Kheer Gad), a left-bank (south-bank) tributary of the Bhagirathi, originates from the glacierised north face of the Srikantha massif (6,133 m).
The higher Himalayas are rapidly warming due to climate change. The 5,000-6,000-metre-high ice fields that nourish the Kheer Ganga have rapidly lost ice mass over the past half century. The ice cores of the moraines in the upper catchment of the Kheer Ganga have also melted almost completely in recent decades, making those moraines more prone to slope failure.
Kaul says, “As a result of climate change-induced thermal destabilisation, the glacierised and periglacial slopes in the upper catchment of the Kheer Ganga have become potent amplifiers of flash floods generated by extreme rainfall events during the monsoon.”
Over the past half century, extreme rainfall events during the monsoon season have become more common in the Garhwal Himalayas as well as the wider Western Himalayan region. Over the same period, monsoon storms from the Arabian Sea have also become more intense and frequent.
The August 5 debris flood at Dharali seems to have been triggered by an extreme monsoon rainfall event whose intensity exceeded the 99th percentile for three-day rainfall in July-August as well as the 99th percentile for daily rainfall in July-August. The extreme rainfall seems to have induced extremely rapid thawing of snow and ice on the north face of the Srikantha massif, causing catastrophic discharges of meltwater from the glacier and snowfields of the Kheer Ganga catchment.
As things would have it, the upper catchment of the Kheer Ganga witnessed an anomalously warm and dry early winter (December 2024-January 2025), followed by an abnormally snowy and sleety spring and early summer(March-June 2025), leading to a deeper/greater but more fragile snow cover.
Kaul reckons that “the extreme rainfall also seems to have triggered extremely rapid erosion of the thermally destabilised moraines in the upper catchment, and the consequent entrainment of tremendous volumes of morainic sediment in the glacial flood, converting it into a viscous debris flow.”
As the debris flow made its way down the Kheer Ganga channel, it gained volume picking up excessive sediment-rich discharges from several small snow-fed tributaries.
“Although a glacial lake outburst flood does not seem to have contributed to the debris flood, the sudden drainage of one or more small, externally invisible subglacial or englacial meltwater pools cannot be ruled out,” he says.
Senior geologist Yaspal Sundriyal, formerly with Hemvati Nandan Bahuguna Garhwal University, suspects “some breach from the higher reaches”.
Instead of a cloudburst contributing to the disaster or at least, in a significant way, it could be that snow melt accumulated in small lakes, held in place by fragile dams of ice and rock; rain melted ice in the dams holding the lakes back, which breached due to them sitting on high gradient. The high-energy flow carried small stones, big boulders, sand, clay and debris and all of that swamped Dharali.
Another possibility, he says, is that a landslide from a hanging glacier. As the name suggests, hanging glacier ends at or near the top of a cliff. The landslide might have formed a lake, which burst. These places have a history of floods. Yet, people built their houses along the river.
Kaul details the geography of Chisoti, in the Kishtwar Himalayas. It too is located on the banks (alluvial fan) of a steep glacier-and-snow-fed mountain torrent, Jasnai or Chisoti Nallah, very near its confluence with the Bhut Nallah, a significant right-bank (northeast-bank) tributary of the River Chenab (Chandrabhaga).
The Chisoti Nallah, a right-bank (northwest-bank) tributary of the Bhut Nallah, originates from the glacierised southeast face of the Arjuna massif (6,230 m). The 4,500-6,000-metre-high ice fields that nourish the Chisoti Nallah have rapidly lost ice mass over the past half century. The story of the Kheer Ganga is repeated here as well. As a result, the loose debris, steep slopes and heavy rain combined to induce catastrophe.
Kaul says the August 14 event at Chisoti also seems to have been triggered by a monsoon storm from the Arabian Sea. It may have led to an extremely rapid thawing of snow and ice on the southeast face of the Arjuna massif, and catastrophic discharges of meltwater from the glaciers and snowfields of the Chisoti Nallah catchment. All of this culminated in a massive flood of debris and meltwater swelled by discharges from smaller snow-fed tributaries.
This region, too, had an anomalously warm and dry early winter (December 2024-January 2025), followed by an abnormally snowy and sleety spring and early summer (March-June 2025).
“Although a glacial lake outburst flood does not seem to have contributed to the debris flood, the sudden drainage of one or more small, externally invisible subglacial or englacial meltwater pools cannot be ruled out,” Kaul says.
Professor Sundrial advocates for finding out sites and zones that could be flooded by debris flows as happened in Dharali and Chisoti, sensitising people there and halting construction along the river course.
Rivers have memories
