Glacial Lakes and the GLOF Threat in the Himalayas

GS Paper 1/3 │ Glacial Lake Outburst Floods │ Disaster Management │ Himalayan Geography

Why in the News? According to satellite data (from ICEYE, PlanetScope, and LISS-IV) analysed by geospatial intelligence firm Suhora Technologies, 4 out of 5 lakes in the Mago Chu basin of Tawang district, Arunachal Pradesh, have expanded in area. Sanhapo Lake recorded the maximum growth — expanding from 78.07 hectares in 2019 to 88.81 hectares by June 2026, an increase of roughly 10 hectares. The National Disaster Management Authority (NDMA) has placed it under the ‘high risk’ category.

What Is a Glacial Lake Outburst Flood (GLOF)?

A GLOF occurs through a sequential process:

Step 1 — Glacial Melt: Rising global temperatures cause high-altitude glaciers to melt rapidly.
Step 2 — Natural Accumulation: Meltwater accumulates behind rocks, soil, and debris (moraine) left behind by the retreating glacier.
Step 3 — Lake Formation: As accumulation increases, a temporary glacial lake forms and continues to grow in size.
Step 4 — Pressure Build-Up: As the lake’s water volume peaks, pressure on the weak, debris-formed natural dam rises sharply.
Step 5 — Trigger Event: A sudden landslide or avalanche sends large amounts of ice or rock crashing into the lake.
Step 6 — Outburst: This sudden shock breaches the natural dam, and the accumulated water surges out at extreme speed.
Step 7 — Consequence: Catastrophic, fast-moving floods (GLOFs) devastate downstream areas and infrastructure.

In short: when a lake’s water volume becomes too great, or an avalanche/landslide strikes it from above, the weak natural dam suddenly fails — triggering severe, sudden flooding downstream.

Major Global Examples of GLOF Events

Region / Country	Event (Year)	Affected Area	Cause and Impact
Sikkim, India	South Lhonak Lake (October 2023)	Teesta River	Natural moraine dam burst due to a cloudburst/avalanche. The Chungthang hydroelectric dam was completely destroyed, with heavy loss of life and property.
Uttarakhand, India	Chorabari Tal / Kedarnath (June 2013)	Mandakini River	Dam of the Chorabari glacial lake burst due to excessive rainfall and glacier melting, causing unprecedented devastation in the Kedarnath valley and thousands of deaths.
Uttarakhand, India	Chamoli Disaster (February 2021)	Rishiganga and Dhauliganga	Flash floods triggered by a large portion of the Nanda Devi glacier breaking off, destroying major hydropower projects including Tapovan.
Nepal	Dig Tsho (August 1985)	Langmoche Glacier Region	Moraine dam burst due to a massive avalanche — now a global ‘classic case study’ for understanding GLOF hazards in the Himalayan region.
Peru, South America	Palcacocha Lake (December 1941)	Huaraz City	The deadliest GLOF event in recorded history; a huge ice block fell into the lake, bursting the dam. Huaraz city was buried under debris, with approximately 5,000 deaths.
France, Europe	Lake Tête Rousse (1892)	Tête Rousse Glacier	A hidden ‘sub-glacial’ water pocket inside the glacier suddenly burst, devastating the Saint-Gervais resort town and causing 175+ deaths.

Risk vs. Imminent Disaster: Reading Expansion Correctly

Expansion Alone Is Not the Only Threat: A growing lake does not automatically mean it will burst imminently.
Other Critical Factors: The actual threat level also depends on the strength of the surrounding moraine, the probability of earthquakes in the region, and the risk of surrounding landslides.
Significance of Instability: That said, an expanding lake is genuinely considered ‘unstable’ and warrants close, continuous monitoring.

Key Challenges

Lack of Ground-Level Access: These lakes sit in extremely remote, inaccessible Himalayan terrain, making physical access for scientists highly difficult — monitoring relies almost entirely on satellite data.
Translating Science into Policy: India has strong satellite-based capabilities to identify and map dangerous lakes, but the real challenge lies in converting this scientific data into practical disaster management and early-warning systems on the ground.
Lack of an Integrated Statutory Framework: Rules governing the safety of mountain populations and infrastructure remain fragmented across ministries — Urban Planning, National Highways, and the NDMA — which hampers swift, coordinated decision-making.

UPSC Note UPSC Mains linkage: This topic bridges GS 1 (Himalayan geography, geomorphological processes) with GS 3 (disaster management, climate change impacts, infrastructure planning). The Sikkim (2023) and Chamoli (2021) case studies are valuable, exam-ready examples for any disaster management or climate change answer.

Practice Question (Mains) Glacial Lake Outburst Floods (GLOFs) represent one of the most under-prepared-for disaster risks in the Himalayan region. Discuss the scientific process behind GLOFs and evaluate India’s institutional preparedness to translate satellite-based monitoring into effective early-warning systems. (250 Words, 15 Marks)

Practice Question (Prelims – MCQ) With reference to Glacial Lake Outburst Floods (GLOFs), consider the following statements: 1. A GLOF occurs when a natural dam formed by glacial moraine suddenly breaches, releasing accumulated lake water downstream. 2. The expansion of a glacial lake’s surface area is, by itself, sufficient to confirm that a GLOF event is imminent. 3. The 2021 Chamoli disaster in Uttarakhand was triggered by a portion of the Nanda Devi glacier breaking off. Which of the statements given above is/are correct? (A) 1 and 2 only (B) 1 and 3 only (C) 2 and 3 only (D) 1, 2 and 3 Answer: (B) 1 and 3 only — Statement 2 is incorrect: lake expansion alone indicates instability and the need for monitoring, but does not by itself confirm that a GLOF is imminent; this also depends on moraine strength, seismic risk, and landslide probability.