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None of Sikkim’s 16 identified high-risk glacial lakes has a source or direct hydrological connection across the China border, but that doesn’t make the threat any less serious
Sikkim’s experience with the catastrophic South Lhonak GLOF in October 2023 showed how a disaster originating thousands of metres above sea level can quickly become a downstream crisis. (AFP)
As Nepal grapples with the aftermath of a devastating glacier-related disaster, Sikkim is taking a closer look at its own Himalayan vulnerabilities—16 glacial lakes that have been identified as highly vulnerable to Glacial Lake Outburst Floods (GLOFs).
But there is one crucial geographical difference.
None of Sikkim’s 16 identified high-risk glacial lakes has a source or direct hydrological connection across the international border with China. All 16 are located within Indian territory and drain into the state’s river systems.
That does not, however, make the threat any less serious.
Sikkim’s experience with the catastrophic South Lhonak GLOF in October 2023 showed how a disaster originating thousands of metres above sea level can quickly become a downstream crisis, damaging roads, bridges, hydropower projects and other critical infrastructure.
The latest disaster in Nepal has brought that vulnerability back into focus.
What Happened In Nepal?
On August 26, a major glacier and ice-rock collapse near the Nepal-China border triggered a chain of events in the Lhende Khola and Bhote Koshi corridor.
The collapse sent huge quantities of ice, rock and debris into the river, temporarily blocking its flow. When the natural blockage failed, a powerful flood surged downstream. The disaster caused extensive damage to settlements, roads, bridges and hydropower infrastructure, with rescue operations continuing in affected areas.
For Sikkim, the incident is a reminder that Himalayan disasters do not always follow the same script.
A GLOF occurs when water stored in a glacial lake is suddenly released. But a glacier collapse, avalanche or landslide can also plunge into a river, block its flow and create a temporary reservoir that may eventually burst. The trigger may be different. The downstream devastation can look remarkably similar.
Sikkim’s 16 High-Risk Lakes
Sikkim is home to more than 400 glacial lakes, both large and small. Of these, 16 have been identified as highly vulnerable under the National Disaster Management Authority’s Category-A classification.
Thirteen are located in North Sikkim and three in West Sikkim.
The 16 lakes are Tikip, Bhalay Pokhari, Doodh Pokhari, Changsang Lake, North Lhonak Lake, South Lhonak Lake, Lachen Khangse Lake, Lachung Khangse Lake, La Chho, Shako Chho, Yulhe Khangse Lake, Gurudongmar Lake A, Gurudongmar Lake B, Gurudongmar Lake C, Khangchung Chho, and Tenchungka Lake.
Several of these lakes are located at elevations of around 17,000 feet or higher, making detailed scientific assessment challenging. Officials say 13 of the 16 priority lakes have already undergone detailed scientific assessment.
The studies have examined lake depth and volume, moraine stability, surrounding terrain and the possibility of avalanches, landslides and other events triggering a sudden release of water.
“The objective is increasingly to move beyond studying glaciers to actually mitigating the hazards they pose,” Sandeep Tambe, Principal Secretary of the Sikkim Science and Technology Department, said. “Studying glaciers is one thing, but we are mostly working towards glacial hazard mitigation,” he said.
Why South Lhonak Changed Sikkim’s Approach
The South Lhonak disaster in October 2023 was a turning point for Sikkim. A major disturbance involving the glacier triggered the sudden release of water from South Lhonak Lake. The resulting flood travelled through the Teesta basin, carrying water, boulders and debris and causing widespread destruction downstream.
The disaster demonstrated that the danger does not end at the lake.
Infrastructure located many kilometres downstream can remain vulnerable to a sudden flood wave. It also showed why monitoring only the amount of water stored in a lake is not enough.
Scientists now need to assess the entire system—the lake, the moraine holding it back, surrounding slopes, glaciers and avalanche paths, as well as the valleys through which floodwaters could travel.
Since 2023, Sikkim has conducted multiple high-altitude scientific expeditions to assess vulnerable lakes. Bathymetric surveys have been used to determine lake depth and water volume, while Electrical Resistivity Tomography (ERT) and other geophysical techniques have been used to study subsurface conditions and the composition and stability of moraine structures.
Officials said the largest assessed lake holds around 70 million cubic metres of water.
One Threat, Different Solutions
Sikkim is not looking at a single engineering solution for all 16 lakes. Instead, authorities are considering interventions based on the geology and geography of individual lake systems.
At Shako Chu, around 12 km from Thangu hamlet, officials are considering solar-powered pumping. The lake is estimated to hold around 27 million cubic metres of water and is backed by a fragile, permeable terminal moraine. Since it has no natural outlet, directly cutting into or modifying the moraine could itself create a hazard.
The proposal, therefore, is to pump water out rather than disturb the natural barrier.
Officials are considering lowering the lake level by around 20-40 metres, with approximately eight million cubic metres of water to be removed through eight solar-powered pumps. The aim is to create additional freeboard and reduce the volume of water that could potentially be released following an avalanche, landslide or other triggering event.
A Detailed Project Report has been prepared and is under examination at the Centre.
