BharatNotes The Himalayas are the world's youngest and most geologically active mountain system, forming India's northern frontier across approximately 2,500 km. They are home to the planet's third-largest freshwater reserve in glacial ice. Yet this same geological youth — marked by ongoing tectonic uplift, unstable slopes, seismic activity, and now accelerating glacial melt — makes the Himalayan region one of the world's most disaster-prone landscapes. Chamoli (2021), South Lhonak (2023), and Kedarnath (2013) have demonstrated how mountain disasters can cause catastrophic, cascading damage reaching far downstream.
Himalayan Vulnerability: Why the Himalayas Are So Hazard-Prone
| Factor | Significance |
|---|---|
| Young fold mountains | Still geologically active; steep, unstable slopes with ongoing uplift |
| High seismicity | Seismic Zones IV–V cover most of the Himalayan belt; frequent moderate-to-strong earthquakes |
| Glacial systems | Over 9,000 glaciers; glacial melt accelerating due to climate change |
| Intense precipitation | Southwest monsoon delivers heavy rainfall; cloudburst events frequent |
| Permafrost degradation | Warming temperatures destabilise permafrost, triggering rock falls |
| Rapid infrastructure development | Roads, hydropower dams, tunnels weaken slope stability |
| High population density | Major rivers support dense populations far downstream |
The "Third Pole" concept: The Hindu Kush-Himalayan (HKH) region contains the largest concentration of glacial ice outside the polar regions. Accelerating glacier retreat due to climate change is intensifying all categories of mountain hazard.
Glacial Lake Outburst Floods (GLOFs): Mechanism
A GLOF is a sudden, catastrophic release of water from a glacial lake — one of the most devastating mountain hazards.
Types of glacial lakes:
- Moraine-dammed lakes: Water impounded behind moraines (debris piles deposited by glaciers); most common and most vulnerable to outburst
- Ice-dammed lakes: Water impounded by glacial ice; can drain suddenly when ice melts or shifts
- Supraglacial lakes: Pools on the glacier surface; can drain through crevasses
GLOF trigger mechanisms:
| Trigger | Description |
|---|---|
| Moraine collapse | Overtopping or sudden structural failure of moraine dam |
| Avalanche/ice calving | Large displacement wave generated by ice or rock mass entering lake |
| Earthquake | Seismic vibration destabilises moraine dam |
| Glacier melt surge | Rapid meltwater input causes overtopping |
| Subsurface drainage | Tunnelling through ice or permafrost |
Consequences: GLOF floods can travel hundreds of kilometres downstream, carrying boulders and debris, destroying infrastructure, farmland, bridges, and villages — often with little warning time.
Glacial Lake Inventory: India's Himalayan Lakes
The National Remote Sensing Centre (NRSC), under ISRO, has conducted systematic inventories of glacial lakes across Indian river basins using satellite data:
| River Basin | Glacial Lakes Mapped |
|---|---|
| Indus | ~5,335 lakes (≥0.25 ha) |
| Ganga | ~4,707 lakes (≥0.25 ha) |
| Brahmaputra | ~18,001 lakes (≥0.25 ha) |
NRSC publishes Glacial Lake Atlases for all three basins, updated periodically using remote sensing data. These atlases are used by state and national disaster management authorities for GLOF risk assessment.
NDMA's role: The National Disaster Management Authority has identified high-risk glacial lakes in states including Arunachal Pradesh, Himachal Pradesh, Uttarakhand, and Sikkim, and deploys expert teams to assess GLOF risk in critical lakes.
Major Himalayan Disaster Events
Kedarnath Disaster (June 2013)
Date: June 16–17, 2013 Location: Kedarnath, Rudraprayag district, Uttarakhand
| Feature | Detail |
|---|---|
| Trigger | Exceptional cloudburst and heavy monsoon rainfall |
| Mechanism | Flash floods, massive landslides, collapse of natural dam on Chorabari Lake |
| Death toll | Approximately 5,700 confirmed deaths; thousands more missing |
| Infrastructure | Roads, bridges destroyed; helicopter rescue limited by weather |
| Temple | Kedarnath shrine survived — large boulder diverted flood flow; called a miracle by many |
Significance: India's worst mountain disaster in modern history. Exposed critical gaps in early warning, evacuation infrastructure, and land-use planning in ecologically fragile zones. Led to NDMA strengthening disaster guidelines for Himalayan states.
