South Lhonak Lake is a high-altitude glacial lake in North Sikkim, located in the upper reaches of the Teesta River basin. It became nationally important after the October 2023 Glacial Lake Outburst Flood (GLOF), one of the most destructive Himalayan climate-linked disasters in recent years.
The lake lies in a fragile glaciated landscape, where rising temperatures, glacier retreat, unstable moraines and steep river valleys combine to create serious downstream flood risk.
Location
South Lhonak Lake is located in Mangan district/North Sikkim, close to the India-China border region.
It is situated at an altitude of about 17,000 feet in the north-western part of Sikkim. The lake is fed by meltwater from nearby glaciers and drains into the Lhonak Chu, which joins the Teesta River system downstream. A Sikkim district administration note described South Lhonak as a glacial lake at about 17,000 feet that ruptured during the 2023 flood event.
Why It Was in News
South Lhonak Lake was the source of a major GLOF event on the night of 3–4 October 2023.
The sudden outburst released a large volume of water downstream into the Teesta basin. PIB noted that a glacial lake outburst at South Lhonak Lake caused a devastating flood in Sikkim, raising the Teesta River water level by about 50–60 feet.
The flood caused severe damage to settlements, roads, bridges, hydropower infrastructure and military installations downstream.
What Caused the GLOF
A Glacial Lake Outburst Flood occurs when water stored in a glacial lake is suddenly released after the lake’s natural dam fails.
In the case of South Lhonak, the risk had been building for years because the lake had expanded due to glacier retreat. The lake was held back by loose moraine material, which is much weaker than a concrete or rock dam.
Possible triggering factors included:
- rapid glacier melting
- moraine-dam instability
- heavy rainfall
- ice or rock avalanche into the lake
- slope failure around the lake
- sudden displacement of lake water
- high seismic and tectonic sensitivity of the Himalayan region
A 2026 scientific study described South Lhonak as a proglacial lake and identified it as the source of the catastrophic GLOF in the Teesta basin on 4 October 2023.
Impact of the 2023 Disaster
The South Lhonak GLOF turned into a destructive flash flood as water rushed down the Teesta valley.
Major impacts included:
- sharp rise in Teesta River water level
- destruction of bridges and roads
- damage to hydropower infrastructure
- loss of lives and missing persons
- displacement of people in downstream areas
- damage to army establishments and public infrastructure
- heavy sediment deposition in the Teesta channel
A 2026 study reported 24 fatalities, more than 70 missing persons, destruction of 13 bridges, damage to a major hydropower plant in the Chungthang region, and impact on over 60,000 people across four districts of Sikkim.
PIB’s immediate 2023 update reported that, as per the State Government estimate at that time, 142 persons were missing and 26 bodies had been recovered.
Link with Teesta River
The disaster showed how a remote glacial lake can affect an entire river basin.
South Lhonak Lake lies high in the Himalayas, but its outburst travelled downstream through the Teesta River, affecting areas far below the lake. The Teesta is a lifeline river for Sikkim and parts of North Bengal, supporting hydropower, settlements, agriculture, roads and bridges.
The flood also carried huge amounts of debris and sediment. Later reporting from West Bengal noted that the 2023 GLOF had increased sediment accumulation in parts of the Teesta, raising the riverbed and increasing future flood concerns.
Why South Lhonak Was Already Considered Risky
South Lhonak Lake had been identified as a potentially dangerous glacial lake even before the 2023 disaster.
Its risk factors included:
- rapid expansion of lake area
- location below retreating glaciers
- weak moraine dam
- steep downstream valley
- high volume of stored water
- possibility of avalanche-triggered waves
- presence of vulnerable infrastructure downstream
The 2023 disaster exposed the gap between scientific risk identification and actual mitigation on the ground. Knowing that a lake is risky is not enough unless monitoring, early warning, evacuation planning and structural mitigation are implemented effectively.
Government and Disaster Management Response
After the 2023 disaster, India increased focus on GLOF monitoring and early warning systems.
A 2025 government reply noted that the South Lhonak GLOF prompted immediate technical response, post-event assessments and urgent repair of Early Warning System infrastructure at the site. It also mentioned state-led interventions such as installation of Automatic Weather Stations, EWS systems, bathymetric surveys and community evacuation protocols.
These measures are important because Himalayan glacial lakes are difficult to access physically, especially during bad weather. Remote sensing, automated sensors and community warning systems become essential.
Significance
South Lhonak Lake is important because it has become a case study in Himalayan climate risk.
It shows that climate change is not only a slow environmental process. In mountain regions, it can create sudden, high-impact disasters through glacier retreat, unstable lakes and extreme rainfall.
Its significance lies in:
- understanding GLOF hazards in the Himalayas
- improving early warning systems
- planning safer hydropower projects
- protecting downstream settlements
- integrating climate risk into infrastructure planning
- strengthening disaster preparedness in mountain states
- using satellite and field monitoring for glacial lakes
Concerns for Himalayan States
The South Lhonak disaster raised wider concerns for Himalayan states such as Sikkim, Uttarakhand, Himachal Pradesh, Ladakh and Arunachal Pradesh.
Many glacial lakes are expanding as glaciers retreat. Some are located above river valleys where hydropower projects, roads, bridges, military infrastructure and settlements are present.
Key concerns include:
- expansion of glacial lakes
- weak moraine dams
- poor real-time monitoring
- inadequate early warning systems
- hydropower projects in hazard-prone valleys
- road construction in fragile slopes
- limited evacuation time during sudden floods
- lack of updated hazard maps for all high-risk lakes
The lesson from South Lhonak is that Himalayan development must be based on basin-level risk assessment, not only project-level engineering.
