South Lhonak Glacier

The South Lhonak Glacier is a significant Himalayan glacier located in the northwestern region of Sikkim, India, near the border with Nepal and Tibet. It is one of the largest glaciers in the Indian state of Sikkim and forms part of the upper catchment area of the Teesta River, which serves as a vital water source for the eastern Himalayas. The glacier has become an important subject of scientific study due to its rapid retreat, the formation of a potentially hazardous glacial lake, and its connection to recent climate-related disasters.

Geographic Location and Physical Features

The South Lhonak Glacier lies at an altitude ranging between 5,200 and 7,000 metres above sea level in the Kanchenjunga range of the eastern Himalayas. It occupies a remote, high-altitude valley in northwestern Sikkim, situated close to the Nepal-Sikkim border. The glacier flows eastward, feeding into the South Lhonak Lake, which subsequently drains into the Zemu River, a tributary of the Teesta.
The glacier is approximately 7–8 kilometres long and is characterised by its debris-covered terminus, moraines, and ice-cored ridges. Like many Himalayan glaciers, South Lhonak is classified as a retreating glacier, having experienced significant shrinkage in both area and volume over the past decades.

Geological and Climatic Background

Sikkim’s glaciers, including South Lhonak, belong to the eastern Himalayan cryospheric system, which is influenced by both the Indian summer monsoon and westerly disturbances. The glaciers here are typically smaller and more sensitive to climatic variations than those in the western Himalayas.
Studies based on satellite imagery and remote sensing have documented that since the 1970s, the South Lhonak Glacier has retreated by more than 2 kilometres, losing nearly 40% of its surface area. Rising atmospheric temperatures, changing precipitation patterns, and the accumulation of supraglacial debris have accelerated melting.

Formation of South Lhonak Lake

As the glacier has retreated, meltwater has accumulated at its terminus, forming the South Lhonak Lake, a large and expanding glacial lake bounded by loose moraine material. The lake’s growth has raised concerns among scientists and disaster management authorities because such lakes are prone to Glacial Lake Outburst Floods (GLOFs)—sudden releases of large volumes of water that can cause catastrophic downstream flooding.
Satellite data from the Indian Space Research Organisation (ISRO) and other research institutions have shown that the lake’s area increased from around 0.17 square kilometres in 1977 to over 1.6 square kilometres by 2023. This dramatic expansion indicates sustained melting and unstable moraine conditions, increasing the risk of outburst events.

The 2023 Teesta Flood Disaster

On 4 October 2023, a devastating Glacial Lake Outburst Flood originated from the South Lhonak Lake following a partial collapse of the lake’s moraine dam. The outburst released millions of cubic metres of water into the Teesta River, causing severe flash floods in downstream areas of Sikkim and North Bengal.
Key impacts of the 2023 event included:

• Destruction of the Teesta III hydroelectric dam at Chungthang.
• Damage to infrastructure, roads, and bridges across northern Sikkim.
• Loss of lives, with over 40 people confirmed dead and many missing.
• Displacement of local communities and disruption of power supply and transport.

Investigations suggest that heavy rainfall combined with a possible ice or rock avalanche triggered the moraine breach. The event highlighted the growing vulnerability of Himalayan communities to climate-induced glacial hazards.

Environmental and Climatic Implications

The retreat of the South Lhonak Glacier exemplifies the broader pattern of glacial retreat in the Himalayas, driven by global warming. Glaciers in the eastern Himalayas are particularly sensitive to temperature rise, and their accelerated melting has multiple implications:

• Water security: Initial increase in river discharge followed by long-term reduction as glaciers shrink.
• Hazard risks: Growth of unstable glacial lakes and increased probability of GLOFs.
• Ecosystem changes: Altered hydrological regimes affecting alpine and downstream ecosystems.
• Socio-economic impacts: Threats to hydropower projects, agriculture, and settlements.

These changes underline the importance of climate adaptation measures in Himalayan regions that depend on glacial meltwater for sustenance and energy.

Monitoring and Scientific Research

Continuous monitoring of the South Lhonak Glacier and lake has been undertaken by Indian research institutions such as the National Remote Sensing Centre (NRSC), ISRO, the Wadia Institute of Himalayan Geology, and the Sikkim State Disaster Management Authority (SSDMA).
Techniques employed include:

• Satellite remote sensing for mapping glacier retreat and lake expansion.
• Unmanned Aerial Vehicle (UAV) surveys for real-time monitoring.
• Early warning systems for detecting lake level fluctuations and potential moraine failures.
• Hydrodynamic modelling to predict flood behaviour and impact zones.

Despite these efforts, the terrain’s remoteness and extreme weather conditions make ground-based studies difficult, leaving some uncertainty regarding the glacier’s precise dynamics and subglacial processes.

Disaster Risk Management and Policy Response

Following the 2023 flood disaster, authorities have prioritised the installation of automated early warning systems and improved emergency preparedness in Sikkim’s high-altitude regions. Risk reduction strategies include:

• Continuous monitoring of glacial lakes using real-time satellite data.
• Strengthening community-based disaster response mechanisms.
• Regulating infrastructure development in vulnerable mountain valleys.
• Conducting regular moraine stability assessments and exploring controlled drainage of dangerous lakes.

India’s national initiatives such as the National Mission for Sustaining the Himalayan Ecosystem (NMSHE) and the National Disaster Management Authority’s (NDMA) GLOF framework provide institutional support for research, early warning, and adaptation planning in such high-risk zones.

Scientific and Global Importance

The South Lhonak Glacier serves as a crucial case study in understanding the interlinkages between climate change, glacial retreat, and natural hazards in the Himalayas. It highlights the fragility of mountain ecosystems and the urgent need for integrated approaches combining science, policy, and community engagement.
Globally, its evolution mirrors similar phenomena observed in other mountain regions such as the Andes, Alps, and Karakoram, where climate-induced glacier changes have created new hazards and altered hydrological systems.