Chamoli Disaster (February 7, 2021)
Location: Chamoli district, Uttarakhand — Rishiganga and Dhauliganga river valleys
| Feature | Detail |
|---|---|
| Date | 7 February 2021 |
| Trigger | A large rock and ice avalanche from Ronti Peak — a wedge of rock carrying a hanging glacier detached |
| Classification | NOT a classic GLOF — characterised as a rock-ice avalanche-triggered flood (LLOF) |
| Rivers affected | Rishiganga → Dhauliganga → Alaknanda (major Ganga headstream) |
| Deaths/missing | Over 200 killed or missing; 83 bodies and 36 body parts recovered (as of May 2021) |
| Infrastructure destroyed | Rishiganga (13 MW) power project; Tapovan-Vishnugad Hydropower Plant (140+ workers missing) |
Scientific finding (published in Science journal): A massive rock and ice avalanche — material dislodged from Ronti Peak — was confirmed as the cause. The event demonstrated how slope failures can generate flood waves comparable to GLOFs without a glacial lake being involved.
Key lesson: "Not all Himalayan sudden floods are GLOFs." The Chamoli event underscored the need for comprehensive monitoring covering both glacial lakes AND unstable rock-ice slopes.
South Lhonak Lake GLOF, Sikkim (October 4, 2023)
Location: South Lhonak Lake, North Sikkim → Teesta River valley
| Feature | Detail |
|---|---|
| Date | Night of 3–4 October 2023 |
| Trigger | ~14.7 million m³ of frozen lateral moraine collapsed into South Lhonak Lake, generating a 20-metre displacement wave |
| GLOF volume | ~50 million m³ of water suddenly released; peak discharge ~48,500 m³/second |
| Deaths | 55 confirmed deaths; 74 persons missing (including 23 Army personnel) |
| Teesta-3 Dam | 1,200 MW Teesta-III Dam at Chungthang destroyed within minutes as the flood arrived at midnight |
| Infrastructure loss | 25,900+ buildings damaged/destroyed; 31 major bridges lost; 270 km² agricultural land affected |
| Climate link | Climate change confirmed as a key contributing factor (ICIMOD study) — South Lhonak Lake had grown significantly due to glacial retreat |
Significance: The South Lhonak GLOF is now one of the most well-documented GLOF events globally. It demonstrated the cascade disaster potential: glacial lake → GLOF → dam destruction → downstream flooding — a multi-hazard chain. The destruction of a 1,200 MW dam illustrated the economic scale of GLOF risk in the era of Himalayan hydropower development.
Cloud Bursts and Flash Floods
A cloudburst is defined as extremely heavy rainfall exceeding 100 mm in one hour over a small area (typically <100 km²). They are distinct from normal heavy rainfall and are associated with convective storms.
| Feature | Detail |
|---|---|
| Threshold | >100 mm/hour precipitation |
| Common zones | Western Himalayas, Uttarakhand, Himachal Pradesh, J&K |
| Season | Mainly June–September (southwest monsoon) |
| Consequences | Flash floods, landslides, debris flows, bridge collapses |
| Warning time | Extremely limited — minutes to 1–2 hours at best |
Cloudbursts are notoriously difficult to forecast because they result from highly localised convective activity. IMD issues cloudburst alerts, but their spatial resolution remains a challenge.
Landslides
India is the second most landslide-prone country in the world after China. The Himalayan and northeastern regions account for the vast majority of landslide events.
| Factor | Role in Landslides |
|---|---|
| Rainfall | Trigger in ~80% of cases — slope saturation, pore pressure increase |
| Seismicity | Co-seismic landslides in Zone IV–V areas |
| Slope steepness | Young mountains have naturally steep, unstable slopes |
| Deforestation | Removal of root anchoring destabilises slopes |
| Construction activity | Road cutting, blasting, excavation trigger slides |
| Geological structure | Shale, clay-rich formations, thrust zones particularly prone |
National Landslide Risk Management Strategy (2019): NDMA's framework for reducing landslide risk through hazard mapping, monitoring, early warning, and community preparedness.
