Nepal Districts at Risk from GLOF: Downstream Hazard Guide

A glacial lake outburst flood (GLOF) happens when the natural moraine or ice dam holding back a high-altitude glacial lake fails, releasing millions of cubic metres of water and debris in minutes. Because the water follows river channels, the danger is felt far from the lake itself, in the settlements, roads, bridges and hydropower plants strung along the valley below. This is why GLOF is best understood as a downstream, district-level hazard rather than only a high-mountain one.

The scale of the exposure is large. A 2020 inventory by the International Centre for Integrated Mountain Development (ICIMOD) and the United Nations Development Programme (UNDP) mapped 3,624 glacial lakes across the Koshi, Gandaki and Karnali river basins of Nepal, the Tibet Autonomous Region of China and India, of which 2,070 lie inside Nepal. The study flagged 47 potentially dangerous glacial lakes (PDGLs), including 21 within Nepal's own territory, and ranked 31 of them in the highest hazard category (Rank I).

Nepal has already lived through these events. At least two dozen GLOF episodes have been recorded in the country's history according to ICIMOD, including several transboundary floods that began in Tibet and swept into Nepali valleys. Warming in the Himalaya is accelerating glacier retreat and lake growth, so the number of dangerous lakes, and the districts sitting in their path, is expected to keep rising.

Most of Nepal's concentrated GLOF risk maps onto a handful of northern districts, each tied to specific lakes and a specific river that carries any flood toward the plains. The Koshi basin in the east holds the largest cluster of dangerous lakes: 42 of the 47 PDGLs identified in 2020 sit within it, which is why eastern districts dominate the risk list. The Gandaki basin in the centre-west adds a smaller but growing set of threats.

The list below pairs each high-risk district with its principal upstream lake or flood source and the river path a flood would follow. Reading these as connected units, lake to river to district, is the core of any downstream hazard assessment, and each district also links to its wider profile on amarnepal.com.

Solukhumbu, the district that includes the Everest region, carries some of Nepal's most closely watched GLOF risk along the Dudh Koshi. Imja Tsho, one of the fastest-growing lakes in the Himalaya, was judged dangerous enough that in 2016 the Nepal Army drained close to 4 million cubic metres of water, lowering the lake by about 3.4 metres to reduce the threat to an estimated 96,000 people living downstream. It remains classed among the highest-risk lakes.

The district also holds the memory of one of Nepal's most destructive historical GLOFs. On 4 August 1985, Dig Tsho burst above Thame, sending roughly 5 million cubic metres of water down the Bhote Koshi and Dudh Koshi. The flood destroyed a nearly completed small hydropower plant, about 14 bridges and some 30 houses, along with farmland and trails, a stark demonstration of how far downstream damage can reach.

The risk is not only from large, well-known lakes. On 16 August 2024, small supraglacial lakes above Thame village, chief among them Thyanbo (Thyangbo) Lake, breached and sent a flood through the settlement, destroying around 14 structures including a school and a health post and displacing scores of residents. The Thame disaster underlined that even tiny, unmapped ponds can devastate a village, reshaping how Nepal approaches cryosphere risk.

Tsho Rolpa, in the Rolwaling Valley of Dolakha district at around 4,540 metres, is the largest glacial lake in Nepal, roughly three kilometres long and up to about 130 metres deep, holding on the order of 80 million cubic metres of water behind an unstable moraine dam. A full breach could send a damaging flood more than 100 kilometres down the Rolwaling Khola and Tama Koshi, threatening thousands of lives, farmland and hydropower infrastructure, which is why the lake has been studied and monitored for decades.

Dolakha is also the site of Nepal's flagship GLOF mitigation effort. Between 1998 and 2000, engineers cut an open channel controlled by sluice gates to lower the lake by about three metres, buying a measure of safety while a longer-term solution is sought. Tsho Rolpa remains ranked among the most dangerous lakes in the region, so the lowering is treated as a partial reduction of risk rather than an all-clear.

The Tama Koshi valley below the lake carries one of the country's better-developed early warning systems. Sensors near the lake are designed to relay an alarm to warning stations spread through the Rolwaling and Tama Koshi valleys; the network, first installed in the late 1990s, was revitalised after the April 2015 Gorkha earthquake, which had raised fresh fears about dam stability across the high Himalaya.

