How glacial lake expansion adds to the growing concerns about GLOFs
A recent analysis of glacial lakes in the Mago Chu Basin in Arunachal Pradesh’s Tawang district found that four of the five examined lakes, including those categorised as ‘high risk’ and ‘very high risk’, have expanded over the last decade. How does this expansion heighten concerns over the growing threat of Glacial Lake Outburst Floods (GLOFs)? See infographics.
The expansion of glacial lakes across the Himalayas has added to the growing concerns over the threat posed by Glacial Lake Outburst Floods. (Representational image) — Abhinav Rai
As rising global temperatures accelerate glacier retreat and increase meltwater, glacial lakes across the Himalaya are expanding at an unprecedented rate. This expansion has heightened concerns over the growing threat of Glacial Lake Outburst Floods (GLOFs).
India has witnessed some of the disastrous GLOF events in recent years that claimed thousands of lives and caused severe infrastructure damage. A recent analysis of glacial lakes in the Mago Chu Basin in Arunachal Pradesh’s Tawang district found that four out of the five examined glacial lakes – Dharkha Tso, Sanhapo Lake, and three other unnamed lakes – have expanded between 2016 and 2026.
For instance, Sanhapo Lake, a ‘high risk’ lake, increased from 55.87 hectares (ha) in 2016 to 88.81 ha in mid-June 2026, according to the analysis by Noida-based geospatial intelligence firm Suhora Technologies. Among these, two unnamed lakes, which are categorised as ‘very high risk’ by the National Disaster Management Authority (NDMA), are also expanding at modest rates.
Such expansion has raised concerns over the current status of glacial lakes in India and the country’s preparedness to reduce the GLOFs risk. But first, let’s understand how glacial lakes are formed, their types and distribution. Why are some glacial lakes more hazardous than others?
Why Himalayan Glacial Lakes Are a Growing Flood Risk
Formation of glacial lakes
Glacial lakes are formed in the high mountains when glacier-eroded surfaces or depressions are filled with the melting of seasonal snow and glacier ice. The origin, evolution, and location of a glacier lake depend upon local and regional climatic conditions, erosional and depositional activities of glaciers on the valley floors and slopes, and types of the eroded material.
The area, shape and volume of water in a glacial lake are determined by the climate, glacier, snowmelt and local topography. Glaciers are very sensitive to temperature changes. Studies suggest that due to warming, there has been a rise in the number and size of glacial lakes across the Himalayas in the past couple of decades.
According to the International Centre for Integrated Mountain Development (ICIMOD) classification, glacial lakes can broadly be categorised into five classes:
1. Glacial erosion lakes
Erosion lakes, like Cirques and Glacial Valley lakes, are formed in the depressions created by the processes of erosion and abrasion during the movement of glaciers.
2. Moraine-dammed lakes
Receding glaciers leave behind a large amount of unconsolidated rock and debris (moraines). When these glaciers further melt, these moraines act as natural ridges, and water starts accumulating between glaciers and moraine ridges. This is known as a moraine-dammed glacial lake, like Samudra Tapu and Gepang Gath in Himachal Pradesh.
These lakes are most vulnerable to the GLOFs because they are formed by loose, unconsolidated glacial debris, making them unstable and susceptible to sudden collapse even from minor external triggers. Based on the location of moraines, these lakes can be divided into further subclasses, such as end-moraine-dammed and lateral moraine-dammed lakes.
Q1. With reference to glacial lakes, consider the following statements:
Which of the statements given above is/are correct? (a) 1 and 3 only |
3. Ice-blocked lakes
An Ice-blocked lake forms when a surging or advancing glacier blocks the melting water, or when, during glacier retreat, a part of the glacier gets separated from the main glacier and blocks the meltwater. Such lakes are mostly found in the upper Indus and Yarkand basins of the Karakoram region.
4. Supraglacial lakes
Supraglacial lakes are formed on the surface of the glacier, generally in the ablation zone of debris-covered glaciers. For example, there are many such smaller ponds at the debris-covered Barashigri glacier in Himachal Pradesh and Rongbuk Glacier on the northern Mount Everest.
5. Subglacial lakes
Sometimes, there are pond-like depressions created within a glacier or ice sheet that are called subglacial lakes. They often have well-developed sub-glacial channels through which water gets transported to the glacier’s snout. Such lakes cannot be identified using remote sensing alone and require instruments like ground penetrating radar (GPR). More than 140 subglacial lakes have been identified under the Antarctic ice sheet.
Spatial distribution of glacial lakes in Indian Himalayas
According to the glacial lake atlas (2023) of the Indian Himalayan river basins prepared by the National Remote Sensing Centre under ISRO, India has 28,043 lakes with a water spread area of greater than or equal to 0.25 ha. As Indus and Brahmaputra originate in the Tibet region, approximately 73 per cent of these lakes are transboundary lakes, and a total of 7,570 lakes are located within the Indian region.
