Glacial Lakes by River Basin: Koshi vs Gandaki vs Karnali

A glacial lake is a body of water fed by ice-melt and held back by a dam of loose moraine (rock and debris) or bedrock left by a glacier. As Himalayan glaciers retreat under a warming climate, these lakes are growing in number and size, which is why they are counted and monitored so carefully. The most authoritative recent count for Nepal comes from the International Centre for Integrated Mountain Development (ICIMOD) and the United Nations Development Programme (UNDP), whose 2020 (Bikram Sambat 2077) inventory covers the Koshi, Gandaki and Karnali river basins.

That inventory mapped 3,624 glacial lakes, each at least 0.003 square kilometres in surface area, roughly 2.4 times the size of an Olympic swimming pool. The lakes were delineated from Landsat satellite imagery captured in 2015, with elevations taken from ALOS digital elevation models (5-metre resolution over Nepal, 12.5-metre over the Chinese and Indian portions). Of the 3,624 lakes, 2,070 lie within Nepal, 1,509 within the Tibet Autonomous Region of China, and 45 within India, because the three basins are transboundary and their headwaters reach north of the main Himalayan crest.

So when people search 'how many glacial lakes in Nepal', the best-cited answer is that just over 2,000 sit inside Nepal's borders, out of 3,624 in the wider Koshi-Gandaki-Karnali system. The report also notes that 1,410 of these lakes are 0.02 square kilometres or larger, the size band generally considered big enough to produce a damaging glacial lake outburst flood (GLOF).

Breaking the 3,624 lakes down by river basin gives a clear ranking. The Koshi basin, in eastern Nepal, holds 2,064 glacial lakes; the Karnali basin, in the far west, holds 1,128; and the Gandaki basin, in the centre, holds 432. These are whole-basin figures that include the lakes on the Tibetan and Indian sides of each basin, not just the Nepali portion. Even so, the pattern is unambiguous: the Koshi basin alone accounts for well over half of all the glacial lakes in the three-basin region.

The area picture is similar but not identical. The report puts the combined surface area of the mapped lakes at roughly 195 square kilometres, of which about two-thirds lies in the Koshi basin, a little under a quarter in the Karnali, and under a tenth in the Gandaki. Notably, the Karnali basin ranks second in both lake count and lake area despite being the least studied of the three, a reminder that Nepal's far-western glaciers hold significant, under-monitored water. (The 195 km2 total should be read as an approximate figure from the inventory.)

This basin ordering matters because it maps directly onto Nepal's river geography. Each basin is named for the river system it feeds, all three of which are covered in our rivers directory (/rivers): the Koshi (the Sapta Koshi and its tributaries such as the Arun, Tamor, Dudh Koshi and Tama Koshi), the Gandaki (the Kali Gandaki, Marsyangdi, Trishuli and Budhi Gandaki, among others), and the Karnali (the Humla Karnali, Mugu Karnali, Bheri and Seti). The glacial lakes sit high in the headwaters of these rivers, so an outburst upstream can send a flood surge many kilometres downstream into inhabited valleys.

The Koshi basin's dominance is a product of geography. Its headwaters drain the most heavily glaciated stretch of the Nepal Himalaya, including the Everest (Sagarmatha), Makalu and Kanchenjunga massifs, where extensive debris-covered glaciers feed a dense concentration of moraine-dammed lakes. That is why the basin holds 2,064 glacial lakes and, more importantly, 42 of the 47 potentially dangerous glacial lakes identified across the three basins, by far the highest hazard concentration in the country.

Many of Nepal's best-known and most-studied glacial lakes sit in this basin. Imja Tsho, below the Everest region in Solukhumbu district, is one of the fastest-growing lakes in the Himalaya and had its water level lowered as a mitigation measure in 2016. Tsho Rolpa, in the Rolwaling valley of Dolakha district, is often described as one of Nepal's single most dangerous lakes. The basin was also the site of the destructive 1985 Dig Tsho outburst near Langmoche, which washed away a small hydropower plant and infrastructure downstream in the Dudh Koshi system.

The Koshi's potentially dangerous lakes cluster in the high districts of eastern Nepal, several of which have detailed pages in our districts directory (/districts): Solukhumbu (Imja, Lumding and Hongu, in the Dudh Koshi headwaters), Dolakha (Tsho Rolpa, in the Tama Koshi headwaters) and Sankhuwasabha (Lower Barun, in the Arun-Barun system). Because these lakes drain toward densely settled mid-hill valleys and hydropower corridors along the Sun Koshi, Tama Koshi and Arun, the basin combines high hazard with high downstream exposure.

The central Gandaki basin holds the fewest glacial lakes of the three, 432 in total, and just 3 of the region's 47 potentially dangerous glacial lakes. Its glaciers drain the Annapurna, Manaslu and Dhaulagiri ranges and the trans-Himalayan district of Mustang, feeding the Kali Gandaki, Marsyangdi, Budhi Gandaki and Trishuli rivers. Although the lake count is modest, the basin contains one of Nepal's highest-profile hazard lakes.