Geological Survey of India (GSI) maintains landslide hazard zonation maps. Bhuvan (ISRO's geoportal) hosts national landslide inventory data.
Avalanches
Avalanches are rapid flows of snow, ice, and debris down mountain slopes. They are a major hazard in high-altitude areas including:
- Siachen Glacier (world's highest battlefield)
- Rohtang Pass, Manali-Leh highway
- Jammu & Kashmir, Himachal Pradesh passes
Snow and Avalanche Study Establishment (SASE): Originally established as a DRDO laboratory, SASE was in 2020 merged with the Defence Terrain Research Laboratory (DTRL) and renamed Defence Geoinformatics Research Establishment (DGRE), based in Chandigarh.
DGRE functions:
- Avalanche forecasting and warning bulletins for Armed Forces
- Weather and snowpack monitoring at field stations
- Research on avalanche dynamics, mitigation
- Training of army and civilian personnel
NDMA Avalanche Guidelines provide guidance for community preparedness, shelter design, and route management in avalanche-prone areas.
Seismic Risk in the Himalayas
The Himalayan seismic belt is among the world's most active earthquake zones:
| Zone | Coverage |
|---|---|
| Zone V (Very High Risk) | Entire northeastern India, Kutch, parts of Uttarakhand, Himachal Pradesh, J&K |
| Zone IV (High Risk) | Much of the remaining Himalayan foothills, parts of the Indo-Gangetic plain |
Why the Himalayas are so seismically active: The Indian tectonic plate continues to push northward into the Eurasian plate at ~5 cm/year, building stress that releases as earthquakes along the Main Himalayan Thrust, Main Central Thrust, and related fault systems.
Historical major earthquakes: Bhuj 2001 (Gujarat — Zone V), Uttarkashi 1991, Chamoli 1999, Nepal-Bihar 2015 (affected India's eastern Himalayan region).
Climate Change–Mountain Disaster Nexus
Climate change is systematically increasing Himalayan disaster risk through multiple pathways:
| Climate Driver | Mountain Disaster Impact |
|---|---|
| Rising temperatures | Accelerated glacial retreat → larger, deeper glacial lakes → higher GLOF risk |
| Permafrost thaw | Destabilised slopes → more frequent rock falls and landslides |
| Intense precipitation events | More cloudbursts → more flash floods and landslides |
| Reduced snowpack | Seasonal river flow changes; less cushion against drought |
| Glacier surges | Unstable glaciers generating more ice avalanches |
ICIMOD's research on the South Lhonak GLOF (2023) confirmed that climate change played a key role — the lake had expanded significantly over decades as the glacier retreated, increasing the stored water volume and outburst potential.
NDMA's GLOF Guidelines (2020)
The National Disaster Management Authority published comprehensive GLOF Risk Management Guidelines covering:
- Inventory and monitoring of glacial lakes using satellite remote sensing
- Community-based early warning systems in high-risk valleys
- Real-time sensor networks on critical lakes
- Evacuation planning and community preparedness
- Hydropower dam safety in GLOF-prone valleys
- Integration with National Disaster Response Force (NDRF) for rapid response
Role of ISRO and NRSC in Himalayan Monitoring
ISRO/NRSC capabilities for Himalayan monitoring:
| Application | Technology |
|---|---|
| Glacial lake mapping | Optical satellite imagery (Resourcesat, Cartosat) |
| Glacial retreat monitoring | Multi-temporal satellite data comparison |
| Landslide mapping | High-resolution SAR and optical imagery |
| Flood inundation mapping | RISAT SAR (works through cloud cover) |
| Post-disaster damage assessment | Pre- and post-event image comparison |
| Subsidence monitoring | InSAR (Interferometric SAR) |
Bhuvan: ISRO's geoportal provides public access to glacial lake atlases, landslide inventory, and disaster damage assessments.
