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Environment › Climate science
Also: Disaster Management › Disasters in news
1 Sep 2026
Nepal's Bhote Koshi and Trishuli disaster: a glacier and rock collapse that became a cascading debris flow
On 26 August glacier ice and bedrock broke away on Langtang Lirung, near the Nepal Tibet border, and became a debris flow that travelled nearly 100 km down the Bhote Koshi and Trishuli. Nepal counted about 1,340 dead by 7 September.
UPSC has asked
Prelims 2019: matching Himalayan glaciers to their rivers
From a mountain wall to the Gandak
Collapse
Glacier ice and bedrock break away on Langtang Lirung.
↓→
Temporary dam
Debris blocks the Lhende Khola in Tibet, then fails within minutes.
↓→
Bhote Koshi
The surge sweeps back into Nepal, gathering sediment.
↓→
Trishuli
The Trishuli rises by up to 9 metres in under 30 minutes.
↓→
Into India
The flow joins the Narayani system and enters India as the Gandak.
From the Indian Express interview with Austin Lord, 31 August 2026, and Frontline, 1 and 7 September 2026.
Why it is not called a GLOF
A glacial lake outburst flood (GLOF) begins when a lake's natural dam fails, but this began with a slope failure of ice and rock.
A debris flow erodes riverbanks and gains mass and momentum downstream, which is why experts called this one a sediment bomb.
How warming loosens slopes
Meltwater widens crevasses and, at the glacier bed, can destabilise large blocks of ice.
Retreating glaciers strip the snow that cushioned rock walls, leaving rock and ice perched on bare slopes.
Thawing permafrost removes the ice that cements rock and sediment together.
Why the warning came late
Nepal's warning system issues flood alerts when it detects rain, and this flood did not begin with rain.
The site was not on Nepal's list of places at high risk of glacial detachment.
The flow's seismic signal was first read as an earthquake, so environmental seismology is proposed as an added layer of warning.
The regional trend in the Hindu Kush Himalaya
89%Monitored years with a negative glacier mass balance, over five decades
DoubledRate of ice loss after 2000
7 to 38GLOFs per decade in the region, up since 1950
From Frontline, 7 September 2026, citing ICIMOD's report of 19 March 2026 and a review of recorded GLOFs.
For the exam
Prelims: ICIMOD, the Kathmandu based intergovernmental centre that monitors the Hindu Kush Himalaya.
Mains, GS3: cascading hazards, and warning systems that watch only for rain.
Mains, GS3: about a tenth of Nepal's hydropower capacity (431 MW) hit by one event, a caution on building in glacier fed valleys.
Also: Disaster Management › Mitigation and preparedness
3 Sep 2026
After Nepal, the Centre reviews Himalayan States' preparedness for glacial lake outburst floods
On 3 September the Union Home Secretary reviewed how prepared the Himalayan States and Union Territories are for glacial lake outburst floods and snow avalanches, seeking continuous lake monitoring and risk informed planning.
UPSC has asked
Prelims 2017: the source and course of the Teesta
The science
Many glacial lakes are dammed by moraine, loose rock and sediment left by a glacier, rather than bedrock.
Mapping
The National Remote Sensing Centre's Glacial Lake Atlas mapped 28,043 lakes in the Himalayan river basins, almost 73 per cent of them outside India.
Mapped lakes inside India
Ladakh3,219
Arunachal Pradesh2,188
Sikkim733
Jammu and Kashmir546
Himachal Pradesh537
Uttarakhand347
From Frontline, 1 September 2026, citing the NRSC Glacial Lake Atlas (lakes of at least 0.25 hectares).
Ranking risk
Glacial lake risk
Monitoring
CWC tracks water bodies monthly in the monsoon
Ranking
CWC 12 criteria index
NDMA list of 195 high risk lakes
Mitigation
Early warning plan: 30 lakes to at least 60
Dam safety
CEA: 47 dams that GLOFs could affect
From Frontline, 1 September 2026, and the PIB release of 3 September 2026.
