What Caused Nepal's Deadly Flash Floods, and Why the Country Keeps Facing Them

A wave of flash floods tore through a roughly 45 mile stretch of the Trishuli River in Nepal's Rasuwa and Nuwakot districts this week, destroying dozens of settlements and leaving officials scrambling to account for hundreds of people still unaccounted for.

Scientists say the disaster appears to have been triggered by a rare and devastating combination: a massive chunk of glacier breaking off high in the mountains near the Nepal China border, falling thousands of feet and triggering a landslide below it. The combined mass of ice, rock and debris slammed into the river, sending a surge of water and sediment downstream that overwhelmed towns with little warning.

Nepal's death toll has climbed into the hundreds, with hundreds more reported missing, and authorities across the border in China have also reported deaths and missing people tied to the same event. Search and rescue operations remain underway in the affected districts.

The disaster has renewed attention on why Nepal is so consistently vulnerable to this type of flooding. The country sits in some of the steepest, most geologically active terrain in the world, where intense monsoon rainfall, sudden cloudbursts, landslides and unstable glacial lakes can combine with little notice. Researchers have identified more than 3,600 glacial lakes across Nepal, India and Tibet, and classified dozens of them, including 21 inside Nepal, as potentially dangerous and at risk of bursting.

Experts who study the region point to a mix of geological and human factors driving the growing toll from these events, and have called for expanded satellite monitoring, more scientific research into glacier and slope stability, and more resilient infrastructure in the river valleys below Nepal's high mountain terrain.