CHANDIGARH: The devastating flash floods in Nepal’s Rasuwa district and other parts of the Bhote Koshi and Koshi river systems have once again brought the potential threat posed by glacial lakes in Himachal Pradesh into sharp focus.
The number of glacial lakes in the hill state has increased nearly fivefold, from 107 in 1990 to 669 in 2024, while their combined surface area has more than doubled. The rapidly expanding Geepang Lake in Lahaul and Spiti is among the major concerns. Its area has increased from 27 hectares in the 1970s to 101 hectares, an increase of about 274 per cent, raising serious concerns over the risk of a Glacial Lake Outburst Flood (GLOF).
The Kullu, Kinnaur and Lahaul-Spiti districts lie directly downstream of several of these growing glacial lakes. They also host a large number of the state’s hydropower projects, with dams, barrages, tunnels and staff townships located along narrow river corridors. A GLOF could send a high-speed surge of water through these corridors, potentially causing extensive damage downstream.
According to a joint study by IIT Ropar and the University of Technology Sydney, the number of glacial lakes in Himachal Pradesh increased from 107 in 1990 to 669 in 2024 — a 525 per cent rise. Their combined area expanded by 102 per cent, from 5.54 square kilometres to 11.20 square kilometres.
Of the 669 lakes, 94 have an area of at least 0.02 square kilometres, and 88 of these have been identified as susceptible to GLOF triggers. Very small lakes, measuring 0.01 square kilometres or less, recorded the sharpest increase, rising from just nine in 1990 to 487 in 2024.
The study categorised nine lakes as “very high hazard”, 18 as “high hazard” and 26 as “medium hazard”, based on factors including the instability of natural dams. It found that 73 lakes had unstable natural dams, while 58 were vulnerable to earthquakes, 52 to ice-core-related failures and 44 to rockfalls.
Himsu Lake, which covers just 0.046 square kilometres and is fed by a 1.19-square-kilometre glacier, lies about 30 kilometres from Hurla village. An unnamed lake in the Sutlej basin, covering 0.363 square kilometres and fed by a 4.91-square-kilometre glacier, was also identified as high-risk.
Modelling by the researchers estimated that if the natural dams of the two lakes were to fail, peak outburst discharge could reach 1,385 cubic metres per second from the Beas-basin lake and 3,507 cubic metres per second from the Sutlej-basin lake, potentially causing severe flooding downstream.
Under the worst-case scenario in the Beas basin, 94 buildings could be affected, including parts of the Pandoh Dam and the Uhl-III hydropower station. In the Sutlej basin, at least 588 buildings could be exposed to risk, including the Koldam project and the Rampur hydropower station.
The study has urged the state government to integrate GLOF hazards into existing dam-safety frameworks. It has also recommended installing early-warning sensors at least 25 kilometres upstream of Pandoh Dam and 100 kilometres upstream of the Bhakra Dam.
A separate study by the Centre for Development of Advanced Computing (C-DAC) found that some glacial lakes have expanded dramatically. Geepang Lake in Lahaul and Spiti, for instance, has grown from 27 hectares in the 1970s to 101 hectares, while Vasuki Lake in Kullu expanded by 30 per cent between 2017 and 2024.
The C-DAC survey found that high-risk glacial lakes in the state now threaten 34 settlements, 57 bridges and 107 kilometres of roads across the Parbati and Chenab basins, with Kullu and Kinnaur districts among the areas facing the highest risk.
Another study, published in the journal Natural Hazards, highlighted the transboundary dimension of the threat, particularly from lakes originating in the upper Sutlej basin across the border in China. These lakes could pose a threat to downstream areas of eastern Himachal Pradesh.
The study also found that in areas such as central Kullu, climate change could result in a sevenfold increase in the probability of a GLOF being triggered and a threefold increase in the downstream area affected. This could raise the hazard classification from “high” to “very high”. It also emphasised that GLOF risks can extend far downstream from the glaciated headwaters where the threat originates.
Across the wider Hindu Kush Himalaya (HKH) region, there are more than 25,000 glacial lakes, according to studies by the International Centre for Integrated Mountain Development (ICIMOD). Their formation and expansion have been linked to rising temperatures and accelerating glacier retreat.
ICIMOD reports released in 2018 and 2024 mapped and classified more than 25,000 glacial lakes across five major river basins in the HKH region. About 8.3 per cent of these lakes are located below 4,000 metres above sea level.
The HKH is warming at an average rate of 0.28°C per decade and is experiencing unprecedented glacier mass loss. According to ICIMOD, the rate of glacier mass loss has increased by 65 per cent in recent decades compared with earlier periods. Rapid glacier retreat has contributed to the proliferation and expansion of glacial lakes across the region, which spans the Amu Darya, Indus, Ganges, Brahmaputra and Irrawaddy basins.
Meanwhile, the Glacial Lake Atlas prepared by the National Remote Sensing Centre (NRSC) of ISRO has mapped around 7,500 glacial lakes across Ladakh, Himachal Pradesh, Uttarakhand, Sikkim and Arunachal Pradesh, many of which pose a potential risk of dangerous flash floods.
Himachal Pradesh Chief Minister Sukhvinder Singh Sukhu said the formation of any glacial lake carries an inherent risk. He said the state disaster management authorities had conducted a study of Geepang Lake and were keeping a constant watch on it.
“There is always some danger when a glacial lake is formed,” he said.
Sukhu stressed the need for scientific research into cloudbursts, glaciers and other climate-induced phenomena, linking the increasing frequency of disasters across the Himalayan region to climate change.
“We have endured natural disasters over the last few years. The impact of climate change will be experienced across the entire Himalayan region,” he said.
Sukhu had also urged Union Home Minister Amit Shah to commission a scientific study involving experts from IITs and environmentalists. A team subsequently visited the state, but its findings were yet to be shared with the government, he said.
Meanwhile, Himachal Pradesh State Disaster Management Authority Director Pushpendra Rana said important and potentially vulnerable glacial lakes, including Geepang, were being regularly monitored through satellite imagery and remote-sensing data.
In the case of Geepang Lake, a detailed downstream flood-zonation exercise has already been initiated, with red, yellow and green zones being demarcated up to 300 metres along the river.
Rana said the state was also working with C-DAC to strengthen early-warning systems and ensure that monitoring data and hazard assessments could be converted into actionable alerts for downstream communities.
What causes a glacial lake to burst?
Several factors can trigger a Glacial Lake Outburst Flood (GLOF), including ice avalanches, rockfalls, landslides, intense rainfall, cloudbursts and earthquakes.
Water pressure can weaken a natural dam, while erosion can cut into the dam and accelerate its failure, according to the joint study by IIT Ropar and the University of Technology Sydney.
The study also notes that earthquakes can trigger landslides or ice avalanches, which may subsequently cause a glacial lake to burst.