A massive landslide in the high mountains of northern Gorkha appears to have triggered Sunday’s destructive flood along the Budhi Gandaki River, with preliminary satellite imagery and field observations pointing to a possible blockage and sudden release of water and debris.
The disaster began in the remote Chum Valley, where a torrent of mud, rocks and uprooted vegetation swept downstream through tributaries before reaching the Budhi Gandaki. The flood damaged settlements, bridges and hydropower infrastructure, while forcing residents to flee towards higher ground.
A field inspection conducted on Sunday afternoon found extensive signs of erosion and landslide activity in the upper catchment. The river, normally blue or green depending on conditions, was flowing black with mud and debris.
Satellite images from the Sentinel-2 mission, processed using false-colour imagery, also appear to show the landslide and the path taken by the flood. The dark area in the images traces a possible route from below the Shringi Himal region towards the downstream river system.
The Centre for Hydrology and Meteorology Studies reported, following an image analysis, that the debris-laden flood had entered the Syarkhola before flowing through the lower Chum Valley towards Salleri and Jagat.
However, the precise origin of the landslide, the location where the river may have been blocked and the mechanism that triggered the flood have yet to be established through detailed scientific investigation.
Signs of a large-scale mountain slope failure
Initial observations suggest that the landslide may not have occurred entirely in a single episode. Some sections show large masses of rock and earth that appear to have collapsed together, while other areas contain uprooted vegetation and signs of erosion that may have developed over a longer period.
One preliminary hypothesis is that accumulated snow and ice near Chamar Himal may have contributed to a slope failure, bringing down forests, rock and debris. However, investigators have not yet established the exact mountain or the mechanism responsible for the collapse.
The satellite imagery points towards major landslide activity in the upper catchment, but the available evidence is not sufficient to confirm whether snow or ice played a direct role.
Anil Pokharel, former chief executive officer of the National Disaster Risk Reduction and Management Authority, said the images indicate a large landslide in the Himalayan region.
“The large volume of earth and debris suggests that the material may have accumulated and temporarily blocked the river before being released,” Pokharel said.
He added that the flood may have arrived in several surges rather than as one continuous wave. Large amounts of debris may have descended, temporarily slowed or blocked the flow, and then moved downstream again.
The possibility of repeated surges is significant because it means that an apparent reduction in river flow does not necessarily indicate that the danger has passed.
A river that appeared to dry up before surging again
Residents in the affected area reported that the river initially appeared to have dried up or experienced a sharp decline in flow, before a sudden flood arrived. The water then seemed to settle briefly before another surge followed.
Such observations are consistent with a possible temporary blockage upstream, although the precise sequence of events remains unverified.
In steep Himalayan terrain, landslides, rockfalls or large deposits of debris can obstruct a river and create a temporary natural dam. Water accumulates behind the obstruction until it overtops or breaches the barrier, releasing a sudden surge downstream. Further landslides or renewed blockages can produce additional surges.
Pokharel said the incident highlights the need to understand the geology and stability of the entire upper catchment rather than treating a river simply as a channel carrying water.
“The actual origin of this event, the location of the river obstruction and the process that generated the flood require detailed scientific investigation,” he said.
Experts have also called for greater preparedness in other Himalayan river basins where similar combinations of unstable slopes, debris and temporary river blockages could pose a risk.
The Budhi Gandaki corridor and its vulnerabilities
The geography of northern Gorkha helps explain how the flood travelled downstream.
Arughat, situated along the Gorkha-Dhading border, lies approximately 40 kilometres northeast of Gorkha Bazaar. From Arughat, the Budhi Gandaki flows upstream towards Philim, the administrative centre of Chumnubri Rural Municipality, through a remote mountain corridor.
Several kilometres above Philim, the Syarkhola joins the Budhi Gandaki. Farther upstream, the Ungilung Khola flows into the Syarkhola.
Field observations by a team that inspected the area indicated that floodwater travelled from the Ungilung Khola through the Syarkhola before entering the Budhi Gandaki. The water and debris then moved downstream, damaging settlements and infrastructure along the river.
The flood also washed away the bridge connecting Gorkha and Dhading at Arughat, adding to the disruption caused by damage to transport infrastructure elsewhere in the region.
The loss of these connections is particularly serious for remote settlements that depend on a limited number of roads and bridges for food supplies, emergency access and movement between villages.
Hydropower projects and settlements hit
The flood damaged the construction camp and associated structures of the 341-megawatt Budhi Gandaki Hydropower Project at Salleri. Preliminary observations indicated that the camp and other infrastructure had been swept away.
A Bailey bridge built by the 14.15-megawatt Dobhan Khola Hydropower Project was also destroyed.
The bridge at Soti Khola, which connects settlements along the route between Philim and Arughat, was washed away, disrupting access between wards 2 and 3 of Arughat Rural Municipality.
Jagat Bazaar, an important stop for trekkers entering the Manaslu region, also sustained damage.
Local residents fled towards higher ground as the flood swept through the river corridor. Ganesh Karki, a resident of Jagat, described the helplessness of watching homes being carried away.
“We have climbed up towards the hills and are staying there,” he said. “There is nothing we can do except watch our houses being swept away.”
Local representatives said the timing of the flood and the intervals between surges allowed some residents to reach safety. Rescue teams also mobilised helicopters to evacuate people from affected settlements.
Lessons from previous Himalayan disasters
The Budhi Gandaki flood comes less than two months after a major flood in the Bhotekoshi River system following a landslide that blocked the Lhende Khola in Rasuwa on August 26.
Although the scale of damage in the two events cannot be directly compared, the latest disaster has again drawn attention to the vulnerability of Himalayan river valleys to sudden changes in mountain terrain.
Pokharel recalled an earlier assessment of flood and landslide damage in northern Gorkha during the 2021 monsoon. At the time, he was leading the national disaster management authority and visited Jagat and Chumnubri with engineers, geologists, local officials and security personnel.
The team examined damaged roads, bridges and settlements, as well as the geological characteristics of landslides and river erosion.
Three years later, another incident involving a landslide into Birendra Lake in the Manaslu glacial region displaced water and increased the flow of the Budhi Gandaki. The event reinforced the need to assess not only the size of glacial lakes and the condition of their natural barriers, but also the stability of surrounding slopes and the wider upstream catchment.
According to Pokharel, the condition of the entire mountain basin matters because a large landslide or avalanche can displace water, obstruct a river or trigger a chain of events that produces a sudden flood downstream.
Monitoring the entire Himalayan river system
The latest disaster highlights the limitations of monitoring Himalayan rivers solely through rainfall, river levels or glacial lake conditions.
A river in the high mountains is part of a complex system involving glaciers, unstable slopes, rock formations, landslides and large volumes of sediment. Changes in any of these elements can affect the river’s flow, sometimes with little warning for communities downstream.
A sudden fall in river levels may be a warning sign rather than reassurance if a temporary obstruction is forming upstream. Similarly, an initial flood surge may not be the last if debris continues to block and release the flow.
Scientists and disaster management authorities will need to establish the exact origin of the Budhi Gandaki flood, determine whether a temporary natural dam formed and reconstruct how the water and debris moved through the tributaries.
Until those investigations are complete, the precise trigger remains uncertain. But the destruction across northern Gorkha has reinforced an urgent lesson: reducing disaster risk in the Himalayas requires monitoring not only the rivers themselves, but also the unstable mountain slopes and catchments that control their behaviour.
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