The Central Water Commission's risk indexing gives 30 of 100 marks to downstream exposure, since risk lies where hazard meets exposure.
The ₹150 crore National Glacial Lake Outburst Flood Risk Mitigation Programme covers four States.
South Lhonak, 2023
Creeping slope
Parts of a frozen lateral moraine creep for years.
↓→
Collapse
Frozen moraine falls into the lake.
↓→
Wave
A wave overtops and breaches the frontal moraine.
↓→
Outburst
Lake water is released.
↓→
Erosion
The flood scours sediment down the Teesta valley.
From Frontline, 1 September 2026, citing Sattar and others in Science, 2025.
The flood destroyed the Teesta-III dam, though a 2019 modelling study had flagged the valley as at serious risk.
Warning is not enough
In worst case models, a flood triggered by an avalanche in Tibet reaches the Nepal border in about 30 minutes, so warning needs land use zoning and local capacity too.
Evacuation on signs of slope instability kept deaths in the 2025 Blatten, Switzerland, avalanche to one, against over 200 in the 2021 Chamoli ice and rock avalanche.
For the exam
Prelims: NRSC works under ISRO, and the Defence Geoinformatics Research Establishment under DRDO.
Mains, GS3: monitoring saves lives only when tied to thresholds, clear responsibility and evacuation plans.
Mains, GS2: most mapped lakes lie outside India, so data sharing with China and Nepal is part of the warning system.
3 Prelims and 2 Mains questions, on the most important items above.
Prelims
Q1
With reference to the August 2026 disaster on the Bhote Koshi and Trishuli rivers, consider the following statements:
1.It began when the natural dam of a glacial lake failed.
2.The rivers it flowed down join the Narayani system, which enters India as the Gandak.
3.A debris flow gains mass and momentum as it travels by eroding riverbanks.
Which of the statements given above are correct?
(a)1 and 2 only
(b)2 only
(c)2 and 3 onlyAnswer
(d)1, 2 and 3
Show answer and explanationHide answer and explanation
Statements 2 and 3 are correct. Statement 1 is wrong: the disaster began with a slope failure of glacier ice and rock, which is why it is not called a glacial lake outburst flood.
With reference to glacial lakes in the Himalaya, consider the following statements:
1.Most of the lakes mapped in the Glacial Lake Atlas of the National Remote Sensing Centre lie outside India.
2.Among Indian States and Union Territories, Sikkim has the largest number of mapped glacial lakes.
3.The South Lhonak outburst of 2023 began when frozen moraine collapsed into the lake and raised a wave that breached its dam.
Which of the statements given above are correct?
(a)1 only
(b)1 and 2 only
(c)2 and 3 only
(d)1 and 3 onlyAnswer
Show answer and explanationHide answer and explanation
Statements 1 and 3 are correct: almost 73 per cent of mapped lakes lie outside India, and South Lhonak failed after collapsing moraine sent a wave over its frontal moraine. Statement 2 is wrong: Ladakh has the most, 3,219, against Sikkim's 733.
The 2026 disaster in Nepal's Bhote Koshi and Trishuli valleys was a cascading hazard rather than a single flood. Explain how climate warming destabilises high mountain slopes, and examine why early warning in the Himalaya must look beyond rainfall.
Satellites can show a glacial lake growing, but they cannot evacuate a valley. Examine the gaps between monitoring and preparedness for glacial lake outburst floods in the Indian Himalaya, and suggest measures to close them.
Q1 (c) Statements 2 and 3 are correct. Statement 1 is wrong: the disaster began with a slope failure of glacier ice and rock, which is why it is not called a glacial lake outburst flood.
Q2 (d) Statements 1 and 3 are correct: almost 73 per cent of mapped lakes lie outside India, and South Lhonak failed after collapsing moraine sent a wave over its frontal moraine. Statement 2 is wrong: Ladakh has the most, 3,219, against Sikkim's 733.
Q3 (b) Statements 1 and 2 are correct. Statement 3 is wrong: the treaty shares the Ganga's lean season flow at Farakka, not its monsoon flow.