Scientists probe why Nepal floods generated 'remarkable' water volumes

The Nepal-China border was struck by catastrophic flooding this week that has claimed at least 600 lives and left roughly 2,500 people unaccounted for. Among the missing are five New Zealanders. The disaster began high in the mountains of the Tibet Autonomous Region, where a mass of rock and ice spanning more than 600 metres across suddenly gave way.

Graham Leonard, a principal scientist with Earth Sciences New Zealand, has been studying the mechanics of what unfolded. The material that broke loose plummeted about two kilometres down the mountainside at considerable speed. As it fell, it transformed into what geologists call a debris flow—a violent mixture of rock, ice, sediment, and water that gathered momentum and mass as it descended. The flow picked up boulders and earth as it moved, carving through the landscape with tremendous force.

The debris flow travelled roughly 60 kilometres through river valleys before finally dispersing across a wide alluvial fan where it could spread out and lose some of its destructive energy. What puzzles scientists is the sheer volume of water the collapse generated. Leonard and his colleagues are still working to understand why such a remarkable amount of water was produced from what began as a rock and ice failure. An initial tremor was recorded as a 5.2 magnitude earthquake, though the primary driver of the flooding was the physical collapse itself.

While this particular disaster cannot be directly attributed to climate change, Leonard emphasises that a warming planet is creating the conditions that make such events more likely. As global temperatures rise, permafrost thaws, glaciers retreat, and the structural integrity of high mountain slopes deteriorates. The ground becomes less stable. Rock and ice that has been locked in place for centuries becomes mobile. In mountainous regions like Nepal and the Himalayas, these processes are accelerating. "Nepal and high mountainous areas are definitely a hot spot," Leonard said. The warming is generating more frequent landslides, larger debris events, and more severe flooding on the plains below.

The same pattern is visible in New Zealand's Southern Alps, where Aoraki Mt Cook continues to rise due to tectonic forces while its glaciers erode at an accelerating rate. Leonard notes that the recent cluster of earthquakes across New Zealand—in Fiordland, Taumarunui, the East Coast, and Edgecumbe—have stressed the ground around them, though they are not directly connected to one another across distance. They are linked, however, through New Zealand's position on a plate boundary where faultlines and seismic activity are endemic. Globally, earthquakes above magnitude 5 send waves through the entire planet, a reminder of how interconnected Earth's systems are.

Leonard pointed to the 2022 eruption of Hunga Tonga-Hunga Ha'apai, an undersea volcano in Tonga, as a striking example of this interconnectedness. The explosion was so violent that it sent a shock wave through the atmosphere and generated a tsunami that circled the entire globe. The eruption was so powerful it breached the atmosphere itself and erupted into space—comparable in scale to the cataclysmic event that created Lake Taupō. The sound was heard as far away as Alaska and Aotearoa. The atmospheric pressure wave was so intense it generated metre-high tsunamis in the oceans as far away as South America. These are the scales at which planetary systems operate, and they are reminders of both the fragility and the interconnectedness of the world we inhabit.

For those living in earthquake and flood-prone regions, Leonard's message is practical: check your foundations and brick chimneys, maintain emergency supplies, and have a family escape plan. The warming climate is making high mountains less stable. The ground beneath us is more active than we often acknowledge. Preparedness is not optional.