Nepal, Rishiganga and Dharali are signals of a Himalayan crisis

The latest devastating flood in the Bhote Koshi river in Nepal’s Rasuwa region has once again exposed the deep distress of the Himalaya—warning signs that would be perilous to ignore. Preliminary scientific assessments suggest that the sudden surge in the Bhote Koshi was not an ordinary rainfall-induced flood. As in the Rishiganga disaster of February 7, 2021, there are indications that a massive mass of ice and rock may have collapsed in an avalanche, plunged into the river valley and transformed into a destructive debris flow carrying water, boulders and sediment. The catastrophic flood at Dharali in Uttarkashi on August 5, 2025, too, occurred without rainfall intense enough to readily explain the scale of destruction. Scientific investigation into the precise causes of the latest Nepal flood is still under way, but the circumstances surrounding these three disasters show striking similarities. Taken together, they constitute a grave warning for the Himalayan region.

According to Nepal’s Department of Hydrology and Meteorology, the affected area did not receive rainfall of a magnitude that could, by itself, explain such an extreme flood. Preliminary analysis by the International Centre for Integrated Mountain Development or ICIMOD has pointed to the possibility of a massive ice-and-rock avalanche in the upper reaches of the Lhende Khola. Satellite images reportedly indicate that a large mass broke away at an elevation of around 5,200 metres and fell approximately 1,200 metres downslope. It is suspected that the debris temporarily blocked the river, creating a natural dam. When that barrier failed, a huge mixture of water and debris rushed down the Bhote Koshi towards the Trishuli.

Temporary natural dams have formed and collapsed in Himalayan streams and rivers for centuries. What is disturbing today is the apparent increase in the frequency, intensity and destructive potential of such events.

The danger, however, is by no means confined to Nepal. The sequence can be complex: a rock or ice mass breaks away, gathers debris while descending, blocks a river, creates a temporary lake and, when the natural barrier fails, releases an enormous mixture of water, ice, boulders and earth into the valley below. Such cascading disasters cannot easily be placed in any single traditional category—flood, landslide, avalanche or glacial lake outburst flood.

Rivers such as the Bhote Koshi, Sutlej and Brahmaputra, originating on the Tibetan Plateau and in the high Himalaya, have witnessed avalanches, glacial lake outbursts and breaches of landslide-dammed lakes that have caused devastation far beyond national borders. The Bhote Koshi, which originates in Tibet as the Poiqu Chu, descends along an exceptionally steep gradient. In July 2016, a major flood caused severe damage to Nepal’s Tatopani region and the Arniko Highway.

In 2000, the failure of the Yigong landslide-dammed lake in Tibet triggered a devastating flood in the Siang Valley of Arunachal Pradesh. In 2005, the breach of the Parechu lake in Tibet caused serious damage along the Sutlej Valley in Himachal Pradesh. Excessive sedimentation and sudden changes in river courses further magnify Himalayan flood hazards. The 2008 Kusaha disaster is a striking example: the Kosi breached its embankment, shifted towards an old channel and unleashed widespread devastation across Bihar. These events demonstrate one fundamental reality—water-related disasters originating in the Himalaya do not recognise political boundaries.

The Rishiganga flood of February 7, 2021, shattered a long-held assumption in Himalayan disaster science. It showed that a catastrophic flood does not necessarily require a cloudburst or even heavy rainfall. At times, the collapse of a mountain mass of rock and ice, or a major landslide, can itself trigger a deluge.

In the Rishiganga-Dhauliganga disaster, approximately 27 million cubic metres of rock and ice broke away from the extremely steep slopes of Ronti Peak and crashed into the Rishiganga valley below. The enormous energy generated by the fall transformed the mass into a rapidly moving debris flow. Carrying huge boulders, mud and sediment, it surged down the Rishiganga into the Dhauliganga. More than 200 people were killed or went missing. The 13.2 MW Rishiganga hydropower project was virtually destroyed, while the under-construction 520 MW Tapovan-Vishnugad project suffered extensive damage.

The Dharali disaster of August 5, 2025, in Uttarkashi district is equally significant in this context. Water and debris from the Kheer Gad swept into Dharali within minutes. Satellite analysis showed a massive debris deposit at the confluence of the Kheer Gad and the Bhagirathi, spread over roughly 20 hectares and measuring about 750 metres in length and 450 metres in width.

According to the India Meteorological Department, nearby Harsil recorded only about eight millimetres of rainfall on the day of the Dharali disaster. Subsequent studies also failed to establish evidence of local rainfall intense enough to explain such an enormous debris flow on its own. Scientists consequently turned their attention to the upper Kheer Ganga catchment, the steep slopes below the Srikanth Glacier, unstable moraines and the large volume of loose mountain debris accumulated there. Water may have acted merely as the final trigger that mobilised material already close to instability.

It would be scientifically unsound to attribute every such disaster directly to climate change. Yet it would be equally mistaken to ignore the rapidly changing environmental background in which these events are occurring. The Hindu Kush-Himalayan region is among the world’s rapidly warming mountain systems. As glaciers retreat, slopes that were long stabilised by ice become exposed, new glacial lakes emerge and expand, moraines become unstable, and freeze-thaw cycles and the penetration of water into fractured rock undergo profound changes. A warming climate may not be the immediate trigger of every disaster, but it is certainly altering the physical system in which such disasters develop.

The challenge, therefore, is not to halt development but to redesign it for a new generation of Himalayan risks. Assessing the safety of hydropower projects can no longer be limited to calculating the probability of conventional river floods. Planners must also ask what would happen if, ten or twenty kilometres upstream, millions—or tens of millions—of cubic metres of rock and ice were to collapse suddenly. Where are the workers’ settlements, tunnels, barrages and control rooms located? And if an early warning is issued, how many minutes would people actually have to escape?

These conditions make comprehensive Himalayan monitoring an urgent necessity. Satellite imagery, automated weather stations, seismic sensors, glacier surveillance, river-level gauges and real-time monitoring of unstable slopes must be integrated into a single warning system. Rapid sharing of information on high-altitude lakes, glaciers, landslides and river blockages among India, Nepal, Bhutan and China is no longer merely a matter of scientific cooperation; it has become a matter of saving lives.

Rishiganga sounded the warning in 2021. Dharali repeated the question in 2025. Nepal has now placed it before us once again. Until now, whenever a flood threatened, we looked towards the river. In a changing Himalaya, we may also have to keep watch thousands of metres above the river—on the mountain slope where the next deluge may begin not with water, but with a falling rock mass, a block of ice, or an increasingly unstable mountain face.

Views expressed are the author’s own and don’t necessarily reflect those of Down To Earth