Hydropower Risk in the Himalayas
India has ambitious hydropower development plans in the Himalayan states — but dams in GLOF-prone areas face existential risk:
| Concern | Detail |
|---|---|
| Cascade failure | Destruction of one dam can trigger failure of downstream dams (as in South Lhonak 2023) |
| Siting risk | Projects often in narrow gorges — maximum GLOF impact zones |
| Design inadequacy | Older projects designed without GLOF risk in their hydrological assessment |
| Regulatory gaps | No mandatory GLOF risk assessment for all Himalayan hydropower projects until recently |
Post-Chamoli and South Lhonak reviews have called for mandatory GLOF risk assessment for all hydropower projects in glaciated river basins, controlled-speed construction, and real-time early warning systems linked to dam operations.
Recent Developments (2024–2026)
South Lhonak GLOF (Sikkim, October 2023) — Post-Event Learnings (2024)
The 4 October 2023 glacial lake outburst flood (GLOF) from South Lhonak Lake in Sikkim remains the most significant Himalayan disaster in India in recent years. The GLOF killed 92+ people, destroyed the Teesta-III dam at Chungthang (a 1,200 MW project), washed away 15 bridges, and caused economic damage estimated at thousands of crores. The lake area had grown from 0.2 km² (1976) to 1.67 km² (2023) due to glacier retreat accelerated by climate change.
A 2024 study in Science journal confirmed that the GLOF was triggered by the collapse of 14.7 million cubic metres of frozen lateral moraine into South Lhonak Lake — caused by permafrost thaw. The study confirmed climate change's significant role. Post-disaster investigations found: no real-time GLOF early warning system was operational for South Lhonak; the Teesta-III dam's emergency protocol was not activated in time; downstream communities had not been informed of the GLOF risk. NDMA subsequently mandated real-time monitoring of 188 high-risk glacial lakes using ISRO satellite data.
UPSC angle: Prelims — South Lhonak GLOF: 4 October 2023; Teesta-III dam destroyed; 92 deaths; ISRO monitoring 188 high-risk lakes. Mains (GS3) — climate change and Himalayan risk; hydropower vulnerability; early warning failure; GLOF risk assessment in dam licensing.
ISRO Glacial Lake Monitoring — 188 High-Risk Lakes (2024)
Following the Sikkim GLOF, NDMA directed ISRO's National Remote Sensing Centre (NRSC) to develop a real-time monitoring system for glacial lakes across India. The NRSC-ISRO Himalayan Cryosphere Monitoring programme identified 188 high-risk glacial lakes in the Indian Himalayan Region — distributed across Uttarakhand, Himachal Pradesh, Sikkim, and Arunachal Pradesh. These are monitored using SAR (Synthetic Aperture Radar) satellite imagery for changes in lake area, outburst risk indicators, and drainage pathway analysis.
The monitoring data feeds into NDMA's Early Warning Dashboard, which shares alerts with state disaster management authorities in real-time. In 2024, the system issued two early alerts for lake expansion events in Uttarakhand and Sikkim — enabling pre-emptive action. ISRO has published a Glacial Lake Atlas (2024 edition) documenting 10,000+ glacial lakes in the Indian Himalayas, of which 188 are classified as high-risk.
UPSC angle: Prelims — ISRO NRSC: 188 high-risk glacial lakes; SAR monitoring; Glacial Lake Atlas 2024; NDMA Early Warning Dashboard. Mains (GS3) — technology in GLOF risk management; ISRO's dual-use (civilian DM) capacity; science-governance linkage.
Wayanad Landslide 2024 — Mountain Disaster Risk in the Western Ghats
The Wayanad landslide (30 July 2024) — which killed 400+ in the Mundakkai-Chooralmala area — highlighted that Himalayan-focused landslide policy has neglected the Western Ghats, which hosts equally high-risk geological formations. The Western Ghats are classified as a UNESCO World Heritage Site (Biodiversity Hotspot) and are subject to HLWG (Kasturirangan Report 2013) ecological sensitivity designations. High-altitude tea and coffee plantations — which replaced native forest — have dramatically reduced slope stability.
