The story so far: A devastating flash flood in the Bhote Koshi–Trishuli river system has killed at least 389 people in Nepal and three in Tibet, while more than 1,400 people are reported missing. Hundreds of people have been displaced in affected districts including Rasuwa, Nuwakot, Dhading, Gorkha and Chitwan No deaths have been reported in India so far, but 288 Indian nationals in Nepal remain uncontactable, with 84 Indians rescued and authorities in Bihar and Uttar Pradesh monitoring downstream flood risks. Roads, bridges, homes and hydropower infrastructure have been destroyed, and search, rescue and relief operations are under way.
What caused the flood?
The evidence now points to a large collapse of ice and rock high in the Himalayas near Langtang Lirung, rather than heavy rainfall. A mass of glacier ice, rock and debris appears to have detached at about 5,200 metres and fallen roughly 1,200 metres into the Lhende Khola system by momentum, gathering more rock, sediment, ice and water and morphing into a fast-moving debris flow. The rubble temporarily blocked the river and the resulting surge then travelled downstream through the Bhote Koshi and Trishuli systems, affecting communities nearly 100 km away.
Why was an earthquake initially suspected?
Seismological instruments detected a strong ground signal at about 8.37 a.m. on August 26. It was initially interpreted as an earthquake of roughly magnitude 4.4. Because the flood followed almost immediately, the natural explanation was that an earthquake had shaken loose a landslide or glacier, which then blocked the river.
Editorial | Danger from above: On the Nepal flood
That interpretation has since changed. Analysis by the U.S. Geological Survey (USGS) found that the seismic signal originated in a glaciated mountain cliff and was generated by the collapse itself. A second seismic event, about three hours later, produced energy equivalent to a magnitude-4.2 earthquake. A preliminary assessment by the National Remote Sensing Centre (NRSC) of ISRO provides important evidence about the origin of the Nepal floods. NRSC compared a pre-event Sentinel-2 image from August 24 with a post-event Resourcesat-2A image acquired on August 26, as well as with an earlier Resourcesat-2A image from April. It provides independent visual evidence of a major physical change at the suspected source area, complementing the seismic evidence that places the ground-vibration source in a glaciated mountain cliff on the north side of Langtang Lirung. However the precise mechanics of the collapse are still being reconstructed.
So how can a non-earthquake produce a “4.2”?
A seismometer does not identify an earthquake by itself. It records ground vibrations. Earthquakes are the most familiar source, but large landslides, volcanic eruptions, explosions and glacier collapses can also shake the ground.
The magnitude figure therefore describes the size of the seismic energy recorded, not necessarily the cause. In this case, the collapsing mass was so large and moved so rapidly that it generated seismic waves comparable to those produced by an earthquake. USGS estimates that the initial collapse released energy equivalent to about a magnitude-5.2 earthquake.
What is an ice-rock avalanche?
It is essentially a rapidly moving mixture of ice and rock released from a steep mountain slope and in the process of descent incorporating snow, water, soil, boulders and other material. The result can behave less like a conventional landslide and more like a highly destructive, mobile debris flow. In Nepal, the material entered a river, temporarily dammed it and then generated an enormous downstream flood.
Was this like the South Lhonak GLOF in October ‘23?
In the 2023 South Lhonak disaster in Sikkim, a glacial lake burst, releasing stored water and producing a flood downstream, draining about 105 hectares of the lake.. A GLOF, or glacial lake outburst flood, requires a lake and the sudden failure of its natural dam whereas the Nepal event appears instead to have begun with the collapse of ice and rock. The two hazards can nevertheless overlap wherein an ice or rock avalanche can fall into a glacial lake, generate a wave and cause the lake to breach.
Why can’t glacier collapses be predicted?
Because scientists can often identify hazardous conditions without knowing when failure will occur. This is like predicting which specific straw will break the proverbial camel’s back. A steep glacier may remain apparently stable for years and then fail suddenly. Temperature, meltwater, cracks, bedrock instability, permafrost and freeze-thaw processes can all influence stability, but there is no reliable equivalent of an earthquake forecast that says a particular glacier will collapse at a particular time.
Can satellites track glaciers?
Yes. Satellites can repeatedly measure glacier area, movement, elevation, snow cover and the growth or shrinkage of glacial lakes. Radar satellites can also observe through clouds, while optical imagery provides detailed before-and-after pictures. ISRO, for example, uses satellite data to monitor glaciers, glacial lakes and landslide-dammed water bodies. For all this, their important limitation is that they mostly observe change rather than predict sudden failure. Satellite revisit times, cloud cover, resolution and the difficulty of obtaining measurements in inaccessible high-altitude terrain constrain monitoring.
Published – August 28, 2026 11:49 am IST
