Kathmandu, Aug. 27 -- Nepal's devastating flash floods on August 26 were the result of several factors that make the Himalayan nation particularly vulnerable to natural disasters. The country's exposure is linked not only to a warming Himalaya, which is creating unstable water bodies, but also to the ongoing collision of the Indian and Eurasian tectonic plates.
At around 9 am on August 26, a huge surge of water swept down the Bhote Koshi River in Nepal's Rasuwa district, destroying settlements close to the Tibet border before moving downstream into the Trishuli River. By Wednesday evening, more than 150 deaths had been reported in Nepal, while around 400 people were missing.
The disaster was the second catastrophic flood to hit the same 30-kilometre stretch of river in just 13 months.
What may have triggered the floods
There are three accounts about how the disaster unfolded, although they point towards a broadly similar chain of events.
Nepal's Foreign Minister Shishir Khanal told the House of Representatives that an earthquake at around 8:37 am triggered a major landslide, which blocked the river. The accumulated water then broke through the obstruction. The US Geological Survey initially recorded a magnitude 4.4 event around 47 km north of Gosainkunda in Tibet, but later clarified that the tremors were caused by a landslide rather than an earthquake.
Scientists have suggested another possibility. According to Saswata Sanyal of the International Centre for Integrated Mountain Development, the worst flooding occurred along the Lhende Khola, a tributary of the Bhote Koshi. An avalanche involving ice and rocks may have blocked the river before the accumulated water suddenly surged downstream.
Glaciologist Rijan Bhakta Kayastha also offered a similar preliminary assessment based on discussions with scientists in Nepal and China. He noted that the Lhende Khola had experienced a comparable blockage the previous year. Scientists are also examining satellite images from August 24 for evidence of changes in water movement in the upstream glacial region. Despite differences in the explanations, all three accounts suggest that an obstruction blocked a Himalayan river, water accumulated behind it and the barrier eventually gave way.
Nepal has faced similar disasters before
Nepal is particularly vulnerable to glacial lake outburst floods, sometimes described as "inland tsunamis". Research cited in the report shows that the rate of ice loss across the Hindu Kush Himalayas has doubled since 2000, increasing risks for around two million people living downstream of glaciers and the rivers fed by them.
The region has experienced similar disasters repeatedly. A flood last year was linked to a glacial lake outburst in Tibet. In July 1981, a lake in Tibet's Zhangzangbo valley also drained suddenly. Research later linked that event to the deaths of more than 200 people, the destruction of the Friendship Bridge and severe damage to the Sun Koshi hydropower station. Major floods in 1964 and 1983 were also linked to the same lake system.
The history of such disasters extends much further back. The World Bank has recorded 24 major outburst floods in Nepal, including 14 originating within Nepal and 10 in Tibet. One such event occurred around 450 years ago in the Seti Khola river system after a moraine holding back a glacial lake collapsed, sending huge amounts of debris into the Pokhara Valley.
The July 2025 flood near the Tibet-Nepal border was later attributed to the sudden drainage of a supraglacial lake that had formed around 35 km upstream of the border at an elevation of approximately 5,150 metres.
Hundreds of glacial lakes pose a risk
As glaciers melt, water can accumulate in hundreds of lakes on or around the ice. These lakes can become unstable and release huge volumes of water suddenly.
A 2020 inventory by the UN Development Programme identified 3,624 glacial lakes across the Koshi, Gandaki and Karnali basins. Of these, 47 were classified as potentially dangerous: 25 were located in Tibet, 21 in Nepal and one in India.
Monitoring these lakes remains difficult because supraglacial lakes can form, expand and disappear within only a few seasons. This means existing inventories can become outdated, and some potentially dangerous lakes may not be identified before they trigger a disaster.
Lack of cross-border warning system
Another challenge is the limited availability of real-time information from the Tibetan side of the border.
Following the 2025 flood, Nepal's Department of Hydrology and Meteorology had said there was no mechanism for obtaining data from the Tibetan region. While Nepal receives information about rainfall from China, glacial lake outburst floods do not necessarily depend on rainfall.
In 2025, Nepali officials reportedly received around an hour's warning before floodwaters entered the country, and that information came from border security personnel rather than a forecasting agency. After that disaster, Nepal's federal government decided to work with China on early-warning and preparedness measures. A grant of around $36 million was also approved by the UN Green Climate Fund in July 2025 to help Nepal address the growing threat. However, there is still no real-time warning system operating across the border.
The Himalayas remain geologically active
Glacial melting is only one part of Nepal's vulnerability. The country lies along the boundary where the Indian and Eurasian tectonic plates are moving towards each other at around 20-21 mm every year.
The collision of these plates began roughly 50 million years ago and led to the formation of the Himalayas. The process continues today, with the mountain range rising by approximately 5 millimetres annually. The continuing movement creates enormous geological pressure, which is released through earthquakes. Even relatively small tremors can destabilise already weakened mountain slopes.
In the August 26 disaster, the movement associated with a landslide appears to have been enough to bring down a slope and block a river, eventually producing a sudden surge of water.
Construction adds to the risks
The location of infrastructure is another factor increasing the potential impact of disasters in the region.
Research cited in the report found that development along China-Nepal trade corridors has not always accounted for the threat posed by glacial lake outburst floods. Dry ports and customs facilities have been constructed along river plains that could be exposed to such flooding. Research has estimated that glacial lake outburst floods could affect around 103 km of the China-Nepal highway, 103 bridges, two major dry ports and 3,301 buildings across the two countries.
The customs post at Timure near the Rasuwagadhi border crossing has now been destroyed twice in roughly 13 months. The region is also important for Nepal's hydropower sector, with fast-flowing mountain rivers being used to generate electricity. The latest disaster resulted in a loss of 430 MW from Nepal's national power grid, accounting for more than a tenth of the country's generating capacity. Floodwaters eventually flow downstream into the Trishuli, which becomes the Narayani at Devghat before entering India as the Gandak. This means that a disaster originating from an unstable glacial lake in Tibet can have consequences far beyond the immediate mountainous region.
Published by HT Digital Content Services with permission from Millennium Post.