The Same Himalayan Avalanche Cascade Hits Nepal Again

A rock avalanche off Mt. Chamar likely set off Nepal’s October 11 flood on the Budhi Gandaki after debris blocked a river and the dam burst. Five people remain missing in Gorkha, and the surge came 46 days after the Langtang collapse, less than 100 km away.

There was no heavy rain in the catchment that weekend. The water arrived as black slurry, rolling house-sized boulders as far as Soti, about 40 km down-valley.

What Triggered the Budhi Gandaki Flood?

University of Zurich professor Christian Huggel said the collapse likely began on the southern slopes of the Chamar massif, where peaks rise above 7,000 metres. Cloud hid the exact scar. Ice on the slope could not be ruled out. He put the fallen mass at several tens of millions of cubic metres, using satellite change and a magnitude 5.1 signal read by seismologist Göran Ekström.

Huggel said the avalanche ran about 7 km before it stopped in the main valley and plugged the Syar. When that plug failed, water, sediment and rock went downstream. The International Centre for Integrated Mountain Development, working from reports up to 11:30 a.m. Nepal time on October 11, reached the same middle step: a landslide near Chumchet that blocked the river, then a breach.

Mt. Chamar stands 7,165 metres, the high point of the Sringi Himal. The rock or ice that moved sat near 5,500 metres. Videos from below Chumling show a debris flow up to 200 metres high. The Syar meets the Budhi Gandaki at about 2,000 metres, so the face drops some 5,500 metres into a narrow gorge.

Satellite frames taken 10 days before the collapse showed no earlier break, which is odd for a failure this large. Central Nepal had dumped more than 3 metres of snow earlier in October, enough that Manaslu teams packed up after four climbers were killed. The 7.8 magnitude earthquake of 2015 was centred only 10 km to the southwest, and that shaking still sits in these slopes.

a landslide at 04:22 local time near Chumchet, with the dam going around 6 a.m., matches the flood that then moved down the Budhi Gandaki. Helicopter film shows dust plastered across adjacent slopes, the sort of cloud a fast rock avalanche throws, and possibly an air blast.

The black mud that hit the bazaars does not match a small valley-floor slide. It matches a high, glaciated source. The Chumchet blockage is the plug. The Chamar face is the start of the chain.

A flash flood swept through Nepal's Budhi Gandaki river system early on 11 October, carrying mud, sediment and debris downstream.

Preliminary accounts indicate that the failure of a temporary landslide blockage near Chumchet, Gorkha, may have triggered the event.
With search,… pic.twitter.com/RX3EsITpfE

— ICIMOD (@icimod) October 11, 2026

WHAT WE KNOW

• The weather: No heavy weekend rain fell on the Budhi Gandaki catchment, so this was not a monsoon river rise.
• The clock: Seismic records mark a slope failure at 04:22 and a dam breach about two hours later.
• The path: Debris ran the Ungilung into the Syar, then the Budhi Gandaki; water above that join stayed normal.

WHAT IS UNCONFIRMED

• Ice share: Scientists cannot yet say how much glacier ice rode with the rock.
• The five: NDRRMA still lists them missing, with no confirmed deaths in the Monday situation report.
• Hydro losses: The 341-megawatt camp at Salleri is damaged; a full account of plant losses is not in.

Qianggong Zhang, ICIMOD’s strategic lead for climate and environmental risks, said the first read is a debris-heavy flood from a failed landslide dam, and that slopes and rivers have to be watched as one system.

Chamoli, Langtang, Then Gorkha

On 7 February 2021, a slab failed on Ronti Peak in Chamoli, Uttarakhand. A study led by University of Calgary earth scientist Daniel Shugar found about 27 million cubic metres of rock and ice fell, turned into a long debris flow, and wrecked two hydropower plants. More than 200 people were killed or listed missing. Early talk of a glacial lake was wrong. There was no lake to drain.

On 26 August 2026, the north face of Langtang Lirung went. Huggel and colleagues at Zurich called that collapse one of the largest documented rock-and-ice avalanches. First reports again said glacial lake outburst. Later work, including the U.S. Geological Survey’s, put the trigger on ice and rock, not a lake. Huggel’s working volume for that fall is 110-130 million cubic metres. Nepal’s National Disaster Risk Reduction and Management Authority, as of October 5, put the toll at 1,455 confirmed dead and 5,285 missing.

THREE ROCK-AVALANCHE FLOODS

None of the three was a glacial lake outburst. The same river had already seen a cousin of this chain. In April 2024, ice from the Manaslu glacier dropped into Birendra Lake and shoved water over the outlet. ICIMOD’s study of that pulse found it was not a typical glacial lake outburst flood, and the moraine dam did not fail.

ICIMOD director general Pema Gyamtsho said, “Coming so soon after the Guirong-Rasuwa disaster of 26 August, Sunday’s event is another reminder of how exposed mountain communities and river corridors remain to sudden, cascading disasters.”

Five People Missing After the Dam Burst

NDRRMA’s October 12 situation report said the five unaccounted for are two women and three men from Ungilung in Chumchet, Chumnubri Rural Municipality-6. They had been staying at a sheepfold by the stream. Searchers had not reached them as of 10 p.m. on October 11.

Four army helicopters lifted 95 people, including nine foreign nationals. By later on October 12 the authority had 703 army, police and armed police on the ground. Relief for Chumnubri, Dharche and Arughat was being stacked for 21 days.

