Growing Glacial Lakes in Arunachal: The Rising GLOF Threat

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Relevance: GS-I Geography (Glaciers) · GS-III Disaster Management & Environment

Source: Suhora glacial-lake study, June 2026

1 · What happened

A new satellite study (by the firm Suhora Technologies) finds that four of five high-risk glacial lakes in the Mago Chu basin of Tawang district, Arunachal Pradesh, have been growing over the past decade.

The standout is Sanhapo Lake, which swelled from about 56 hectares in 2016 to nearly 89 hectares by June 2026. Growing lakes mean more trapped meltwater — and a rising risk of a Glacial Lake Outburst Flood (GLOF) in the eastern Himalayas.

2 · The basics: glacial lakes & GLOFs

As a glacier melts and retreats, the water pools in the hollow it leaves behind, forming a glacial lake. This lake is often held back by a moraine — a natural wall of loose rock, soil and debris dumped by the glacier. Because it is loose, not solid, this dam is fragile.

A GLOF happens when that natural dam suddenly fails — sending a wall of water and debris crashing downstream. It can be set off by an avalanche or ice falling into the lake, a landslide breaching the dam, the sheer build-up of meltwater overloading it, or slow water seepage eating the dam from within.

3 · When is a growing lake actually dangerous?

A bigger lake is a warning sign — but size alone is not the whole story. The real danger is a combination:

The lake How fast it’s growing More meltwater means more pressure on a weak moraine dam — which is why lakes like Sanhapo are watched closely.	The slope above Unstable terrain A lake sitting below steep, landslide- or avalanche-prone slopes is the real trigger risk — one collapse can breach the dam.
Downstream exposure Who’s in the path Villages, highways and dams below turn a flood into a disaster. In Sikkim (Oct 2023), a South Lhonak Lake breach washed away the Chungthang (Teesta-III) dam and killed dozens.	The answer Warn & lower Water-level sensors and Early Warning Systems buy time downstream; where possible, siphoning drains a lake to ease the pressure.

4 · India’s safety net

Mapping the danger: the NDMA (National Disaster Management Authority) classifies high-risk lakes — 27 in Arunachal alone, and about 189 across the Himalayas are now monitored after the 2023 Sikkim disaster.
Watching the cryosphere: the NMSHE (National Mission for Sustaining the Himalayan Ecosystem), under the NAPCC, tracks the health of Himalayan glaciers.
The strategic stake: in Arunachal, a GLOF threatens not just villages but border roads and hydro-dams — making this a frontier-security issue too.

UPSC Value Box

Glacial lake	Meltwater pooled where a glacier has retreated, often dammed by a moraine.
Moraine	A ridge of loose rock, soil and debris left by a glacier; a fragile natural dam.
GLOF	Glacial Lake Outburst Flood — sudden flood when a glacial lake’s dam fails.
Mago Chu basin	Basin in Tawang district, Arunachal; holds the studied high-risk lakes (Sanhapo, Dharkha Tso).
Sikkim GLOF (2023)	South Lhonak Lake breach; destroyed the Chungthang (Teesta-III) dam; the key warning event.
NDMA	National Disaster Management Authority; apex body, chaired by the Prime Minister.
NMSHE	National Mission for Sustaining the Himalayan Ecosystem; a mission under the NAPCC.

MCQ Practice Question

Q. With reference to glacial lakes and Glacial Lake Outburst Floods (GLOFs), consider the following statements:

Many glacial lakes are dammed by moraines — ridges of loose rock and debris — which makes such dams prone to failure.
The October 2023 GLOF in Sikkim, caused by a breach of the South Lhonak Lake, destroyed the Chungthang (Teesta-III) hydropower dam.
The National Disaster Management Authority (NDMA), which maps high-risk glacial lakes, is chaired by the Union Home Minister.

Which of the statements given above is/are correct?
(a) 1 and 2 only (b) 2 and 3 only (c) 1 and 3 only (d) 1, 2 and 3

Answer: (a) 1 and 2 only

Statement 1 — Correct: Moraine dams are made of loose debris, which is exactly why they can fail.
Statement 2 — Correct: The 2023 Sikkim GLOF from South Lhonak Lake washed away the Chungthang (Teesta-III) dam.
Statement 3 — Incorrect (the trap): The NDMA is chaired by the Prime Minister, not the Home Minister.

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