Nepal flash floods: What triggered the disaster, and why the country is so vulnerable

The cause of the latest event, however, is unclear. Experts initially pointed to the possibility of an an upstream avalanche triggered by an earthquake. The US Geological Survey had reported a 4.4 magnitude earthquake in the region, which was widely believed to have triggered the landslide.

However, on Friday morning, the USGS updated its report saying that the seismic energy that was captured by its sensors was not generated by an earthquake but rather by a glacial collapse and debris flow. “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,” USGS said in its update.

What, however, is a flash flood, and how can it be caused by an earthquake? And how does Nepal’s unique topography make it more vulnerable to such disasters? We explain.

How does a flash flood happen?

A flash flood is a sudden (and often violent) surge of water that inundates low-lying areas in very small periods of time.

In mountainous terrains, such floods carry greater energy, obliterating all the structures in its path. The sheer velocity of the flood leaves communities that live downstream with little to no warning or window for evacuation.

The latest flash flood instantly destroyed heavy infrastructure such as hydropower plants and communications hardware, cutting off affected border areas from the outside world. While the reason for this calamity is yet to be officially stated, official channels are considering the possibility that an avalanche triggered by an earthquake was the catalyst.

How can an earthquake trigger an avalanche?

The peaks of the Himalaya mountains are capped with gargantuan glaciers and precariously balanced rock formations. An earthquake can disturb this fragile balance, violently displacing rock, ice, mud and other solids.

Before-and-after satellite images show flooding in the areas around the Trishuli river. (Special Arrangement)

According to Reuters, authorities suspect that an ice-rock avalanche was triggered by such an earthquake in the Lhende Khola river. This narrow, high-altitude tributary of the Bhote Koshi carried the bulk of the avalanche down its waters. Somewhere along the way, the debris likely formed a dense and temporary dam. In such cases, water can back up rapidly behind this unstable barrier until the constantly increasing pressure gives way to a rupture in the dam wall.

This rupture can send a wall of water in combination with the solid debris from the avalanche hurtling downstream towards settlements.

However, experts now say that the seismic event hat was captured was not actually an earthquake but triggered by glacial collapse and debris flow.

Nepal’s topographic conditions

Nepal’s extreme topography includes some of the highest peaks on Earth alongside narrow but deep river gorges. This combination creates funnels that naturally accelerate the speed of the water, further increasing its destructive capacities.

While this creates a host of geological complications, some of the immediate hurdles are:

• The naturally rough terrain coupled with a disaster makes emergency response services difficult. Officials stated on Wednesday that rescue helicopters would not be able to land in the affected areas until the floodwater had receded. The narrow nature of habitable valleys also creates highly concentrated settlements. In such cases, even a single flood can geographically isolate such communities from aid.

Rescuers use an earthmover to carry a victim of flash flooding to safety along the Trishuli River in Nepal’s Nuwakot district, Aug. 26, 2026. (AP Photo)

• The incline and altitude for the flood on its way down also amplify the dangers. As water hurtles down the steep gradients, it also dislodges huge amounts of earth and boulders lying on the riverbed and the banks. This flood sediment has created logistical challenges for the 574 rescue personnel China has deployed to Gyirong port’s land crossing.

Within the geography of the Himalayas, the Lhende Khola is a transboundary river that runs through both Nepal and China. It assumes the role of a high-altitude tributary of the Bhote Koshi river—part of the region’s interconnected hydrology network which includes the Trishuli and Narayani rivers.

All three carve out deep river basins for themselves through the mountains before flowing downstream into the plains. This interconnected flow is why flood alerts were immediately issued in downstream Indian states like Bihar and Uttar Pradesh.

Furthermore, the fast-flowing rivers are highly prized for hydropower generation, drawing foreign investment and workers — such as eight South Koreans who remain unaccounted for after a hydropower plant was struck during the floods.