Glacial Lake Outburst Floods (GLOFs)
The Teesta River swells. Brown water rushes through Chungthang town. October 4, 2023. The South Lhonak glacial lake bursts. 41 dead, 100 missing. Sikkim's worst flood in 50 years. No warning.
[TOPIC CLASSIFICATION]
- Topic type: Natural Disaster + Physical Geography (cascading hazard)
- PYQ frequency: Moderate (3-4 questions in last 10 years, mostly Prelims)
- Exam stage: Prelims (concept + examples) and Mains GS-1 (geographical phenomena) + GS-3 (disaster management)
- Primary GS paper: GS-1 (Physical Geography) + GS-3 (Disaster Management linkage)
[EXAMINER REASONING]
- Primary trap. Candidates confuse GLOF with regular flash floods. GLOF is specifically a glacial moraine-dammed lake burst caused by glacial retreat, not monsoon flooding.
- Most confused point. The difference between proglacial lakes (formed at glacier snout) and supraglacial lakes (on glacier surface). UPSC asks about moraine-dammed lakes specifically for GLOF risk.
- Key anchor. The Sikkim 2023 GLOF was triggered by a landslide into South Lhonak Lake, not by thermal erosion of the moraine. A cascading hazard: unconsolidated moraine collapsed by seismic/tectonic activity.
- Current affairs hook. India's glacial lake inventory (SAC-ISRO 2023 update) identified 7,000+ glacial lakes, 300+ at high risk. Uttarakhand 2021 (Chamoli disaster) was different — it was a rock avalanche, not a GLOF.
- Mains hinge. Disaster preparedness varies across Himalayan states. Sikkim had a risk list but no monitoring or early warning. Compare with Swiss Alps automated monitoring systems. The question: institutional capability vs technical solution.
Core Concept
Glacial Lake Outburst Floods happen when a glacial lake's natural dam fails. The dam is usually an ice-cored or unconsolidated moraine (rock debris left by the glacier). When the dam fails, the lake drains rapidly, releasing a massive flood wave downstream. Peak discharge can exceed 10,000 cubic meters per second — higher than the average flow of the Brahmaputra at full monsoon.
The trigger can be internal (gradual thermal erosion of the ice core within the moraine, hydrostatic pressure buildup) or external (landslide into the lake creating a displacement wave, earthquake shaking, heavy rainfall, or avalanche from surrounding slopes). Climate change amplifies every mechanism: more ice melts, more lakes form, more water fills existing lakes, and extreme rainfall events become more frequent.
The Himalaya holds the largest concentration of glacial lakes outside the poles. The ISRO's 2023 satellite inventory mapped 7,213 glacial lakes in the Indian Himalayan region. Of these, 304 are at high risk. Sikkim has the highest density, followed by Himachal Pradesh and Uttarakhand. The size of glacial lakes in India increased 33% between 2017 and 2023.
GLOF risk is not just about the lake. Downstream vulnerability depends on population density, infrastructure, and warning systems. The Sikkim 2023 event destroyed the Chungthang dam (1,200 MW Teesta-III project), washed away 14 bridges, and damaged the NH-10 highway — Sikkim's lifeline. The Teesta riverbed rose by 3 meters in places due to sediment deposition.