Glacial Lakes of Nepal & GLOF Risk: The Potentially Dangerous Lakes

A glacial lake outburst flood, or GLOF, is the sudden release of water from a lake formed at the snout, surface or margins of a glacier. Most large glacial lakes in the Nepal Himalaya are impounded by end moraines — unstable natural dams of loose boulders, gravel and buried ice left behind by retreating glaciers. Because these dams were never engineered, they can fail with little warning, sending a wall of water, mud and debris down narrow valleys at destructive speed.

Common triggers include ice or rock avalanches crashing into a lake and sending waves over the dam, melting of the ice core inside a moraine, internal seepage and piping, heavy rainfall, and strong earthquakes. A single event can destroy bridges, hydropower plants, trails and settlements for tens of kilometres downstream, as Nepal has experienced repeatedly since the 1980s.

GLOF risk in Nepal is rising with climate change. Glaciers feeding the big valley lakes are retreating fast — the Trakarding Glacier above Tsho Rolpa lost about 66 metres per year between 1957 and 2000, and the Imja Glacier about 74 metres per year between 2001 and 2006 — so the lakes behind their moraines keep growing in area, depth and volume.

The authoritative count comes from the 2020 (2077 BS) inventory of glacial lakes in the Koshi, Gandaki and Karnali river basins, published by the International Centre for Integrated Mountain Development (ICIMOD) and the United Nations Development Programme (UNDP). Using Landsat satellite imagery from 2015, it mapped 3,624 glacial lakes of at least 0.003 square kilometres across the three transboundary basins.

Of these, 2,070 lakes lie inside Nepal, 1,509 in the Tibet Autonomous Region (TAR) of China, and 45 in India. Because the Koshi, Gandaki and Karnali systems drain almost the entire Nepal Himalaya, 2,070 is the standard answer to 'how many glacial lakes does Nepal have'. The figure changes between inventories because new lakes keep forming as glaciers retreat and because mapping methods and size thresholds differ.

Counting lakes in the neighbouring headwaters matters as much as counting Nepal's own: floods originating in Tibet flow straight down the Bhotekoshi, Trishuli and other rivers into Nepali districts, which is why the inventory covered whole basins rather than stopping at the border.

From the 3,624 mapped lakes, ICIMOD and UNDP identified 47 potentially dangerous glacial lakes (PDGLs): 21 in Nepal, 25 in the Tibet Autonomous Region of China, and 1 in India. The screening looked at lake size and expansion, the condition of the damming moraine, the stability of surrounding slopes, avalanche and landslide exposure, and the state of the mother glacier.

The distribution is heavily skewed toward eastern Nepal: 42 of the 47 dangerous lakes sit in the Koshi basin, against 3 in the Gandaki basin and 2 in the Karnali basin. Each lake was assigned a hazard rank, from Rank I (large, growing lakes behind weak moraines needing urgent attention) to Rank III (routine watching).

Notably, both Tsho Rolpa and Imja Tsho remain in Rank I even though their water levels were artificially lowered in 2000 and 2016 — the lakes keep expanding, so lowering reduced but did not remove the danger. Six Nepali lakes are consistently treated as the highest national priorities for investigation and mitigation: Tsho Rolpa, Imja Tsho, Thulagi, Lower Barun, Lumding Tsho and Chamlang South Tsho in the Hongu valley.

Tsho Rolpa, at about 4,580 metres in the Rolwaling valley of Dolakha district, is Nepal's largest glacial lake and its longest-running GLOF worry. Fed by the retreating Trakarding Glacier, it grew from roughly 0.23 square kilometres in the late 1950s to about 1.65 square kilometres, around 135 metres deep, holding an estimated 86 million cubic metres of water. A breach would tear down the Rolwaling and Tamakoshi valleys, where thousands of people live and major hydropower assets, including the 456 MW Upper Tamakoshi plant, sit downstream.

Tsho Rolpa was the site of Nepal's first engineered GLOF mitigation: in 2000 (2057 BS), a project supported by the Governments of Nepal and the Netherlands cut an open channel and gate through the moraine, lowering the lake by 3 metres. An early warning system with sirens in downstream villages along the Rolwaling and Tamakoshi rivers, installed in 1998, stopped functioning by 2002 during the conflict years; the Department of Hydrology and Meteorology (DHM) and partners re-established monitoring and warning systems after the 2015 earthquake.

Experts had recommended lowering the lake by up to 20 metres for lasting safety, but funding never materialised. ICIMOD field expeditions in 2019 confirmed the lake is still expanding toward the glacier, with continuing rockfall from its side walls — which is why Tsho Rolpa remains a Rank I potentially dangerous lake.

Imja Tsho sits at about 5,010 metres below Island Peak in Solukhumbu district, close to the Everest Base Camp trail. In the early 1960s it was only a cluster of small meltwater ponds; by a 2014 bathymetric survey it had become a lake of about 1.28 square kilometres, up to 148.9 metres deep, holding roughly 75 million cubic metres of water — one of the fastest-growing lakes in the Himalaya.

