Glacial lake hazards

Glacial lake hazards | Flooding | Case Study 1 | Case Study 2 | Summary

Increasing glacial lakes

In recent decades glacial lake hazards have increased due to accelerated glacial mass loss and retreat leading to the expansion of existing glacial lakes and formation of new lakes 1,2.

As a result, glacial lakes now exist in all glaciated regions, forming behind moraines, other glaciers, landslide deposits, in bedrock depressions, in cirques and through the coalescence of supraglacial ponds 3,4 (Figure 1, for example).

Glacial lake hazards in Bhutan
Figure 1: Rapid growth of glacial lakes in the Bhutan-Himalaya in response to retreating glacier termini. Photo: NASA/USGS, Wikimedia Commons.

Why are glacial lakes a hazard?

Generally, whether glacial lakes represent a hazard to people and the environment depends on two factors.

First, their ‘probability’ of outburst, i.e. how likely they are to burst and cause flooding. Second, their potential flood ‘magnitude,’ i.e. the volume of outburst, distance of travel, inundation depths 5.

In recent decades, the number of people and amount of infrastructure and services close to glacial lakes has increased due to tourism, hydroelectric projects, and agriculture expansion. As a result, outbursts from glacial lakes could have much larger social and economic implications.

It is therefore important that we understand which lakes are a hazard, and, just how hazardous they are.

Flooding from glacial lakes

Generally, the terms Glacial lake outburst floods (GLOFs) and jökulhlaups are used to refer to the rapid discharge of water under pressure from a glacial system 6 and are among the most far-reaching glacier hazards 2.

Glacial flooding is rather complex, and each event is distinct (see this post for more detail). However in all cases, the peak discharge of a glacial flood is several times higher than hydrometeorological floods 7. As a result, glacial related flooding can have devastating and far-reaching geomorphic and socioeconomic consequences where they interact with human populations.

Below are two examples of glacial flooding, the first as an illustration of large human impact and the second a large environmental impact.

Case study 1: South Lhonak Lake GLOF

Background information

Location map of Sikkim, and aerial image showing the glacial lake hazard of South Lhonak lake
Figure 2: (A) Map of the Indian state of Sikkim showing the location of the South Lhonak Lake and the two major settlements along the valley; Google Earth image showing the bird-eye view of (B) the tongue of the South Lhonak glacier and its associated proglacial lake; and (C) in detail the frontal moraine damming the South Lhonak Lake (Sattar et al., 2021).

The Teesta basin in Sikkim Himalaya is home to numerous glacial lakes, including one of the largest and fastest-growing lakes – South Lhonak lake.  

South Lhonak lake is a moraine-dammed proglacial lake located at an elevation of 5200 m a.s.l. The lake is fed by meltwater from the lake terminating South Lhonak glacier, which has been retreating over the past 30 years, shrinking by ~0.96 km2 8. At the same time, the lake has exhibited significant growth- expanding from 0.42 km2 in 1990 to 1.35 km2 in 2019 8.

The lake drains into the Teesa river through a narrow channel in the moraine as can be seen in Figure 2c. The moraine itself is unconsolidated, contains internal ice and is also classified as unstable.

Scientists previously reported there was a 42% chance that Lhonak lake could burst 9, so the glacial lake hazard was known.

The Sikkim GLOF

All of a sudden on 4th October 2023 a significant GLOF was released from South Lhonak Lake.

The GLOF travelled down the valley and overwhelmed the Teesta 111 HEP dam, which collapsed as a result, releasing an even more catastrophic flood downstream. Consequently, the outburst significantly impacted downstream communities, resulting in; 42 confirmed deaths, 150 missing individuals, severe damage to settlements 2,400 people evacuated, 7,600 people displaced, and 15 bridges washed away or submerged.

What caused the outburst and why was it so devastating?

In this case, there are several contributing factors to the Sikkim GLOF disaster, which can be split into physical factors and human factors.  

Physical factorsHuman factors
Rapid growth of the lake and unstable moraine  Construction of the Teesta 111 HEP dam has increased populations and settlements throughout the valley as people move for work  
Narrow river valley creating a ‘funnel’ for the water, increasing flow velocities  Removal of vegetation along the river valley to make way for infrastructure reduced the natural buffer to flooding  
Continuous heavy rainfall prior to outburst triggering a landslide into the lake  Poor regulations for infrastructure and tourism
Exposure development as a result of the HEP construction. Irrespective of the changing glacial lake hazard, exposure changes can be crucial for overall impact.

Figure 3: Evolution of the Chungthang town to illustrate exposure increase. The rapid growth of the infrastructure can be seen after the establishment of the hydropower setup from 2009 and without a doubt contributed to the scale of the impact of the GLOF (Sattar et al., 2021).

Case study 2: Vatnajökull Jökulhlaup 

Background information

Vatnajökull is the largest glacier in Iceland covering more than 10,000km2 10.

On 30th September 1996 eruptions firstly began beneath the glacier at Gjálp volcano, located 8km northwest of Grímsvötn caldera. Volcanic activity continued for two weeks until 14th October when the eruptions stopped.

The Vatnajökull Jökulhlaup 

Aerial image of Vatnajokull ice cap
Figure 4: Aerial satellite imagery to illustrate the extent of Vatnajökull ice cap, Iceland. Photo credits: Wikimedia Commons 

As a result of the eruptions, glacial ice melted and flowed into the Grímsvötn caldera. Over the first 4 days of eruption meltwater was produced at a rate of 5000 m3 s−1 so by 4th November, the lake level had risen to 1,510m – the highest ever recorded 10.

At this point, for the first time in the observational history of Grı́msvötn, the ice dam was floated off the bed, triggering rapid drainage.

About 10.5 hours later water suddenly emerged from the margin of Skeiðarárjökull as a flood wave, in the most rapid jökulhlaup ever recorded from Grímsvötn in which 3.2 km3 of water drained from the lake within a period of 40 hours 11.

What were the impacts of the Jökulhlaup?

Physical impacts

Over 100 million tons of volcanic material and clay was carried more than 15km from the coast over Skeiðarársandur, altering the outwash landscape through erosion and deposition.

Human impacts

Figure 5: Twisted remains of a bridge following the outburst. Photo credits: Chris 73 from Wikimedia Commons  under the creative commons 3.0 license.

By comparison to other recorded glacial floods, the impacts of the 1996 outburst were minimal. The flood waters destroyed large parts of the main road Hringvegur, two bridges as well as some communication installations.

The flood plain has been uninhabited for hundreds of years and authorities closed the road before the flooding, thus unlike the Sikkim GLOF, no fatalities or injuries occurred.


To summarize, glacial lake hazards are becoming increasingly important for downstream communities as glaciers retreat and glacial lakes expand. The case studies of the Sikkim GLOF and Vatnajökull Jökulhlaup have been used to demonstrate how glacial lake hazards can have very different impacts. Understanding glacial lake hazards is vital if we are to prevent future flood disasters.

Extra Reading

See this site for a collection of videos on GLOFs: Video E-Library – GLOFCA


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3.          Song, C. et
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9.          Babu
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I am a glaciologist and natural hazard scientist at Newcastle University. My research focusses on the risk of Glacial Lake Outburst Floods (GLOFs), to help communities better prepare for, respond to, and live alongside hazards.

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