The Brahmaputra basin has the highest number of glacial lakes (64.19%), followed by the Indus basin (19.02%) and the Ganga basin (16.78%). State-wise, Arunachal Pradesh (28.90%), Sikkim (9.68%), Himachal Pradesh (7.09%), and Uttarakhand (4.58%) have the most glacial lakes. Among these, 299 glacial lakes have a lake area greater than 50 ha lake area and most of them are found in the Brahmaputra Basin (207) and the Union-Territories of Jammu & Kashmir and Ladakh (24).
While the growing number and expansion of glacial lakes underscore the impact of climate change in the Himalayas, it also raises concerns about the potential risk of GLOFs.
Q2. Consider the following pairs:
Which of the pairs given above are correctly matched? (a) 1, 2 and 4 only |
The growing threat of GLOFs
Since glacier lakes are generally dammed structures made of ice or moraines, they are weak and thus vulnerable to breaching due to triggering factors such as snow avalanches or rockfalls, seismic events, increased volume of meltwater, and heavy precipitation.
When such dams breach, water from glacial lakes is suddenly released and causes catastrophic floods with extreme peak discharges and very high erosion and sediment-transport potential. Such events are called GLOFs. They pose potential flood risks and can be catastrophic, degrading the valley, its infrastructure, and hydropower and other such projects, and physical and socio-economic conditions of downstream communities.
India has experienced some of the disastrous GLOF events in recent years. In June 2013, a compound event, including GLOF at Chorabari glacial lake (near Kedarnath temple, Uttarakhand) and cloudbursts, landslide, claimed more than 5,000 lives and caused severe infrastructure damage. A decade later, in October 2023, a GLOF at Sikkim’s South Lhonak lake killed 55 people, and several were injured.
Last year’s GLOF events at Rasuwagadhi, Tilgaon and Upper Mustang in Nepal Himalayas, where lakes were smaller (< 1 sq. km area), show that even the smaller and short-lived glacial lakes can cause much damage.
Several studies suggest that climate change may intensify the GLOF risks in the high Himalaya by elevation-dependent warming, a phenomenon where high-mountain environments experience more rapid changes with increasing temperature compared to lower elevation regions, increasing the number, area, and volume of glacial lakes.
India’s strategies to mitigate GLOFs
The NDMA has formulated a guideline (2020) to raise awareness, improve administrative response, develop early warning mechanisms and has suggested suitable actions to reduce the risk and costs associated with these glacial hazards. Based on this, a standard operating procedure (SOP) has been developed in coordination with various central ministries, including the Ministry of Home Affairs (MHA), the Department of Science and Technology (DST), and the Ministry of Jal Shakti (MoJS).
Additionally, the Central Water Commission (CWC) has finalised a criterion for the ‘risk indexing’ of glacial lakes (2024) and is currently ranking 681 Glacial Lakes (greater than 10 Ha) based on the likelihood of failure and potential damage due to GLOF. In July last year, CWC published “Guidelines for structural measures to mitigate adverse effects of GLOF on Dams” for authorities to incorporate GLOF considerations and risk reduction strategies into infrastructure planning of these vulnerable Himalayan states.
The Central Government also launched the National Glacial Lake Outburst Flood Risk Mitigation Project (NGRMP) in 2025 with a budget of Rs. 150 crore in the four Himalayan states of Arunachal Pradesh, Himachal Pradesh, Sikkim and Uttarakhand. The NGRMP is aimed at reducing risk and strengthening the early warning and monitoring capacities.
| Q3. Which one of the following best explains the phenomenon of elevation-dependent warming?
(a) Mountain regions experience less warming because of permanent snow cover. (b) Higher elevations warm faster than lower elevations, accelerating glacier retreat and glacial lake expansion. (c) Warming occurs only above the snowline. (d) Temperature increases uniformly across all elevations. |
Future challenges and way forward
Due to rugged terrain and harsh climatic conditions, availability of field-based glaciological and climate data is very sparse in the high-Himalayan region and most studies rely on remote-sensing data, which has its own issues related to resolution and availability.
There is a need for dynamic glacial lake inventories with seasonal monitoring, which can be updated regularly, especially for supra-glacial lakes and other potentially dangerous glacial lakes (PGDLs). In the long term, an institutional trans-boundary cooperation mechanism between countries of the Hindu Kush Himalayan region is needed for effective monitoring, real-time data sharing and establishing early warning systems to reduce the potential threat of glacial lake outbursts.
Post read questions
1. What are glacial lakes? How has the climate change transformed glacial lakes from freshwater reservoirs into potential disaster hotspots?
2. Discuss the spatial distribution of glacial lakes in the Indian Himalayan Region. Why do certain basins and states face greater GLOF risk?
3. The increasing frequency of GLOFs has emerged as one of the most significant climate-induced hazards in the Indian Himalaya. Examine the causes, impacts and mitigation measures.
4. Examine the vulnerability of Himalayan infrastructure, hydropower projects and downstream communities to Glacial Lake Outburst Floods. Suggest measures to enhance resilience.
5. Recent observations from the Mago Chu Basin in Arunachal Pradesh indicate continued expansion of several high-risk glacial lakes. Discuss the implications of such findings for Himalayan disaster preparedness and infrastructure planning.
(Abhinav Rai is a Doctoral candidate at the Department of Geography, Delhi School of Economics, University of Delhi.)
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