In 2016 (2073 BS), the Government of Nepal and UNDP, with Global Environment Facility funding, carried out the highest lake-lowering operation ever attempted. About 40 Nepal Army personnel and more than 100 local high-altitude workers spent six months cutting a 45-metre outlet channel with a regulating gate, lowering the lake by 3.4 metres and safely draining more than five million cubic metres of water.

The project also installed a GLOF early warning system: sensors at the lake feed automated sirens in six downstream settlements and an SMS alert chain along roughly 50 kilometres of the Imja–Dudh Koshi corridor, protecting residents and trekkers. Even so, Imja continues to expand toward its glacier and remains on the potentially dangerous list.

Thulagi (also called Dona Lake) is the Gandaki basin's most-watched dangerous lake. It lies at about 4,044 metres at the snout of the Thulagi Glacier on the southern flank of the Manaslu massif, draining into the Marsyangdi River. About 2.5 kilometres long and roughly 76 metres deep, it holds around 35–36 million cubic metres of water. Studies estimate a worst-case outburst could directly affect over 160,000 people and threaten the Marsyangdi hydropower cascade, including the 69 MW Marsyangdi and 70 MW Middle Marsyangdi plants.

Lower Barun, in the Barun valley of Sankhuwasabha district below Makalu, is the deepest glacial lake yet surveyed in Nepal: about 205 metres deep with an estimated 112 million cubic metres of water. It has grown from around 0.04 square kilometres in the early 1960s to roughly 1.8 square kilometres, and it drains into the Arun River — the corridor that hosts the 900 MW Arun-3 hydropower project — making it a top candidate for structural mitigation.

Lumding Tsho, in a remote Solukhumbu valley draining to the Dudh Koshi, expanded from about 0.2 square kilometres in 1962 to 0.77 square kilometres by 2000 and has kept growing rapidly since, yet has no mitigation works. The neighbouring Hongu (Hunku) valley, inside Makalu Barun National Park, holds another cluster of Rank I lakes, including Chamlang South Tsho and the Hongu/Seti Pokhari lakes; their upper valleys are uninhabited, but their floodwaters would join the Dudh Koshi system downstream.

Reporting around the 2020 inventory recorded 26 GLOF events affecting Nepal since 1977, 11 of them transboundary floods originating in Tibet; newer satellite studies counting small events have documented even more. National attention was first fixed by the Dig Tsho outburst of 4 August 1985 (2042 BS) in the Khumbu, when an ice avalanche sent a wave through the moraine and the flood destroyed the nearly completed Namche small hydropower plant 11 kilometres downstream. In 1981, a GLOF from the Zhangzangbo (Cirenmaco) lake in Tibet had already wrecked infrastructure on the Arniko Highway corridor.

Recent years show the threat accelerating. In July 2016 a GLOF from Tibet damaged the Upper Bhotekoshi hydropower plant. On 16 August 2024 (2081 BS), the small Thyanbo glacial lake failed above Thame village in Solukhumbu, destroying around 20 buildings including a school and health post and displacing about 135 people, with no deaths thanks to daytime timing. On 8 July 2025 (2082 BS), a supraglacial lake that had formed only months earlier in Tibet's Lende Khola catchment burst, killing at least nine people in Rasuwa, sweeping away the Miteri Bridge at Rasuwagadhi and damaging four hydropower plants on the Bhotekoshi–Trishuli corridor.

A sobering lesson from Thame and Rasuwa is that neither flood came from the 47 listed lakes: both originated in small or newly formed lakes below the inventory's radar. Scientists now warn that thousands of small supraglacial ponds, which can appear and burst within a season or two, must be watched alongside the big named lakes.

Responsibility for glacial lake monitoring and flood warning in Nepal sits mainly with the Department of Hydrology and Meteorology (DHM), working with ICIMOD, UNDP, the Nepal Army and local governments. Monitoring combines repeat satellite mapping with Landsat and Sentinel imagery, published through the ICIMOD Regional Database System, periodic field expeditions with bathymetric surveys, and automated stations tracking lake levels, weather and river discharge.

Proven risk-reduction measures fall into three groups: structural mitigation, such as cutting controlled outlet channels to lower lake levels (done at Tsho Rolpa in 2000 and Imja in 2016); early warning systems linking lake sensors to downstream sirens and SMS alerts; and preparedness — hazard mapping, evacuation drills, land-use planning and GLOF-resilient design of bridges and hydropower intakes. Proposals to lower Thulagi, Lower Barun, Lumding and the Hongu lakes have been discussed under national adaptation planning, but as of the mid-2020s none of these lakes had been drained.

For residents and trekkers the advice is simple: know whether your valley lies below a listed lake, learn the local warning sirens, and move immediately to high ground if a river suddenly rises, turns muddy or roars unusually. Because Himalayan temperatures are projected to keep rising faster than the global average, the number of glacial lakes in Nepal — and the population exposed to GLOFs — will continue to grow, making the 2020 inventory a living document rather than a final list.