The devastating flash flood in Nepal has once again drawn attention to a little-understood hazard in the Himalayas - the collapse of hanging glaciers and the cascading floods they can trigger.
Around 9 am on August 26, a massive ice-rock avalanche reportedly blocked the Lende Khola, a tributary of the Bhote Koshi near the Nepal-China border. The sudden obstruction and subsequent release of water sent a violent surge of water, ice, sediment and boulders downstream, causing widespread destruction in Nepal.
The exact cause of the glacier collapse is yet to be established. But the disaster has highlighted growing concerns over the stability of glaciers in a warming Himalaya, where ice loss has accelerated sharply - nearly doubling the rate recorded three decades ago. The United Nations had declared 2025 the International Year of Glaciers’ Preservation to highlight the importance of glaciers, snow and ice to the climate system and global water cycle.
What triggered the Nepal flash flood?
According to the US Geological Survey (USGS), a large glacier collapse occurred at an elevation of about 5 km near the Nepal-China border. The event generated ground vibrations equivalent to a magnitude 5.2 earthquake and sent a huge mass of ice and rock crashing into the Lende Khola gorge.
The debris blocked the river, creating conditions for a sudden surge downstream. The resulting flow carried mud, rocks, ice and large boulders, turning the river into a destructive, debris-laden torrent. An initial suggestion that an earthquake had triggered the collapse was subsequently abandoned, leaving scientists to determine what caused the glacier mass to fail.
Nepal’s National Disaster Risk Reduction and Management Authority cited satellite imagery from Planet Labs and reported that a large section of the glacier on the northern side of Langtang Himal detached and fell from an elevation of nearly 5.2 km to the river at about 3 km on the Nepal side.
A preliminary assessment by the International Centre for Integrated Mountain Development (ICIMOD) also indicated that a substantial ice-rock avalanche descended from high elevations into the Lende Khola, producing a rapid downstream surge.
This event generated an unprecedented surge of water mixed with sediments and large boulders, sending water levels to a height of 9 metres within 30 minutes and impacting areas more than 100 km downstream. The surge ripped through villages and towns, carried away 19 motorable bridges and 42 km of highways, and damaged 13 hydropower plants, wiping out 430 MW, or over 12 per cent of Nepal’s power grid. Mountain towns and trading posts such as Timure, Rasuwagadhi and Syabrubesi suffered catastrophic damage under thick mud and debris, with Rasuwa district being the hardest hit. The violent river flow swept through the Bhote Koshi corridor all the way to the Gandak Barrage in India, covering a distance of more than 200 km in just a few hours.
How big was the ice mass?
Estimates of the size of the collapsing ice mass have varied considerably. Some estimate it to be between 0.2 km and 1.5 km wide, descending from about 5 km above the river surface. An expert on the Tibetan Plateau, analysing satellite images on a social media platform, stated that the ice chunk was likely 1.5 km wide and 1.4 km deep - almost twice the height of the Burj Khalifa. The total area of the ice chunk could be equivalent to 250 football fields, while claims have put its volume at about 30 million cubic metres.
India has seen a similar disaster
The Nepal event has echoes of the February 2021 Chamoli disaster in Uttarakhand. On February 7, 2021, a massive ice-rock avalanche in the Rishi Ganga valley triggered a devastating flash flood. The disaster killed at least 200 people, including those reported missing, and destroyed infrastructure, including hydropower facilities. The Chamoli tragedy demonstrated how an avalanche originating high in the mountains can rapidly transform into a major downstream disaster when it enters a river valley.
Threat of hanging glaciers
Most Himalayan glaciers have been experiencing negative mass balance, meaning they are losing more ice than they gain. An analysis by ICIMOD shows that mean mass loss has almost doubled from the pre-2000 period (1974-1999) to the post-2000 period (2000-2023). Scientists estimate that the annual mass loss has intensified to approximately 9 cm of water equivalent per decade.
The Indian Institute of Science’s Divecha Centre for Climate Change (DCCC) has conducted a comprehensive study of hanging glaciers on the Indian side of the Himalayas. Over the past two years, researchers assessed a total of 858 hanging glaciers. The findings revealed that 99 glaciers are located in the Yamuna basin, 219 in the Alaknanda basin, 261 in the Bhagirathi basin and 279 in the Kali basin. The cumulative water stored in these glaciers amounts to 3,236 gigatonnes.
Dr Anil V Kulkarni, a leading glaciologist in India, noted that predicting the extent of damage caused by the melting and breaking of glaciers is challenging. Localised effects also affect hanging glaciers, in addition to the degradation of permafrost, which includes soil, rock or sediment that remains frozen at or below zero degrees Celsius.
Melting of glaciers in HKH region
The HKH region is an extensive 3,500-km mountain system spanning eight countries - Afghanistan, Bangladesh, Bhutan, China, India, Myanmar, Nepal and Pakistan. It is home to over 63,700 glaciers, covering nearly 55,782.02 square km and storing around 5,735.79 cubic km of estimated ice reserves, according to 2020 Landsat satellite imagery.
A study by ICIMOD indicates that glacier loss in the HKH is widespread and accelerating, threatening the sources of at least ten major Asian river systems.