Glacial lake outburst flood

Geography glossary

Also called: GLOF, Glacial burst · Topic: Natural Hazards and Disaster Management in India · NCERT: Class 7, Ch 3 "Climates of India"; Class 9, Ch 2 "Shaping of the Earth's Surface"

Meaning

A glacial lake outburst flood (GLOF) is a sudden release of water from a lake held back by a glacier (an ice dam) or by glacier debris (a moraine dam), which happens when that dam breaks. Class 7 NCERT calls it a "glacial burst".

  • It matters because a GLOF sends a huge volume of water and debris down a valley within minutes or hours, with very little warning. It can kill people and wreck dams tens of kilometres downstream.
  • In India, GLOFs caused the Kedarnath disaster (2013) and the South Lhonak flood in Sikkim (2023), so the topic is often in the news.

Explanation

Key terms

  • Glacial lake: a lake fed by glacier meltwater. It usually forms at or near the snout (the front end) of a glacier.
  • Moraine: the heap of rock, gravel and soil that a glacier carries and then drops.
  • A moraine dam is made of this loose material, so it is weak.

  • Water can seep through it, cut into it or push it over.

  • Ice dam: a wall of glacier ice that blocks a valley and holds back water behind it.

How a GLOF happens (cause → process → result)

  1. The lake grows - Warming melts the glacier. - The meltwater collects behind a moraine dam or an ice dam. - The lake gets bigger every year.

  2. The dam breaks. One of these triggers can break it: - Overfilling: too much meltwater, mostly in summer. - Heavy rain or snow adds a lot of water in a short time. - An avalanche or landslide falls into the lake. It pushes up a big wave that goes over the dam or breaks it. - An earthquake shakes the loose dam apart.

  3. The flood rushes down - A huge volume of water and debris surges down the valley within minutes or hours. - It often turns into a flash flood (a flood that rises within minutes or hours, with very little warning) or a debris flow (a fast, thick mix of water, mud, rock and boulders that pours down a steep channel).

Conditions that favour GLOFs

  • Season: summer and the monsoon, when there is the most meltwater and rain.
  • Warming: lakes grow bigger, and glacier ice and permafrost get weaker.
  • Fragile slopes: the Himalayas are young, steep, earthquake-prone mountains with plenty of loose debris.
  • Exposure: hydropower dams, roads and big crowds of pilgrims sit inside the flood path.

GLOF as part of a cascade

  • A cascade hazard is one hazard setting off the next, like falling dominoes.
  • A typical chain:
  • A slope collapses or an avalanche falls.
  • The falling mass hits a lake, and the lake bursts.

  • A debris-laden flood rushes down the valley.

  • In the Himalayas, avalanches, GLOFs and cloudbursts often join up like this.

Where it occurs

  • South Lhonak lake, north Sikkim (3-4 October 2023)
  • It is a moraine-dammed lake.
  • The flood went down the Teesta. The Teesta rises in the glaciers of north Sikkim, flows south through Sikkim and West Bengal, then enters Bangladesh and joins the Brahmaputra (Jamuna).
  • It destroyed the Teesta III dam at Chungthang and killed many people, including soldiers.

  • Lesson: a dam built downstream of a known high-risk lake became part of the disaster.

  • Chorabari lake, Uttarakhand (16-17 June 2013)

  • The lake sits above Kedarnath, at the snout of the Chorabari glacier.
  • The early monsoon met a western disturbance (a low-pressure system that comes from the Mediterranean region and travels east). Together they brought extreme rain, and the lake filled and breached.
  • Floods and debris rushed down the Mandakini, which joins the Alaknanda at Rudraprayag.

  • About 6,000 people died, many of them Char Dham pilgrims.

  • Who monitors GLOFs in India:

  • NDMA Guidelines on Management of GLOFs (2020).
  • A national GLOF risk-mitigation programme (2024) for high-risk lakes (verify current). It aims to assess lakes, install automatic monitoring and early-warning systems, and lower lake levels.
  • NRSC (National Remote Sensing Centre, ISRO) keeps an inventory of glacial lakes mapped from satellite images.

  • CWC (Central Water Commission) monitors glacial lakes and water bodies.

  • Shared river basins: Himalayan glacial lakes feed rivers shared with Nepal, China and Bhutan, so managing GLOF risk needs cooperation across borders.

Don't confuse with

  • Cloudburst: rainwater falling from the sky. The IMD defines it as 100 mm or more of rain in one hour over about 20-30 sq km. A GLOF releases water that was already stored in a glacial lake. Example: Leh 2010 was a cloudburst, not a GLOF.
  • Avalanche: a sudden fall of a large mass of snow (sometimes with rock and ice) down a slope. In a GLOF, the thing that moves is lake water. An avalanche can trigger a GLOF, but it is not a GLOF.
  • Chamoli flood (7 February 2021): this was a rock-and-ice avalanche from Ronti peak that turned into a debris flow. It was not a lake outburst. About 27 million m³ of rock and glacier ice collapsed from the steep north face of Ronti Peak [2].
  • Ordinary river flood: it rises slowly over days from rain over a whole basin. A GLOF comes from a single dam breaking, rises within minutes or hours, and carries heavy debris.

Prelims Hooks

  • "Glacial burst" (Class 7 NCERT) means the same thing as a GLOF.
  • South Lhonak lake (north Sikkim) → Teesta → Teesta III dam at Chungthang destroyed (October 2023).
  • Chorabari lake → Mandakini → Kedarnath disaster (June 2013). Trap: the river was the Mandakini, not the Bhagirathi.
  • Chamoli 2021 trap: it was a rock-and-ice avalanche (Ronti peak → Ronti Gad → Rishiganga → Dhauliganga), not a GLOF [2].
  • NDMA GLOF Guidelines came out in 2020. NRSC (ISRO) keeps the glacial-lake inventory, and CWC monitors glacial lakes.
  • Moraine dams are loose and weak. Main GLOF triggers: overfilling, heavy rain or snow, an avalanche or landslide into the lake, and earthquakes.

Mains Points

  • Cascading risk (GS-III)
  • In Kedarnath 2013 and South Lhonak 2023, a trigger high in the mountains became a debris flood far downstream.

  • So planning must cover the whole valley, not just the lake.

  • Development in hazard zones (GS-III)

  • Hydropower projects such as Teesta III, Char Dham roads and pilgrim crowds raise exposure.

  • Possible responses:

    • carry out cumulative impact assessment for each river basin;
    • keep dams out of the paths of high-risk lakes;
    • cap pilgrim numbers.

  • Early warning and climate link (GS-I/III)

  • Warming is making glacial lakes bigger, so GLOFs are becoming more frequent.
  • What is in place: NRSC's inventory, CWC monitoring, the NDMA 2020 guidelines and the national programme for high-risk lakes.
  • What is still needed:
    • automatic sensors and lake lowering;
    • sirens and evacuation drills in villages, in line with the Sendai Framework's push for "early warning for all";
    • cooperation with Nepal, China and Bhutan on shared river basins.

Related concepts

Read more

Sources

  1. 1Class 7, Ch 3 "Climates of India"; Class 9, Ch 2 "Shaping of the Earth's Surface" (primary)
  2. 2A massive rock and ice avalanche caused the 2021 disaster at Chamoli, Indian Himalaya (Science, 2021), USGS Publications Warehousepubs.usgs.gov · tier 2