40 high-hazard glacial lakes identified in Sikkim

The eastern Himalayan state of Sikkim has identified 40 high-hazard glacial lakes, with 16 of them classified as being at the highest risk of catastrophic outbursts that could threaten downstream settlements.

The data is significant as it comes against the backdrop of the recent flash flood in Nepal, triggered by a glacial collapse near the Nepal-Tibet border on August 26 that left a trail of destruction with hundreds of people dead and scores missing.

Though the catastrophe was not linked to a Glacial Lake Outburst Flood (GLOF), the flash flood has highlighted a broader transformation across the Himalayas as glaciers retreat, new lakes are forming in their place, while existing ones are expanding.

Glacial lakes are formed by the melting of glaciers and accumulation of meltwater in depressions on or near the glacier’s surface. GLOFs occur when these lakes suddenly burst open due to factors such as excessive water accumulation or triggers like earthquakes.

These floods can cause significant destruction and pose serious risks to people and the environment downstream.

One such outburst in Sikkim in October 2023 resulted in at least 60 fatalities and extensive damage in the Mangan, Gangtok, Pakyong and Namchi districts. It also destroyed the Chungthang dam, a crucial part of a major hydropower project.

At a media sensitisation workshop on GLOF Risk Mitigation held here on Thursday, Principal Secretary Sandeep Tambe said remote sensing and satellite imagery were being used to monitor changes in glacial lakes.

Of the 40 high-hazard lakes, 16 are in Category A (highest risk), two in Category B (high risk) and nine in Category C (moderate to low risk), while 13 remain unclassified, he said.

The workshop focused on strengthening public understanding of GLOF risks and ensuring that verified, science-based information reaches the public.

Yougan Tamang, the press secretary to Chief Minister Prem Singh Tamang, said media sensitisation was crucial to communicating the scientific measures being undertaken to mitigate GLOF risks.

Sikkim’s Department of Science and Technology was designated the nodal department for GLOF hazard assessment and mitigation studies in February 2025, with the work being carried out through an interdisciplinary approach, Tamang said.

The state is also working towards what could be the country’s first dedicated GLOF Risk Mitigation Protocol, aimed at creating a standardised scientific framework for assessing, preparing for and mitigating glacial flood risks.

It could also serve as a model for other Himalayan regions, an official release said.

Field studies cover water-level and discharge monitoring, Electrical Resistivity Tomography, geophysical and differential global positioning system surveys, bathymetry, flood modelling, hazard assessment and feasibility studies for lowering lake levels and constructing flood-retention structures.

Tambe said a uniform strategy could not be applied to all lakes, and separate proposals had therefore been prepared for the Lhonak Complex, Shako Chho Lake and Gurudongmar Complex, according to the release.

The Sikkim government has identified Shako Chho as most vulnerable to GLOF.

The Detailed Project Report (DPR) for Shako Chho has been submitted to the National Disaster Management Authority, and a preliminary project report has been sent to the Jal Shakti Ministry.

The Central Water Commission is preparing a DPR along with the Sikkim government for a flood-retention structure at Dolma Sampa near Muguthang in the Lhonak Complex.

For the Gurudongmar Complex, a moraine plug wall at the lake outlet has been proposed. The Department of Science and Technology and Brahmaputra Board are jointly working on the proposal, taking into account both GLOF risk and the lake’s cultural and religious significance.

The Science and Technology Department’s principal director-cum-secretary DG Shrestha said Sikkim’s devastating 2023 GLOF had underscored the urgency of strengthening preparedness.

At the policy level, Sikkim is pursuing a whole-of-government approach, clearer business rules, measures to bridge technical expertise gaps and a proposal for a commission on glacial hazards.

Various lake-level lowering techniques are being examined, including siphoning, pumping, improving existing channels, artificial channels, moraine cutting with safety walls and tunnelling.

Shrestha outlined a four-stage mitigation process — preliminary assessment, comprehensive assessment, designing mitigation measures and execution — and stressed the need to strengthen institutional, technical and financial capacity.

He also said additional early-warning sensors had been proposed across Sikkim following a virtual meeting with Union Home Secretary Govind Mohan, who gave a positive assurance on the proposal.

On Thursday, Union Home Secretary Govind Mohan reviewed the preparedness of Himalayan states and Union Territories in dealing with GLOF risks and snow avalanches.

The chief secretaries and State Disaster Management Authorities of Uttarakhand, Himachal Pradesh, Sikkim, Arunachal Pradesh and Jammu and Kashmir, along with senior officers of the Ministry of Home Affairs, National Disaster Management Authority, Central Water Commission, India Meteorological Department and the Defence Geoinformatics Research Establishment attended the meeting.