Glacial lake outburst floods, a latent danger in high mountain areas

On December 13, 1941, the city of Huaraz (Peru) woke up like any other day. Chronicles tell us that the inhabitants were enjoying a few summer days after a short but intense rainy season. The Auqui and Paria rivers had overflowed in some areas, but people assumed it was due to the rain. They were mistaken. A large part of Lake Palcacocha was about to come crashing down on them, in what is known as a glacial lake outburst flood.

What is a glacial lake outburst flood and how does it occur?

A glacial lake outburst flood can be considered as a sudden flood or flash flood caused by the breaking of the terminal moraine of a glacier. But to better understand it, let’s first refresh the knowledge you acquired in school.

A brief science lesson

A glacier is a mass of ice that forms on the Earth’s surface. Far from remaining stationary, the slope causes it to move slowly, at a rate of 10 to 100 meters per year.

In the following image (1), you can observe its main parts.

For the purposes of this article, the elements that interest us the most are moraines and glacial lakes.

Moraines, for example, are materials, mainly rock fragments, that the ice picks up and carries along. They accumulate in various parts of the ice mass: sides, front/terminal part, bottom, etc.

Glacial lakes, on the other hand, are formed when water occupies the depression created by the retreating ice.

The outburst of a glacial lake, an explosive phenomenon

This phenomenon, also known as a GLOF (Glacial Lake Outburst Flood) or violent overflow of a glacial lake, occurs when the moraine that damns the glacial lake gives way under the pressure of accumulated water. Imagine a reservoir you know and replace the concrete dam with a pile of rocks and soil with little cohesion to get an idea of the result.

This type of event has three main characteristics:

  • They involve the sudden (and sometimes cyclical) release of water.
  • They happen rapidly.
  • They result in large downstream flows.

These sudden outbursts are not an extraordinary phenomenon. In fact, studies carried out on continents like the Americas have identified up to 165 events since the beginning of the 18th century (1).

However, in recent years, the risk has noticeably increased. An analysis of satellite imagery conducted in 2020 revealed that the number of glacial lakes had increased by 53% during the period 1990-2018 (2). One of the reasons for this is the progressive global warming experienced by the planet, which is causing rapid glacier melting.

As a result, lakes like Palcacocha (see image), which we mentioned at the beginning of this article, have increased their volume 34 times since 1970, once again threatening the 120,000 inhabitants of Huaraz.

And, obviously, the risk is not limited to the Andean mountain range alone. Local communities in other high mountain areas like the Himalayas are also paying more attention to what is happening in the mountains.

Life-saving technology

In March 2021, following the incident in Chamoli, India, we took the opportunity to explain the usefulness of early flood warning systems.

Although, as we mentioned at the time (and subsequently confirmed), the cause of the flood in India and a glacial flood are different, this technology can be relied upon in both cases. After all, the purpose is the same: to alert and evacuate people before the arrival of the flood, thereby minimizing losses, especially human lives.

In fact, areas prone to glacial lake outburst floods, such as the Hunza Valley in Pakistan, are beginning to deploy early warning systems in anticipation of future events.

Hunza is a mountainous valley in the Pakistani region of Gilgit-Baltistan. Located in the northernmost part, it is part of the China-Pakistan Economic Corridor.

But it’s not just about alerting the local population to the danger of a flood. Continuous monitoring is essential. The reason is to recognize that glacial areas are the main water resource for large parts of the planet. Therefore, technologies such as automatic weather stations equipped with video cameras and sensors that collect information on precipitation are more than useful.


Glacial lake outburst floods, as we have seen, are not something new but rather a situation that worsens over time. Considering the projections regarding the increase in global temperatures and what it means in terms of ice, wouldn’t it be advisable to entrust part of the monitoring tasks to technology?

Sources consulted:

  • (1) Schoolmeester, T., Johansen, K. S., Alfthan, B., Baker, E., Hesping, M., & Verbist, K. (2018). Atlas de Glaciares y Aguas Andinos: El impacto del retroceso de los glaciares sobre los recursos hídricos. In Ediciones UNESCO. ISBN 9789233001039 Disponible en
  • (2) Shugar, D. H., Burr, A., Haritashya, U. K., Kargel, J. S., C, S. W., Kennedy, M. C., Bevington, A. R., Betts, R. A., Harrison, S., & Strattman, K. (2020). Rapid worldwide growth of glacial lakes since 1990. Nature Climate Change, 10(10), 939–945.

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