For the South-North Lhonak lake cluster, authorities are studying a different approach. Instead of directly modifying the lake, officials are examining Dolma Sampang, around 24 km downstream. The area is a broad, relatively flat stretch of around 300 hectares that naturally helped slow and spread the 2023 South Lhonak flood.
Authorities are now examining whether a stone-masonry retention structure could strengthen that natural buffer. The proposed structure would not necessarily stop a GLOF. Instead, it would aim to slow the flood wave, reduce its energy and allow water and debris to disperse more gradually.
The proposal has been submitted to the Ministry of Jal Shakti, with the Central Water Commission involved in the technical process.
At the Gurudongmar lake complex, which comprises three lakes, authorities are considering a moraine-plug safety wall or stone-masonry structure to regulate discharge.
The proposal has been developed with the Brahmaputra Board and the Lachen Dzumsa, while taking into account the cultural and religious importance of Gurudongmar.
The broader principle behind the three interventions is the same: reduce the hazard before a flood gains momentum. Once water starts rushing down a steep Himalayan valley, it can rapidly pick up boulders and debris, making the resulting flood far more destructive and intervention much more difficult.
Sensors, Weather Stations And Early Warnings
Engineering measures are being accompanied by an expanding monitoring and early-warning network.
Sikkim State Relief Commissioner Rinzing Chewang Bhutia said vulnerable lakes are being continuously monitored, with sensors installed at most sites to track changes in water levels and lake volume. “We are continuously monitoring the lakes and we have installed sensors in most of the lakes to find out whether the volume of the glacial lake is increasing or not,” Bhutia said.
Automated Weather Stations and early-warning systems have also been installed at critical locations, particularly in North Sikkim. Monitoring stations are operational at Shako Chu, Kerang and Dolma Sampang, while additional monitoring of South Lhonak and Shako Chu has been supported by Swiss development partners.
The state is also exploring locations for Doppler radar in coordination with the India Meteorological Department and the Ministry of Earth Sciences.
But officials acknowledge that the warning system is still evolving, according to NP Sharma, scientific officer with the Sikkim Science and Technology Department.
Satellite observations can be affected by cloud cover, while earthquakes, landslides and cloudbursts cannot always be predicted with sufficient precision. That makes ground-based sensors, reliable communication systems, power supply and human monitoring crucial.
What Nepal’s Disaster Tells Sikkim
The geographical contrast between Nepal and Sikkim is important.
Sikkim’s 16 vulnerable lakes have no cross-border source. Nepal’s latest disaster, meanwhile, began near the Nepal-China border.
But the Nepal incident demonstrates something bigger: a hazard may originate in a remote mountain location while its consequences travel much farther downstream.
The August 26 glacier and rock collapse entered the Lhende Khola, temporarily blocked the river and created an accumulation of water and debris. When that natural blockage failed, the resulting flood moved rapidly through downstream river corridors.
Infrastructure including hydropower projects and roads was damaged, while difficult terrain and blocked access complicated rescue operations.
Sikkim sees a direct lesson in the incident. Its own South Lhonak disaster showed that the source of a Himalayan hazard can be remote, while the impact can extend across an entire river valley.
For authorities, therefore, the challenge is not simply to identify dangerous lakes. It is to understand what can trigger a disaster, how the resulting flood will behave and what lies in its path.
From Reacting To Disasters To Anticipating Them
The South Lhonak disaster has also prompted Sikkim to strengthen its institutional response.
A high-level steering committee headed by the Chief Secretary oversees policy-level preparedness, while a multidisciplinary task force works on field-level implementation. The Science and Technology Department has been designated the nodal department for GLOF hazard assessment and mitigation.
The state is also working with agencies, including the Sikkim State Disaster Management Authority, Department of Mines and Geology, Central Water Commission, Sikkim University, C-DAC and the Brahmaputra Board.
Chief Minister Prem Singh Tamang (Golay) has said the Centre has asked Sikkim to further develop and strengthen the model it built after the 2023 disaster. That experience is now also being offered to Nepal.
Sikkim has sent humanitarian assistance to disaster-hit Nepal and expressed its willingness to share its experience in monitoring, preparedness and GLOF mitigation. Golay has said Nepal and others had supported Sikkim after the 2023 disaster and that the state’s current outreach was a humanitarian gesture in return.
One Himalayan Warning
The comparison between Nepal and Sikkim is not about identical disasters.
Nepal’s latest event was driven by a glacier and ice-rock collapse followed by a river blockage. Sikkim’s defining disaster was the South Lhonak GLOF.
But both underline the same fundamental vulnerability of the Himalayas.
A glacier can collapse in an almost inaccessible location. A landslide can block a river. A glacial lake can expand silently for years. An avalanche can suddenly displace huge quantities of water. And once water, ice and debris begin moving downhill, the distance between a remote mountain hazard and a human disaster can shrink dramatically.
Sikkim now has 16 highly vulnerable glacial lakes on its watchlist. None has a cross-border source.
But the lessons from Nepal, and Sikkim’s own experience, go beyond borders.
In the Himalayas, the question is not simply whether a mountain hazard will occur. It is whether authorities can detect it early enough, understand its likely path and act before the mountain moves.
Gangtok (incl. Upper Tadong), India, India
September 02, 2026, 15:48 IST
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