40 high-hazard glacial lakes identified in Sikkim, 16 at highest risk of outburst
The Sikkim government has classified 40 glacial lakes in the state as high-hazard, of which 16 fall in the highest-risk category for a Glacial Lake Outburst Flood (GLOF)
Shako Chho has been identified as the single most vulnerable lake, with a Detailed Project Report on it submitted to the National Disaster Management Authority and a preliminary report sent to the Ministry of Jal Shakti
Assessment combined remote sensing and satellite monitoring with field studies — water-level monitoring, geophysical surveys, bathymetry and flood modelling
The state is developing what is described as the country's first dedicated GLOF Risk Mitigation Protocol, with lake-level lowering and flood-retention structures under consideration for specific lakes, including separate proposals for the Lhonak Complex, Shako Chho and the Gurudongmar Complex
Glacial Lake Outburst Flood (GLOF) — Definition and Mechanism
A GLOF is the sudden release of a large volume of water impounded in a lake formed by glacial meltwater, most commonly dammed by unstable glacial moraine (loose rock and sediment debris) rather than solid bedrock. Failure can be triggered by an ice or rock avalanche falling into the lake, moraine dam erosion, permafrost thaw destabilising the dam, or an earthquake, producing a fast-moving flood and debris flow down the valley.
Sikkim's hazard classification exercise is a direct GLOF-risk assessment — grading lakes by the likelihood and consequence of exactly this dam-failure mechanism, distinguishing "high-hazard" (susceptible to failure) from "high-risk" (failure plus high downstream exposure, such as Shako Chho).
South Lhonak Lake GLOF, October 2023 — The Reference Disaster
On the night of 3-4 October 2023, a landslide (linked to permafrost thaw) fell into South Lhonak Lake in north Sikkim, triggering a moraine dam breach and a GLOF that swept down the Teesta river valley. The flood destroyed the 1,200 MW Teesta III hydropower dam at Chungthang and caused over 55 confirmed deaths, with dozens more reported missing, across Mangan, Gangtok, Pakyong and Namchi districts, and the flood wave was traceable more than 380 km downstream into Bangladesh.
Key Details
- South Lhonak Lake sits at roughly 5,200 m altitude in north Sikkim and had been flagged as high-risk by Sikkim's own Information and Public Relations Department as early as May 2022 — over a year before the disaster
- The disaster is documented in peer-reviewed studies (Science, 2024) as a "multihazard cascade" combining permafrost degradation, an avalanche/landslide trigger, and moraine dam failure
- It remains India's most severe GLOF event on record and is the direct trigger for the current systematic hazard-mapping exercise across Sikkim's glacial lakes
The 2023 South Lhonak disaster is the precedent driving the state's shift from reactive to proactive GLOF risk management — the 40-lake hazard classification and the proposed GLOF Risk Mitigation Protocol are a direct institutional response to that event.
Institutional Framework for GLOF Risk Management in India
Disaster risk reduction for glacial lakes in India involves a layered institutional structure spanning national, state and technical agencies, operating under the Disaster Management Act, 2005.
Key Details
- The National Disaster Management Authority (NDMA), headed by the Prime Minister, is the apex statutory body for disaster management policy under the Disaster Management Act, 2005, and has separately identified a larger national inventory of high-risk glacial lakes across Himalayan states (including Sikkim, Uttarakhand, Himachal Pradesh and Arunachal Pradesh)
- Sikkim designated its Department of Science and Technology as the nodal department for glacial lake monitoring (February 2025)
- The Central Water Commission and the Brahmaputra Board provide hydrological and basin-level technical inputs, while the Ministry of Home Affairs oversees the overall disaster management framework and the Ministry of Jal Shakti is involved given the river-basin and dam-safety dimension
- Mitigation techniques under consideration for high-hazard lakes — controlled lake-level lowering via siphoning/channels and downstream flood-retention structures — are standard global GLOF mitigation approaches, also used in Nepal and Bhutan
The state-level classification exercise feeds directly into this multi-agency structure — the Shako Chho Detailed Project Report routed to NDMA and Jal Shakti illustrates how a single high-hazard lake moves from scientific assessment to a funded mitigation project.
Geographic and Hydrological Context — Teesta Basin
Sikkim's glacial lakes lie within the Teesta river basin, which originates from glaciers near the Sikkim-Tibet border (including the Zemu glacier, source of the Teesta) and is a key tributary of the Brahmaputra system before it enters Bangladesh.
Key Details
- The Teesta basin supports major hydropower infrastructure in Sikkim, including cascade projects along its course — the 2023 GLOF's destruction of the Teesta III project illustrates the basin's exposure to upstream glacial hazards
- Sikkim lies in the Eastern Himalaya, a seismically active zone (Zone IV), which compounds glacial hazard risk through landslide-triggering potential
- The state's terrain ranges from subtropical valleys to high-altitude glaciated zones near Kangchenjunga, India's highest peak (8,586 m), giving it one of the highest densities of glacial lakes among Indian Himalayan states
Because nearly all of Sikkim's high-hazard lakes drain into the Teesta system, a single GLOF event has cascading downstream consequences for hydropower infrastructure, settlements and transboundary river flow into Bangladesh — which is why the state is prioritising basin-wide, lake-by-lake risk classification rather than addressing incidents case by case.
- High-hazard glacial lakes identified in Sikkim: 40 (16 in the highest-risk category)
- Most vulnerable lake identified: Shako Chho
- South Lhonak GLOF: night of 3-4 October 2023; destroyed the 1,200 MW Teesta III dam at Chungthang; over 55 confirmed deaths
- Flood wave traced over 380 km downstream along the Teesta into Bangladesh
- Sikkim's nodal department for glacial lake monitoring: Department of Science and Technology (designated February 2025)
- HKH region glacial lakes mapped by ICIMOD (2018): more than 25,000
- Apex disaster management body: NDMA, under the Disaster Management Act, 2005