Bhutan: glacial flood

A glacial lake outburst is a flood with no rain in it. Meltwater collects behind a moraine or an ice dam, something displaces it — a rockfall, an avalanche, a thaw — and the dam fails in minutes. The Himalaya holds tens of thousands of these lakes, India monitors a fraction of them, and the warning systems are almost entirely unbuilt.

28,043 — Glacial lakes mapped across the Indian Himalayan river basins. Lakes above 0.25 ha. THE BASINS, NOT THE TERRITORY: these river basins extend into Pakistani, Chinese and Nepali ground, so this is not a count of lakes inside India and is routinely quoted as though it were. ISRO / NRSC

63,000 — Glaciers across the Hindu Kush Himalaya. The source region for river systems supporting billions of people. ICIMOD

25,614 — Glacial lakes mapped across the Hindu Kush Himalaya. A 2005 Landsat baseline, not a current count — lakes have grown and formed since, so this understates today's number. India's own basin atlas counts 28,043 lakes above 0.25 ha, but across whole river basins including Pakistani, Chinese and Nepali territory, so it is not a count of lakes in India. ICIMOD

2,485 above 10 ha — Glacial lakes CWC monitors. Plus 358 other water bodies, monitored June to October on Sentinel imagery. The portfolio grew from 902 in 2025. Of these, 681 glacial lakes lie inside India: Arunachal 324, Ladakh 179, Jammu & Kashmir 76, Sikkim 72, Himachal 17, Uttarakhand 13. Central Water Commission

15 million — People exposed worldwide. Living BOTH within 50 km of a glacial lake AND within 1 km of a potential flood path — the two conditions together, which is the part most often dropped in quotation. India holds the largest exposed population of any country, about 3 million. India leads on exposure; on the study's danger index, which weights vulnerability, China and Pakistan rank higher. Nature Communications

1,203 → 2,963 — Lakes at high or very high risk, now and by 2100. Outburst risk across the Third Pole is projected to roughly triple, with the eastern Himalaya — the transboundary basins above Bihar and Bengal — remaining the primary hotspot under every scenario. Nature Climate Change

766 events, 1533–2025 — Recorded outbursts across High Mountain Asia. About 59 of them in India, per the peer-reviewed version of the database. ICIMOD's own caveat: the record is an underestimate, and the apparent rise over time is mainly better observation rather than more events. ICIMOD

+65% — Glacier mass loss, one decade against the last. From −0.17 to −0.28 metres water equivalent a year, between 2000–2009 and 2010–2019, with high confidence. Losses are steepest in the eastern Himalaya. ICIMOD

none — End-to-end warning systems commissioned in India. The government's own position as of 2026, under a ₹150 crore four-state programme approved in July 2024, of which ₹27.73 crore of ₹135 crore had been released by mid-2025. Two automatic weather stations are confirmed installed, in Sikkim. Press accounts describing twenty or more instrumented lakes contradict the government's own answer. Ministry of Home Affairs / NDMA

189 — High-risk glacial lakes assessed across the Indian Himalayan Region. From the 2025 assessment. India's National Disaster Management Authority separately lists 195 high-risk lakes; the two exercises are not the same and this page prints both rather than choosing between them. 2025 assessment, Indian Himalayan Region

56 — Of those, classified VERY HIGH RISK. The same figure a 2026 parliamentary reply gave for the highest risk class. 2025 assessment, Indian Himalayan Region

9 — Potentially dangerous glacial lakes in the Alaknanda Valley. The highest concentration in Uttarakhand, and the valley carrying the road to Badrinath. The Dhauliganga Valley follows with 6. 2025 Uttarakhand valley assessment

713 — Glacial lakes in Chamoli district alone. The highest concentration of any Uttarakhand district in this inventory. Chamoli is where the 2021 rock-ice avalanche happened. ISRO / IIRS

25 — Potentially dangerous, of 426 larger Uttarakhand lakes assessed. Six of the 25 fall in the most hazardous class, Category A. 2025 Uttarakhand valley assessment

1,200-1,350 — Glacial lakes identified in Uttarakhand. The range is real: the count depends on which inventory and which minimum size threshold is used. ISRO / IIRS

195 — Lakes on India's high-risk list. Himachal 48, Sikkim 41, Ladakh 34, Arunachal 32, Jammu & Kashmir 27, Uttarakhand 13. By type, 103 are moraine-dammed. A 2026 parliamentary reply put 56 of them in the highest risk class. National Disaster Management Authority

Figures in circulation that do not hold up

• 'About 200 glacial lakes in the Hindu Kush Himalaya are dangerous' — the figure appears in an ICIMOD press release but nowhere in the report it is attributed to, and its likely ancestor is an inventory built partly on topographic maps from 1960–1982.
• '178 dead at South Lhonak' — contradicted by every readable source; the disputed range runs from 55 dead with 74 missing to about 102.
• 'India has the highest GLOF danger in the world' — India has the highest exposed population. On the danger index, which weights vulnerability, China and Pakistan rank above it.
• '67 Indian glacial lakes have expanded by more than 40%' — absent from the ISRO page it is attributed to. ISRO's published finding is that 130 lakes in India expanded notably since 1984.
• '47 potentially dangerous glacial lakes in Nepal' — the 47 is a three-country figure. Nepal's share is 21, China's 25 and India's 1.

Glacial lake breach

The lake, before and after. An emptied lake shows at 10 m, and the area change is measurable — usually settled within days of the first clear pass.

Ice or rock avalanche

A detachment scar on the slope above, and seismometers, which time a rock-ice collapse to the second. This is how Chamoli 2021 was reclassified.

Extreme rainfall

Gauge or radar rainfall in the hours before. A true outburst has no rain in it, so a heavy-rain record is evidence AGAINST one.

Landslide blockage and collapse

A temporary lake upstream, and a discharge record that rises, falls, then rises higher. Often established only after the second flood.

Thawing permafrost

Ground temperature and modelling of the slope. A condition rather than a trigger — it was the finding at Sikkim in 2023.

The mechanism in general

Trigger

A rockfall or avalanche into the lake, a surge over the moraine, or permafrost loosening the debris behind it. Minutes long, and no rain in it.

Conditions

Retreating glaciers leave meltwater ponded behind loose moraine, never engineered. Himalayan mass loss rose 65% from the 2000s to the 2010s, so the lakes are more numerous, larger and younger.

What science says

Outburst risk across the Third Pole roughly triples by 2100, high or very high risk lakes going from 1,203 to 2,963. ICIMOD's caveat: higher hazard has not yet produced more floods.

Human factors

What was built in the path decides the disaster. Sikkim 2023 destroyed a 1,200 MW dam and about 25,900 buildings on the Teesta. Nepal's Bhote Koshi warning gives roughly six minutes, its sensors all below the border the water starts above.

The mechanism for this kind of event, not a finding about this one. Himalayan disasters are often reclassified once fieldwork is done.