A massive flood in the mid-north Himalayan mountains of Nepal bordering China has left around 500 people dead or missing, and the scientific community said the flood was caused not by an earthquake but by a spontaneous glacier collapse. While the Nepal government suspects an earthquake triggered the deluge, analysis found the Himalayan mountain region collapsed on its own without a quake.
According to preliminary findings reported to parliament on the 26th, the day of the disaster, by Nepal Minister of Foreign Affairs Sishir Khanal, an earthquake struck at 8:37 a.m., and a flash flood was reported three minutes later. Minister Khanal said, "An earthquake on the Tibet side triggered a large landslide that blocked the flow of the Bhotekoshi River," while adding, "We need to further verify the exact cause."
Nepal's Department of Hydrology and Meteorology (DHM) said the heavy rainfall typically observed right before an ordinary landslide did not occur in Rasuwa, the disaster area. Narendra Pariyar, the chief district officer overseeing Rasuwa, also told China's Xinhua News Agency, "There is no rainfall record for this area." Pariyar said a natural phenomenon upstream pushed the water down. The Nepal government inferred an earthquake as the cause of this disaster on the grounds that it did not rain in the area.
However, Eos, a science outlet published by the American Geophysical Union (AGU), on the 26th (local time) identified the cause of this disaster as a "rock-ice avalanche," not an earthquake. A rock-ice avalanche is when bedrock in a high mountain area and the overlying glacier collapse together as a single mass. Dave Petley, a landslide analyst and geologist, pinpointed the failure location for Eos at 28.2765 degrees north latitude and 85.5194 degrees east longitude. He said an overlay of satellite images from the 25th and 26th showed the summit's form changed within a day. He analyzed that the outburst flood resulted from a combination of ice melted by collapse energy, moisture contained in failed debris, and river water.
The U.S. Geological Survey (USGS) also corrected its observation record the same day. The USGS first detected an energy signal at 8:37 a.m. local time at a point 55 kilometers northwest of Kodari, Nepal. The USGS initially announced the energy signal as a magnitude 4.4 earthquake, then revised it to say "the energy signal came from a landslide, not an earthquake." This suggests that rather than an earthquake causing a landslide, a glacier collapsed on its own, and the vibrations generated by rock and ice produced a shock akin to an earthquake.
Still, Nepal-based researchers have not ruled out the possibility that a lake situated high on the mountain played some role before the glacier failed on its own. They suggest that as the glacier collapsed, lakes' water resources also came into play, amplifying the disaster. Tribhuvan University geology professor Ranjan Kumar Dahal told Nepal outlet Khabar on the 26th, "At this stage, we cannot say which lake burst or how it burst." Dahal said, "For a flood of this scale, a large snow avalanche and a glacial lake outburst must occur together." He pointed to three glacial lakes in the upper reaches of the Lende River as potentially involved.
Amrit Thapa, a researcher at the University of Alaska Fairbanks, also told the same outlet, "In recent years, small lakes on the Tibet side have merged into one," adding, "Satellite analysis after last year's flood also showed marked changes." In the Rasuwa area where the disaster occurred, a glacial lake on the Tibet side burst in Aug. last year, killing nine. The International Centre for Integrated Mountain Development (ICIMOD) classifies the Poiqu basin, where the water surged this time, as particularly vulnerable to glacial lake outbursts.
Nepal's National Disaster Risk Reduction and Management Authority (NDRRMA) officially announced, based on satellite imagery, that snow and rock collapsed together at a point 20 kilometers northeast of the Rasuwagadhi border checkpoint. The NDRRMA said the debris poured into the Lende River, which passes Timure village and flows into the Trishuli River, triggering a mud-laden flood. The same day, Xinhua reported that a landslide on the China Tibet Autonomous Region side killed three and left 265 missing in the Gyirong trade zone.
As the precise cause remains unclear, the toll continues to rise. According to Reuters on the 26th, at least 160 people have died in the flood, and 403 are missing, including 341 foreigners. Power infrastructure also collapsed. Down to Earth said the flood knocked out 430 megawatts of electricity supply, equivalent to one-eighth of Nepal's total generation capacity.