In the Himalayas, one disaster can prepare the ground for another, scientist says

Daniel Parsons says the Nepal-China flood shows how altered slopes, rivers and sediment can reshape the risks of future disasters

The devastating floods that swept through the Nepal-China border region were first and foremost a human tragedy, with hundreds of people killed and many still missing.

But the disaster also offers a stark lesson about how natural hazards behave in high mountain regions, according to Daniel Parsons, pro-vice-chancellor for research and innovation and professor of geosciences at Loughborough University.

Writing in The Conversation, Parsons argues that mountains and rivers do not simply reset after an extreme event. A landslide alters a slope, a flood reshapes a river, while a retreating glacier can expose unstable ground and vast quantities of sediment. In the Himalayas, he says, these changes can turn individual hazards into destructive cascades.

Early satellite and seismic evidence suggests the latest disaster began high in the mountains with the collapse of part of a slope and a steep glacier in Tibet. A huge mass of ice and rock then rushed into the valley below, mobilising water, sediment and debris that spread through the river system and into Nepal.

The precise mechanics of the event will take time to establish, Parsons noted, but calling it simply a flood obscures the longer chain of processes behind it.

Parsons is involved in a UK-Indian academic project examining the proposition that hazard risk in the Himalayas is not static.

Retreating glaciers can leave unstable slopes and large stores of loose sediment behind. Such deposits have been described as “sediment bombs” because vast quantities of material can remain in a valley until released by another landslide, intense rainfall or a flood.

Parsons cautioned against concluding that climate change caused this particular disaster. Individual events require careful attribution, while landslides, avalanches and floods have always occurred in the steep Himalayan terrain.

But climate change is altering the conditions in which such hazards occur. Glaciers are retreating, slopes they once helped support can become unstable, new glacial lakes are forming and large quantities of loose sediment are being exposed. Warmer air can also hold more moisture, increasing the potential for intense rainfall.

More significantly, Parsons said, these changes are making hazards increasingly interconnected.Changes in glaciers can affect slope stability; slope failures can destroy glaciers and alter river drainage; and altered rivers can mobilise sediment, amplifying subsequent floods.

Mountain landscapes also retain the physical legacy of previous disasters. A landslide can fill a river system with debris that takes years or decades to move downstream, altering channels and influencing how subsequent floods behave.

Thus, the same rainfall can produce very different consequences depending on the state of the landscape. Heavy rain falling on a relatively stable catchment with a clear river channel is one thing; the same rainfall after a major landslide has deposited millions of tonnes of sediment and narrowed the channel is quite another.

The latest disaster will itself have changed the landscape, Parsons wrote. Sediment will have been redistributed, river channels altered and some slopes potentially destabilised. Embankments that once guided water and sediment may have been damaged or destroyed.

Communities can also carry the effects of previous disasters into subsequent ones. The same river corridor experienced severe flooding in 2025, when people were killed, a Nepal-China friendship bridge was destroyed and transport and trade disrupted. Some communities affected by the latest disaster were still dealing with the consequences of the earlier one.