The National Disaster Management Authority issued guidelines in 2024 specifically for Western Ghats landslide risk, calling for: mandatory geological surveys before plantation extension; strict enforcement of building prohibitions in high-risk zones; and the creation of a Western Ghats Landslide Early Warning System separate from the Himalayan GLOF monitoring system. The post-Wayanad review committee (chaired by a State Relief Commissioner) called for relocating 5,000+ families from the highest-risk zones.
UPSC angle: Prelims — Wayanad landslide: 30 July 2024; 400+ deaths; Western Ghats; Kasturirangan Report 2013. Mains (GS3) — landslide risk in Western Ghats vs Himalayas; development-environment trade-off; relocation vs risk acceptance in mountain communities.
Key Terms
| Term | Meaning |
|---|---|
| GLOF | Glacial Lake Outburst Flood — sudden release from a glacial lake |
| Moraine | Debris deposited by a glacier; moraine dams impound glacial lakes |
| Cloudburst | Rainfall >100 mm/hour over a localised area |
| DGRE | Defence Geoinformatics Research Establishment (formerly SASE) — DRDO's snow/avalanche unit |
| NRSC | National Remote Sensing Centre — ISRO's disaster monitoring arm, Hyderabad |
| Third Pole | Hindu Kush-Himalayan region — world's third largest ice reserve |
| InSAR | Interferometric SAR — satellite technique for detecting ground deformation |
Previous Year Questions (PYQs)
Prelims
Consider the following: (UPSC Prelims 2022 type question) Which of the following correctly defines a GLOF? A sudden, large discharge of water from a glacial lake due to failure of its natural dam — (Correct option)
The Chamoli disaster of February 2021 was primarily caused by:
- (A) A classic GLOF from a moraine-dammed lake
- (B) A rock-ice avalanche from Ronti Peak
- (C) An earthquake in Zone V
- (D) A cloudburst event
- Answer: (B)
Snow and Avalanche Study Establishment (SASE) has been renamed and is now known as: (A) NRSC (B) DGRE (C) NDMA (D) GSI — Answer: (B) DGRE
Kedarnath disaster (June 2013) was primarily triggered by:
- (A) A GLOF (B) An earthquake (C) Cloudburst-triggered flash floods (D) Avalanche
- Answer: (C)
Mains
The Chamoli (2021) and South Lhonak (2023) disasters highlight the growing threat of Himalayan mountain hazards. Examine the causes, consequences, and management strategies for Glacial Lake Outburst Floods (GLOFs). (GS3, 15 marks)
"Hydropower development in the Himalayas is a double-edged sword." Discuss the disaster risks associated with hydropower projects in glaciated valleys and suggest safeguards. (GS3, 15 marks)
Critically examine the role of ISRO and NRSC in disaster risk management in the Himalayan region. How can technology be better leveraged to improve early warning for GLOFs and landslides? (GS3, 10 marks)
The Sendai Framework for Disaster Risk Reduction emphasises understanding disaster risk. With reference to the Himalayan region, examine how climate change is altering the risk landscape and what policy responses are needed. (GS3, 15 marks)
Exam Strategy
For Prelims:
- Know GLOF mechanism: moraine-dammed lake → trigger → sudden drainage
- Chamoli 2021: rock-ice avalanche (NOT a classic GLOF), Ronti Peak, Rishiganga-Dhauliganga, 200+ dead
- South Lhonak 2023: Sikkim GLOF, Teesta-3 destroyed, 55 dead, October 4
- Kedarnath 2013: cloudburst + flash floods, ~5,700 deaths
- SASE renamed DGRE; headquartered in Chandigarh (under DRDO)
- Cloudbursts = >100 mm/hour
- India = 2nd most landslide-prone after China
For Mains:
- Emphasise the cascade disaster concept (GLOF → dam failure → downstream flood)
- Connect climate change to increased GLOF frequency and intensity
- Hydropower risk is a contemporary issue — use South Lhonak 2023 as case study
- Link to Sendai Framework: understanding risk, investing in resilience
- Use three actors: ISRO/NRSC (monitoring), NDMA (guidelines), DGRE (avalanches)
- Mention Bhuvan geoportal for disaster data democratisation