Some of the worst floods in these districts do not begin in Nepal at all. On 5 July 2016, a GLOF from Gongbatongshacuo, a lake in Tibet's Poiqu basin, surged down the Bhote Koshi into Sindhupalchok, damaging the Arniko Highway and the Upper Bhotekoshi hydropower project and causing losses estimated around 70 million US dollars. The Bhote Koshi corridor, a key trade route to China, is especially exposed because a flood can cross the border and strike Nepali infrastructure with almost no warning.

Rasuwa faces the same transboundary pattern on the Trishuli side. On 8 July 2025, a flood tied to a glacial lake in Tibet swept down the Lende Khola and Bhote Koshi at Rasuwagadhi, the main Nepal-China border crossing. It killed at least nine people, left roughly 19 missing, destroyed the friendship bridge and disrupted four hydropower stations that together account for around 230 megawatts, close to eight percent of Nepal's electricity capacity at the time.

These events show why 'sindhupalchok glacial lake' and 'rasuwa flood' spike as search terms after the monsoon: the hazard is real, recurrent and often crosses an international boundary. Because the source lakes lie outside Nepal, effective protection depends on transboundary data sharing with China as much as on domestic monitoring, a gap that both floods exposed.

In eastern Nepal, Lower Barun lake sits inside Makalu Barun National Park in Sankhuwasabha at around 4,540 metres. It has grown dramatically, nearly doubling in area since the 1960s to roughly 2.2 square kilometres, and drains via the Barun into the Arun. Modelling suggests a major breach could threaten settlements along the Arun in Sankhuwasabha, Bhojpur, Dhankuta and Sunsari, yet as of 2020 no dedicated early warning system had been installed there, leaving a long, populated corridor exposed.

In the Gandaki basin, Thulagi lake below Mt Manaslu, in Manang district, is one of central Nepal's leading GLOF concerns. The lake has expanded past one square kilometre as its glacier retreats by tens of metres a year, and a burst would run down the Marsyangdi toward hydropower projects and towns such as Besisahar and Dumre; one assessment estimates well over 100,000 people could be affected. Thulagi is a priority target for lowering under Nepal's new national GLOF programme.

Mustang, in the rain-shadow of the Annapurnas, shows that even the arid trans-Himalaya is not safe. On 13 August 2023, a flood linked to a high ice-dammed lake tore through the Kagbeni area near Muktinath, along the Kali Gandaki, destroying around 29 houses and several bridges with damage estimated near 7.4 million US dollars. Follow-up assessments have confirmed GLOF as the driver of more than one recent Upper Mustang flood, adding the district to Nepal's active watch list.

Nepal's response combines two tools: lowering the most dangerous lakes to shrink the volume that can escape, and building early warning systems (EWS) so that downstream communities get minutes to evacuate. So far, only Tsho Rolpa (lowered about three metres by 2000) and Imja Tsho (lowered about 3.4 metres in 2016) have been physically drained, both flagship projects led by the Department of Hydrology and Meteorology (DHM) with international partners. Warning networks are strongest on the Tama Koshi below Tsho Rolpa and in the Dudh Koshi basin, and thinnest in transboundary corridors and around fast-growing lakes like Lower Barun.

The largest step forward is a seven-year programme backed by a 36.1 million US dollar Green Climate Fund (GCF) grant, led by DHM with UNDP, that aims to benefit around 2.3 million people. It will lower four high-risk lakes, Thulagi (Manang), Lower Barun (Sankhuwasabha) and Lumding Tsho and Hongu (both Solukhumbu), while expanding hazard monitoring, early warning systems and downstream protection such as check dams and riverbank planting.

For residents and travellers, the practical takeaways are consistent across districts. Know which river your settlement sits on and which lake feeds it, treat sudden rises or unusual roaring in a glacial river during clear weather as a warning sign, and follow instructions from local disaster authorities and the DHM. GLOFs move faster than people can outrun on foot, so moving to higher ground immediately, rather than toward the river to look, is the single most important response.