Sikkim flags 40 high-hazard glacial lakes, 16 at highest risk

Sikkim has identified 40 glacial lakes as high-hazard sites, including 16 categorised as being at the highest risk of a potentially catastrophic glacial lake outburst flood (GLOF), raising concerns for communities and infrastructure downstream.

The assessment comes amid growing concern over the impact of a rapidly changing Himalayan landscape. A flash flood near the Nepal-Tibet border on August 26, triggered by a glacial collapse, killed hundreds and left several people missing. Although the Nepal disaster was not classified as a GLOF, it has drawn attention to the risks posed by retreating glaciers and the formation and expansion of glacial lakes across the region.

Glacial lakes form when meltwater accumulates in depressions created by retreating glaciers. A sudden release of the stored water, caused by factors such as excessive accumulation, earthquakes or other disturbances, can produce a GLOF capable of sweeping away settlements, roads, bridges and other infrastructure downstream.

Sikkim has already experienced the consequences. A GLOF in October 2023 killed at least 60 people and caused widespread destruction across Mangan, Gangtok, Pakyong and Namchi districts. The disaster also destroyed the Chungthang dam, a key component of a major hydropower project.

At a media sensitisation workshop on GLOF risk mitigation on Thursday, Sikkim Principal Secretary Sandeep Tambe said satellite imagery and remote sensing were being used to track changes in glacial lakes.

Of the 40 high-hazard lakes identified by the state, 16 fall under Category A, denoting the highest risk. Two are in Category B, or high risk, while nine are in Category C, classified as moderate to low risk. The remaining 13 have not yet been classified.

The state is developing lake-specific mitigation plans rather than adopting a single strategy for all sites, Tambe said.

Three major areas — the Lhonak Complex, Shako Chho Lake and the Gurudongmar Complex — have been given separate proposals based on their individual risk profiles.

Shako Chho has been identified by the Sikkim government as the lake most vulnerable to a GLOF. Its Detailed Project Report has been submitted to the National Disaster Management Authority, while a preliminary project report has been sent to the Ministry of Jal Shakti.

At the Lhonak Complex, the Central Water Commission and the Sikkim government are preparing a DPR for a flood-retention structure at Dolma Sampa near Muguthang.

For the Gurudongmar Complex, authorities have proposed a moraine plug wall at the lake outlet. The Department of Science and Technology and the Brahmaputra Board are jointly examining the proposal, taking into account both flood risks and the site's cultural and religious importance.

Sikkim's Department of Science and Technology was designated the nodal agency for GLOF hazard assessment and mitigation studies in February 2025. Officials said the work involves experts from multiple disciplines and includes water-level and discharge monitoring, geophysical surveys, bathymetry, flood modelling and hazard assessment.

Authorities are also examining options to lower lake levels, including siphoning, pumping, improving existing drainage channels, creating artificial channels, cutting through moraines with protective walls and constructing tunnels.

Science and Technology principal director-cum-secretary D G Shrestha said the scale of the 2023 disaster had highlighted the need to improve preparedness and institutional capacity.

He said Sikkim was working on what could become the country's first dedicated GLOF Risk Mitigation Protocol, which would establish a standard scientific framework for assessing and managing glacial flood risks. The framework could potentially be adopted by other Himalayan states, officials said.

The proposed approach covers four stages — preliminary assessment, detailed assessment, design of mitigation measures and their implementation. The state is also considering a broader institutional framework, including a possible commission on glacial hazards, while seeking to address gaps in technical expertise and clarify responsibilities between government departments.

Additional early-warning sensors have also been proposed across Sikkim. Shrestha said the proposal received a positive response from Union Home Secretary Govind Mohan during a recent virtual meeting.

Mohan separately reviewed the preparedness of Himalayan states and Union territories for GLOFs and snow avalanches on Thursday. Chief secretaries and State Disaster Management Authorities from Uttarakhand, Himachal Pradesh, Sikkim, Arunachal Pradesh and Jammu and Kashmir attended the meeting along with senior officials from the Ministry of Home Affairs, National Disaster Management Authority, Central Water Commission, India Meteorological Department and the Defence Geoinformatics Research Establishment.

Officials at Thursday's workshop also stressed the role of the media in communicating GLOF risks accurately. Yougan Tamang, press secretary to Chief Minister Prem Singh Tamang, said greater public awareness would be important as the state expands its scientific monitoring and mitigation measures.