When a mountain falls: How ice and rock triggered Nepal's catastrophic flood
The catastrophic flood that killed hundreds of people in Nepal on Aug. 26, 2026, and left many more missing likely began with falling ice and rock crashing into the river below, but that wasn't the end of the disaster. Days later, a new lake pooled behind the debris and then burst, sending more water streaming through the valley.
A catastrophic flood in Nepal on August 26, 2026, killed hundreds and left many more missing, likely triggered by falling ice and rock into a river below. This disaster was not the end; days later, a new lake formed behind debris and burst, sending more water through the valley. The disaster highlights how rising temperatures, thawing permafrost, and melting ice are increasing the risk of deadly floods in high mountain regions worldwide.
Geologists from an international team are analyzing satellite images and field data to understand the risks from climate change. Evidence suggests that a large mass of bedrock and glacier ice broke off a slope near Langtang Lirung, slamming into the valley 1,200 meters below. This caused a slurry of melting ice, rock, and water to flow into the river. The landslide generated a magnitude 5.2 earthquake, and the resulting slurry likely contributed to the flood.
The initial ice and rock avalanche briefly dammed the valley, allowing water to back up. When this dam broke, a wall of water rushed downstream, reaching towns and destroying infrastructure. Within a day, a new barrier lake formed behind avalanche and flood debris, and on August 28, it broke, causing the river to rise again. This demonstrates how temporary dams can form and fail with little warning, posing significant risks.
Other recent disasters, such as a rock and ice avalanche in India's Uttarakhand and a rock fall in Switzerland's Alps, also killed or left people missing. The increasing number of glacial lakes due to climate change and the thawing of permafrost further raise the risk of outburst floods. Thawing permafrost has led to rock collapses into lakes, causing waves that burst through barriers holding back glacial lakes, resulting in massive water releases that damage infrastructure.
The risks are exacerbated by climate change, population growth, and the increasing use of mountain valleys for hydropower. As countries build more infrastructure in these areas, the hazard risk rises. The Himalayas, often called the "third pole" due to their large ice volume, are experiencing accelerated ice loss rates since 2000.
Permafrost thawing weakens slopes, and meltwater can lubricate rock and ice, leading to sudden landslides. The recent disaster in Nepal occurred in a valley with multiple hydropower plants, bridges, and a major border crossing, making it a significant catastrophe.
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