THE MONDAY COUNT

• Houses: 204 damaged, 43 in Chumnubri, 39 in Dharche, 64 in Arughat, 58 in Tripurasundari in Dhading.
• Bridges: Two concrete spans, two suspension bridges and one Bailey bridge gone, which cuts the Manaslu trail and the Gorkha-Dhading road.
• Other losses: One school, three hydropower sites, and the river at Arughat at 8.4 metres against a 5-metre danger mark before the gauge washed out.

The missing were on the low ground beside the Ungilung, the same strip these valleys have always used for sheds, trail stops and winter herds. The gorge walls are the safe ground. The floor is where the next slurry will run.

Phone Alerts Bought Hours Before the Surge

People in Tsum Valley phoned down-valley. NDRRMA pushed alerts. At Dobhan the river stopped, which is the tell of a dam upstream, and residents climbed. The August Trishuli flood was still close enough that few treated the messages as noise. All Nepali and foreign trekking groups on the trail were reported safe because of that lead time.

The slurry itself moved slower than the August wave, which ran wetter and faster. Thick black mud still carried enough force to take a steel highway bridge at Soti. The difference was hours, not a gentler mountain.

SUNDAY MORNING ON THE BUDHI GANDAKI

1. October 11, 04:22: Seismic records mark the Chumchet-area slope failure.
2. October 11, about 6:00 a.m.: The landslide dam breaches and the debris flood starts down-valley.
3. October 11, 6:36 a.m.: Army helicopter search begins; NDRRMA tells people to stay high and keep off the river as far as Trishuli Dobhan.
4. October 11, by 11:00 a.m.: Four helicopters, including an Mi-17, are in the air and 95 people are off the river.

In August, river gauges on the Bhote Koshi were built for a slower rise. The first surge wrecked them. Upstream hamlets had minutes. This time the dry river, the Tsum calls and the SMS stack were enough for bazaars to empty before the wall arrived. That is learning. It is also luck of speed and of a thinner corridor.

The 341 MW Camp in Salleri

The flood took the construction camp and kit of the 341-megawatt Budhi Gandaki semi-reservoir project at Salleri in Chumnubri-3. Sushil Thapa, the project’s managing director, said water came down the Ungilung, through the Syar, and into the Budhi Gandaki, while the main stem above that join still looked normal. Two smaller plants in the same corridor, at Dobhan and Machhakhola, lost camps, a Bailey bridge or access road.

A much larger scheme has sat on paper for some 30 years: a 270-metre dam near Benighat, rated at 1,200 megawatts, that would drown the valley for about 40 km. That reservoir was never built. The sediment in Sunday’s pulse would have taken a bite out of any lake that had been there. Putting a work camp on the same floor the Syar uses as a chute is the Chamoli pattern in miniature. In 2021 the dead included plant workers. On October 11 the camp went into the river; the head count, this time, did not.

The Manaslu circuit runs this gorge at peak season. Jagat, Sirdibas and Machhakhola are trail towns. With five bridges gone, northern villages now wait on army helicopters for food and for anyone still above the mud.

Which Glacial Lakes Is Nepal Draining?

After August, officials said they would start design work in the fiscal year that runs to mid-July 2027 to lower four high lakes: Thulagi, Lower Barun, Lumding Tsho and Hongu II. Canals would bleed water into nearby rivers. The build, with sensors and downstream work, could take up to six years. Nepal has logged about 26 glacial lake outburst events in four decades. That list is real. It is also the wrong list for August 26 and for October 11.

THE FOUR LAKES ON THE WORK PLAN

• Thulagi: A central Nepal lake slated for study and a controlled drawdown through gated works.
• Lower Barun: An eastern basin lake on the same design track.
• Lumding Tsho: Listed with the eastern group for canals, sensors and downstream alerts.
• Hongu II: The fourth named lake; none of the four sat on the Chamar or Langtang faces that actually failed.

Draining a lake can cut one class of flood. It does nothing for a rock face at 5,500 metres that gives no lake signal, no slow gauge rise, and no 10-day crack in the satellite record. After August, HKH governments also signed Kathmandu 10 Action Points that call for early-warning triggers built for sudden slope failure, not only for rising water. Sunday was that test, six weeks later, in the next valley.

Flood Alerts Still Wait on River Gauges

NDRRMA’s flood tools still lean on river stations and weather posts that feed the BIPAD portal, then SMS and sirens. Those tools work when a river climbs for hours. They go blind when the first pulse is a slope, and they die when the pulse eats the station, as it did at Arughat when the water hit 8.4 metres. There is still no working remote sensing net on most upper catchments below hanging ice and steep rock.

Events like this demonstrate why monitoring and evaluating unstable slopes and river systems together is essential in steep mountain catchments, where hazards can develop rapidly and affect communities far downstream.

Qianggong Zhang, Strategic Lead for Climate and Environmental Risks, ICIMOD

Gyamtsho said rescue comes first, then the trigger has to be named if the next corridor is to be less exposed. The trigger on October 11, as on August 26 and on 7 February 2021, was a high rock face, a short run to a gorge, and a river that had nowhere to put the debris except through towns and camps. Five people from a sheepfold in Ungilung are still unaccounted for while that face, at 5,500 metres, has no instrument on it.