What Caused the Flash Floods That Killed Hundreds in Nepal-Tibet?

When floods struck the Nepal-Tibet border on Wednesday (26/08), Keshav Prasad Baral was at home with his wife. “A huge amount of mud suddenly surged directly towards us,” said the 68-year-old, who is now being treated at a small hospital in Nepal. As the mudflow burst into his home, Baral tried to grab his wife’s hand. But she was “swept away by the current”. “My wife is still somewhere under the mud,” he said. “She is gone.”

Nepal estimates the disaster has caused a very large number of casualties. At least 469 people have died, while around 910 others remain missing, including about 517 foreign tourists. In Gyirong, China, 558 people including 260 foreign nationals are also unaccounted for, according to Chinese state television CCTV.

Initially, the Nepalese government suspected a 4.4 magnitude earthquake near the border triggered the disaster. However, according to the United States Geological Survey (USGS), the event recorded as seismic activity was actually a glacier collapse followed by a debris flow. The event then caused severe impacts downstream.

The finding aligns with observations previously conveyed by officials from Nepal’s Department of Hydrology and Meteorology to the BBC. From images examined by the department, a large mass of ice likely originated from a glacier that collapsed from a mountain slope on the Tibetan side. The material then surged down the slope together with rocks and sediment.

The Nepalese government suspects the collapse may have released a large volume of water while generating a very powerful debris flow. One theory is that the debris temporarily blocked the Lhende River and formed a natural dam. Water then continued to accumulate behind the dam until it eventually burst, sending a wave of water and material downstream.

However, several experts caution that evidence of river blockage remains limited. Basanta Raj Adhikari, a disaster studies expert from Tribhuvan University in Nepal, told BBC News Nepali that there were no reports of a decrease in the Lhende River’s discharge or water level before the flood arrived. A drop in water level would normally occur if the river flow had been dammed upstream.

According to Adhikari, the flash flood could have occurred directly as a result of the ice and rock avalanche triggered by the glacier collapse. He also warned of the possibility of “second and third waves of danger”. “We still don’t know,” he said. Adhikari said he continues to communicate with colleagues in China who are monitoring the situation. “There is no heavy rain at the moment, but landslides can still occur,” he remarked.

Several glaciology experts say the latest evidence makes the possibility of a glacial lake outburst flood (GLOF) as the primary cause increasingly unlikely. A GLOF occurs when water stored in a glacial lake suddenly escapes after the natural dam holding the lake—which may be made of ice, rock, or sediment—collapses or bursts.

It is not yet known exactly what caused the glacier to collapse on Wednesday (26/08). However, climate change is suspected to have played a role. Simon Cook, a glaciologist from the University of Dundee in Scotland, said similar events in recent years have also occurred in several regions, such as the Himalayas, Switzerland, and the Dolomites in Italy. According to him, global warming is likely causing permafrost—permanently frozen ground that helps maintain the stability of some mountain slopes—to melt and degrade.

Cook added that these conditions make high mountain environments increasingly unstable. As a result, the risk of landslides, debris flows, glacial melt, glacial lake outburst floods, and glacier collapses becomes greater. Adhikari agrees that the impacts of climate change are becoming more visible. “We do observe these changes, but we don’t know that an event like this will happen on a particular day,” he said.

According to the United Nations in 2023, Nepal lost nearly a third of its ice volume in the previous three decades. In the period 2011-2020, glaciers in the country also melted 65% faster than in the previous decade.

Experts say the Nepal-China border region is highly vulnerable to natural disasters due to its geographical conditions. The area is home to some of the world’s highest mountains, vast glaciers, snowfields, and unstable steep slopes. Rivers originating on the Tibetan Plateau flow rapidly down through deep valleys before reaching populated areas of Nepal. According to scientists, these conditions create what is described as a highly hazardous environment.