August 26, 8:40 am. Langtang Lirung Himal—the highest peak in sub-region of Nepalese Himalayas.
It is a humdrum morning in the primordial landscape. There is no significant trace of rainfall. Glacier- and snow-capped mountains, with characteristic vertical reliefs up to 5,500 metres, tower above the Langtang Valley, which has just woken up to the day. The impenetrable guard of the mountains, deep gorges and steep rivers and passes make the valley feel protected.
Perhaps that is why, the people do not cling to bad memories like the April 25, 2015, earthquake of 7.8 magnitude that led to a glacier collapse over Langtang village, on the historic travel route between Kathmandu and Tibet. The disaster killed 400 people and flattened nearly all the houses and infrastructure; but within two years, people rebuilt the village. Then on July 8, 2025, a supraglacial lake outburst in Tibet caused a torrential flood in the Lhende Khola river, a tributary of Bhotekoshi river, sweeping away bridges and infrastructure at Rasuwagadhi. Twenty-two days later, another flood on the Bhotekoshi further battered the area. But people rebuilt after this as well, showing resilience that befits the vulnerability of life in this region.
And on August 26, somewhere at the top of one of the mountains, another geological explosion was imminent. At 8.45 am, a 600-metre-wide section of an unnamed glacier on the north slope of Langtang Lirung and its surrounding bedrock collapsed at an elevation of 5,200 metres, just above the Lhende Khola on the Nepal-Tibet border. According to initial assessments by geologists, the area of the collapse zone was 2.26 sq km, with an additional 2.93 sq km eroded due to an ice-rock meltdown.
The gigantic collapse would have mobilised some 142 million cubic metres of ice, rock, sediments and debris. The tumbling mass scrapped the glacier’s lower ablation area (lowest-altitude part of the glacier tongue where annual ice loss exceeds new snow accumulation), 3,800 metres above sea level. The ice-and-rock avalanche freely plunged 1,100 metres to the valley floor. It generated energy equivalent to that of a 5.2-magnitude earthquake, as per the US Geological Survey. A seismometer in Jilong, 12 km from the affected area, picked up anomalous signals, as did a second station in Zhangmu, according to the International Centre for Integrated Mountain Development (ICIMOD), Nepal.
The cascade of ice, rock and debris emptied into the Lhende Khola. It blocked the river for a while, altering its course and ultimately causing a breach of enormous force. “We have estimated that about 67 million m³ of ice under the debris from that area mixed in the Lhende River. We suppose all snow, ice, and rock brought down to the confluence of the Lhende River and the river were temporally blocked for certain times before it rushed down,” says a joint rapid situation analysis by Nepal-based institutions Youth Alliance for Environment; Himalayan Cryosphere under Kathmandu University’s Climate and Disaster Research Center, and the Society for Conservation Biology.
The destructive wall of high-speed floodwater and debris surged across ravines along the Nepal-Tibet border, overwhelming river valleys in Nepal’s Rasuwa district and in Tibet’s Gyirong County. Pema Gyamtsho, director general of ICIMOD, calls it a “tsunami from the sky” that entered the Bhotekoshi and Trishuli river systems through the Lhende Khola and swept populated valleys.
According to ICIMOD, by 9 am water levels in the Bhotekoshi basin rose by as much as 9 metres within 30 minutes. This high-energy debris-laden flood flattened everything on its way for 100 km downstream. The 30-metre-high and concrete-thick wall of water, ice-rock and debris swept through Timure, Rasuwagadhi, Syabrubesi and other settlements downstream and continued into Nuwakot, Dhading, Gorkha, Chitwan, Tanahun and Nawalparasi districts. It picked up more and more water and rubble as it gushed through the Lhende Khola and Trishuli river valleys, adding to its force. Langtang Lirung’s topography exacerbates floods, given that it is on the crest of the high Himalayas and has very steep valleys on both sides.
Basanta Raj Adhikari, director at the Centre for Disaster Studies, Tribhuvan University, Kathmandu, says, “The flood was highly unique and unprecedented in three distinct ways: its size, its area, and its mechanism.” By September 1, the flood was considered the country’s deadliest natural disaster …
This article was originally published in the September 16-31, 2026 print edition of Down To Earth