Why the Nepal floods are a warning for the Himalayas, and India

Each of them involved a very large and sudden release of waters in the mountainous rivers, either due to a breach in a glacial lake, or because of a large block of glacial ice breaking off and falling into the river.

The breach in the lake or the breaking off of the ice-block can be spontaneous — under accumulated stress over time — or be caused by an extremely heavy rainfall, a landslide or an earthquake.

The end result, however, is the same — a series of cascading events in which a mass of water, stones and soil gushes downstream at great speeds, eroding river banks, uprooting trees, triggering landslides, and destroying buildings, roads and bridges. All the debris being collected by the river on the way further strengthens its force, and its potential to cause damage. The situation continues till the river arrives at a flatter landscape, offering the raging flow an opportunity to spread out and disperse.

What caused the Nepal event

Wednesday’s event, which started in the northeastern district of Rasua in Nepal, is currently understood to have been triggered by a rock and ice avalanche, a result of a large block of glacier in the upper reaches of the Bhotekashi river (the lower side is also called the Trishuli) falling off. What caused this break-off is not very clear as of now. There was no heavy rainfall incident recorded in the area, so that factor is largely ruled out.

The US Geological Survey had reported a 4.4 magnitude earthquake in the region on Wednesday, which was initially believed to be the trigger. On Thursday morning, however, it said the signal that was captured by its sensors was most probably a result of a landslide and not a seismic event.

“This event was initially reported as a magnitude 4.4 earthquake. Additional analysis of nearby seismic stations, long period seismic waves and satellite imagery led to the determination that the seismic event was instead a glacial collapse and debris flow, and that no earthquake had occurred,” said the USGS update.

The deadly path of the Nepal flash flood

That could mean that a large chunk of snow broke off from the glacier on its own, possibly under its own internal stress, and caused a landslide which triggered the chain of events.

But Syed Iqbal Hasnain, a veteran glaciologist and former vice-chancellor of University of Calicut, pointed out that ice-blocks were not visible in any of the videos of the disaster. “The ice that breaks off from a glacier is almost like a solid rock. It does not melt immediately. But no ice block is visible in the videos of the incident. It is possible therefore that the incident was caused not by ice blocks breaking off from a glacier, but from a glacial lake outburst,” he said.

Safi Ahsan Rizvi, an IPS officer who worked with the National Disaster Management Authority (NDMA) before his retirement, suggested that it could have been a result of a breach in a “supra-glacial lake”. Lakes are generally formed at the end of a glacier, but sometimes, mainly as a consequence of climate change, lakes emerge within the glaciers themselves, and are known as supra-glacial lakes. A breach in this kind of lake could explain the lack of ice blocks in the visuals.

Previous events

The actual sequence of events can be ascertained only after scientists are able to complete a comprehensive study. But it unfolded  in a manner very similar to the other incidents in India in the previous years.

“As of now, the Nepal event looks very similar to the Chamoli disaster of 2021 in the way it got triggered. But in terms of its impact downstream, and the destruction it has caused, it is more like the Kedarnath tragedy of 2013,” said Farooq Azam, an associate professor at IIT Indore. Azam currently heads the Cryosphere group at the Kathmandu-based The International Centre for Integrated Mountain Development (ICIMOD), a major research organisation working on the Himalayas.

Chamoli was caused by an ice and rock avalanche, a result of a large ice-block of a glacier, estimated to be 27 million cubic metres in all, breaking away and falling downhill. Kedarnath, on the other hand, was a GLOF (Glacial Lake Outburst Flood) event, the result of a breach in a glacial lake.

A border area between Nepal and Tibet (China), before the flooding (left) and after (right). (Planet Labs PBC via AP)

The Kedarnath incident was much more devastating. The damage potential of an event like this depends on factors like geography, the incline of the river, vegetation, population and urbanisation. The amount of rock and ice that fell off in the Nepal incident is yet to be estimated, but the fact that its fall was captured as a fairly large seismic wave (USGS had revised its 4.4 measurement to 5.2) suggests the involvement of a very big chunk of mass. It is not surprising, therefore, that the impact of the Nepal event appears to be significantly bigger than the Chamoli incident, in which about 200 people were reported to have been killed, or gone missing.

The reason these events in the Himalayan region become so risky is that while they start off as individual events — extreme rainfall or landslide or lake overflow — they soon transform into a multi-hazard disaster, involving multiple landslides, flooding and building collapse episodes.

Disasters in the Himalayas

“The Himalayan region is very fragile. Such events lead to a chain reaction. A surging river with large rocks and ice blocks would erode the banks and slopes, and more soil, mud and rocks would become part of the raging river. It becomes a cascading disaster because the waters do not have any other way to spread out. That is why any such event in the Himalayan ecosystem becomes extremely dangerous,” Azam, currently in China attending a disaster risk reduction conference, told The Indian Express.

A study published in Science magazine last year about a 2023 event in Sikkim captured the cascading impact of such flash flood events in the mountains. This event had been triggered by the collapse of a large chunk of glacier into the South Lhonak lake, situated over 5,000 metres above sea level, in northern Sikkim. This led to a GLOF event that took the shape of flash floods.

The study found that a landslide had resulted in the collapse of nearly 14.7 million cubic metres of rocks, soil and snow deposited around a glacier into the South Lhonak lake, which triggered a series of cascading events. The volume of the flowing sediment downstream had grown to about 270 million cubic metres.

Nepal’s Syapru Besi before (left) and after (right) the floods. (Planet Labs PBC via AP)

“The collapse of approximately 14.7 million cubic metres of frozen lateral moraine into the South Lhonak Lake … on 3 October 2023 generated a 20-m-high displacement wave. The wave overtopped and breached the frontal frozen moraine, causing the sudden drainage of approximately 50 million cubic metres of water… The GLOF eroded approximately 270 million cubic metres of sediment and triggered 45 secondary landslides. Most of the erosion occurred within about 67.5 km of the lake,” the study had found.

Vulnerable Himalayas

Azam said the entire Himalayan region was “out of balance entirely” because of population pressure and climate change.

“The whole ecosystem is vulnerable. Because of climate change and increasing instances of extreme weather events, newer areas are getting exposed. Earthquakes or heavy rainfall events are quite common, and any of them can trigger these kinds of disasters,” he said.

Incidentally, the Himalayan region is also one of the most dangerous places for earthquakes. The region has recorded some of the biggest earthquakes in the past. Nepal itself was the victim of a 7.2 magnitude earthquake in 2015.

Flooding in Timure, Nepal on Aug. 27, 2026. (Planet Labs PBC via AP)

The only way to minimise the damage from such events is to install early warning systems, prepare evacuation plans, and strictly monitor and regulate construction and other activities in the ecologically fragile region so that further instability is avoided.

After the Sikkim disaster, the NDMA had initiated a National GLOF Risk Mitigation Programme (NGRMP) seeking to install early warning systems at glacial lakes at high risk of facing a breach. The Rs 150-crore programme is targeting connecting 195 glacial lakes in Sikkim, Uttarakhand, Arunachal Pradesh, Himachal Pradesh and the union territories of Jammu and Kashmir and Ladakh with early warning systems.

“As of now, no such early warning system exists, not in Nepal as well. The Chinese and the Nepalese sides may have some informal mechanism to exchange information but an established mechanism does not exist. The same is the case in India as well. It will take some time, but most of the risky glacial lakes would eventually be connected to an early warning set-up. This is likely to substantially reduce the risks from such events,” Rizvi said.