At least 359 people have died, and nearly 1,000 others are reported missing after a flash flood struck a mountainous area in Nepal and the Tibet region of China on Aug. 26.
The working theory is that a glacial collapse produced the flood that washed away buildings, cars and people with incredible speed and force, scientists told USA TODAY. The U.S. Geological Survey said a glacial collapse, in which part of a glacier breaks away, produced a seismic event at about 8:37 a.m. local time.
The collapse occurred on the north side of the Langtang Lirung mountain in Nepal next to the Lhende river that is used as the border between Nepal and China. The resulting flood swept into Nepal’s northern Rasuwa district through the Bhote Koshi river system.
As emergency officials continue to search for the dead and missing after a horrific Himalayan river flood, scientists race to assess the risk of even more flooding and piece together how it all began. It could take days or weeks to fully understand the event. Here’s a closer look at what we know so far:
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It appears a massive ice and rock debris flow plummeted from a high altitude near the point where the Bhote Koshi River flows into Nepal, north of Kathmandu.
Two locations on Langtang Lirung were the source of the materials, according to Jeffrey Kargel with the Tucson, Arizona-based Planetary Science Institute, in collaboration with an international group of scientists. One was a rock source very near the peak and the other glacier ice and snow a few kilometers to the east.
Videos captured a rapidly moving slurry with mud, rocks and water cascading more than 60 miles down a river valley, washing away entire villages and wiping out bridges and roads, leaving mud and debris behind. The destruction hampered search efforts, forcing authorities to use helicopters to rescue people.
At least 900 foreigners, including 664 tourists, 65 of them Americans, are missing, according to the Nepal Tourism Board. Reuters reports that at least 540 of the missing are foreigners. Horrifying videos circulating online show people fleeing before the mudslide hits and a deluge crashes down on them.
What started the flood?
The ice broke away at an altitude of about 17,000 feet and fell about 4,000 feet into the upper Lhende Khola River valley. That sent an avalanche of ice and rocks into the Lhende river system and then downstream into Nepal’s Bhote Koshi and Trishuli rivers, USA TODAY reported.
The event was originally reported as a magnitude 4.4 earthquake, the USGS said. Information from seismic stations, long period seismic waves, and satellite imagery showed the event was a 5.2 magnitude landslide from glacial collapse and debris flow, the agency said. No earthquake had occurred.
Satellite images from before and after the event
Nepal warns of additional flood risk
Chinese officials said a barrier lake had formed near the confluence of the Chhochen Khola and Purepu Tsangpo rivers, following the flood. The two rivers merge before entering Nepal to form the Bhote Koshi river.
A barrier lake is a body of water that occurs when a river or stream is suddenly blocked. They’re considered dangerous because the temporary dams can give way and cause a flood.
Authorities in Nepal told residents to stay away from the Bhote Koshi river, citing a risk of more flooding.
As rescue efforts continue, the United States said it will give $500,000 in emergency assistance. The International Federation of the Red Cross and Red Crescent Societies has released $1.4 million in aid.
Source: USA TODAY Network reporting and research; Reuters; U.S. Geological Survey
Contributing: Francesca Chambers, Alberto Cuadra, Michael Loria, Natalie Neysa, Ramon Padilla and Shawn J. Sullivan.
This is a developing story which may be updated.
Learn more: The force of a glacial collapse.