That lake is Thulagi (also spelt Thulagi Tal), a moraine-dammed lake fed by the Dona Glacier in Manang district, sitting above the Marsyangdi valley. Thulagi has been studied and monitored for decades and is one of the four priority lakes now targeted for structural risk-reduction works under a major Green Climate Fund project. Its position above the Marsyangdi, a river lined with hydropower schemes and road settlements, is what makes a relatively small lake a nationally significant concern.

For readers cross-referencing our districts directory (/districts), the Gandaki basin's glacial lakes fall largely in Manang, Mustang and Gorkha, high districts on the northern rim of Gandaki Province. The comparatively low lake count reflects both the basin's smaller high-glacier catchment and the deep rain-shadow of the trans-Himalaya, where less ice accumulates than in the wetter eastern ranges.

The Karnali basin ranks second in the country with 1,128 glacial lakes, yet it carries only 2 of the 47 potentially dangerous glacial lakes, the lowest hazard flag of the three basins. This is Nepal's largest and most remote river basin, draining the high country of Humla, Mugu, Dolpa and Jumla and feeding the Karnali river along with the Humla Karnali, Mugu Karnali, Bheri and Tila.

The Karnali's large lake count but low count of flagged dangerous lakes partly reflects how little field research has reached this far-western frontier compared with the intensely surveyed Everest and Rolwaling areas of the Koshi basin. Access is difficult, glaciological monitoring is sparse, and several lakes here have never been visited on the ground. The two potentially dangerous lakes identified sit in the high, sparsely populated headwaters, where downstream exposure is lower than in the eastern hills but still real for scattered mountain settlements.

For a Nepali audience, the takeaway is that the Karnali holds a substantial share of the country's glacial water in a landscape that is only lightly monitored. As satellite coverage and field surveys improve, the far-western basin, whose districts such as Humla, Mugu and Dolpa are among the most remote profiled in our districts directory (/districts), is the region where the hazard picture is most likely to be revised.

A potentially dangerous glacial lake (PDGL) is one judged, on criteria such as size, growth rate, dam condition and slope stability, to pose a credible risk of a glacial lake outburst flood. Across the three basins, the 2020 inventory identified 47 PDGLs. By country they split into 25 in the Tibet Autonomous Region of China, 21 in Nepal and 1 in India, a reminder that many lakes threatening Nepali valleys actually sit upstream across the border in Tibet.

The report ranked the 47 lakes into three priority tiers: 31 were placed in Rank I (the highest priority for action), 12 in Rank II and 4 in Rank III. By basin the concentration is stark, with 42 of the 47 in the Koshi basin, 3 in the Gandaki and 2 in the Karnali. This is the core statistic behind headlines such as 'Koshi basin glacial lakes' being the most dangerous in Nepal.

The danger is not theoretical. According to the ICIMOD-UNDP report, Nepal has experienced 26 GLOF events since 1977, of which 14 originated within the country and the rest swept down from lakes across the border. Past events like the 1985 Dig Tsho flood destroyed infrastructure and lives, and the risk is rising as warming accelerates glacier melt and lakes expand behind fragile moraine dams.

Nepal has already carried out landmark mitigation works, all in the Koshi basin. At Tsho Rolpa in Dolakha, an outlet drainage canal completed in June 2000 lowered the lake level by about three metres. At Imja Tsho near Everest, the Nepal Army, working with UNDP and the Global Environment Facility, lowered the lake by roughly three metres in 2016 and installed a community early-warning system. These interventions show that engineered lowering, combined with warning systems, is feasible even at extreme altitude.

Building on the 2020 inventory, the Green Climate Fund in 2023 approved about US$36.1 million to help Nepal reduce GLOF risk. That project targets four priority lakes for structural works, early-warning systems and community preparedness: Lower Barun, Lumding and Hongu in the Koshi basin, and Thulagi in the Gandaki basin. This is a direct example of the inventory's basin-level hazard ranking guiding where scarce mitigation money is spent.

When citing these figures, a few caveats keep the numbers accurate. The lake counts are based on 2015 satellite imagery, so today's totals are almost certainly higher as glaciers keep retreating. The per-basin counts (2,064 / 1,128 / 432) are whole-basin figures spanning Nepal, Tibet and India, whereas the 2,070 figure is the lakes inside Nepal alone. And the hazard tally is a snapshot of assessed risk, not a fixed count, since new surveys can reclassify lakes, especially in the lightly studied Karnali. Cross-referenced against our rivers directory (/rivers) and districts directory (/districts), the inventory gives students and planners a durable, basin-by-basin map of where Nepal's glacial water, and its flood risk, is concentrated.