A glacial lake outburst flood (GLOF) is a release of meltwater from a moraine- or ice-dam glacial lake due to dam failure^1,2. GLOFs often result in catastrophic flooding downstream, with major geomorphic and socioeconomic impacts^3,4.

GLOFs have three main features:

• They involve sudden (and sometimes cyclic) releases of water.
• They tend to be rapid events, lasting hours to days.
• They result in large downstream river discharges (which often increase by an order of magnitude).

Why are GLOFs important?

Some of the largest floods in Earth’s history have been GLOFs^5,6. They have caused large-scale landscape change⁵, and even altered regional climate by releasing huge quantities of freshwater to the oceans^7,8.

Today, GLOFs pose a risk downstream communities and infrastructure. In Peru alone, GLOFs were responsible for ~32,000 deaths in the 20^th century^3,4. They have killed hundreds to thousands of people in other mountain regions (e.g. the Himalayas), and destroyed roads, bridges, and hydroelectric developments³.

An increasing hazard

Importantly, the general global trend of glacier shrinkage through the 20^th and 21^st centuries has seen the number and size of glacial lakes increase^9-11, at the same time as human activities have expanded further into glaciated catchments. The study of how GLOFs occur and their impacts is therefore important for future hazard mitigation.

Glacial lake settings

There are two main settings in which glacial lakes form^1,2: (1) behind moraine dams, and (2) behind ice dams.

Moraine-dammed lakes

Moraine-dammed lakes form during periods of glacier retreat from a moraine^1,12. As a glacier margin retreats, water collects in the topographic low between the ice-front and the abandoned frontal and/or lateral moraine. Most existing moraine-dammed lakes (such as the Imja Tsho glacial lake in Nepal; see below) formed when mountain glaciers began to retreat from large moraine ridges constructed during the Little Ice Age¹².

Ice-dammed lakes

In contrast to moraine-dammed lakes, ice-dammed lakes form when drainage is blocked by a glacier that advances or becomes thicker^13,14. Consequently, ice-dammed lake growth is closely related to glacier mass balance and climate. Ice-dammed lakes form wherever a glacier blocks the drainage of meltwater.

The main settings for ice-dammed lakes are shown in the diagram below. These include: where a tributary valley is blocked by a trunk glacier; where a glacier from a tributary valley advances across the main trunk valley; in openings between the lateral glacier margin and ice-free valley sides; and at the point where two glaciers join.

The failure of ice or moraine dams

The failure of glacier and moraine dams depends on two main factors: (1) the integrity of the dam, and (2) the nature of trigger mechanisms^1,2,12,13.

Moraine-dam failures

Moraine dams tend to be narrow and sharp-crested¹². As such, they are more likely to fail than broader dam types, such as ice-contact fans or landslides (see diagram below). Most moraines are made up of loose, poorly sorted, permeable sediment, and some contain ice cores¹². Unconsolidated sediments are susceptible to failure, especially if saturated, while the melting of ice cores may cause moraines to subside over time. Despite these weaknesses, where a moraine is low, wide, and armoured by large rock material it may survive intact for hundreds or even thousands of years¹².

Displacement waves

Outburst floods in moraine-dammed settings are often caused by the sudden input of material into a lake causing displacement of water and overtopping of the dam^12,16. Displacement (or seiche) waves are commonly triggered by avalanches or rockfalls, or calving of a lake-terminating glacier as shown below.

Other triggers include, the rapid input of meltwater from an glacier upstream, heavy rainfall or snowmelt events that rapidly raise the lake level, or earthquakes that destabilise the moraine dam^12,14.

Breach incision

Once overtopped, a process called breach incision can occur. This occurs as water flowing across the dam surface erodes a channel into the moraine, starting a positive feedback process where channel incision allows more water to escape, and the higher discharges leaving the outlet encourage greater rates of erosion (see diagram below). This process allows a moraine dammed lake to drain very rapidly^12,16. It also adds large volumes of unconsolidated sediment to the flood waters, which may then develop into highly destructive debris flows¹².

Moraine degradation

Piping (the seepage of water and sediment through a moraine weakening its internal structure) or the melting of ice cores can trigger moraine collapse, or lower the moraine to the point where a smaller displacement wave will cause overtopping and breach incision¹².

Ice-dam failures

Unlike moraine dam failures, the drainage of an ice-dammed lake does not necessarily end in dam destruction^1,13,17. This is because water can drain through subglacial channels that close up when water discharge drops, or because an ice-margin may temporarily float before regaining contact with the bed^18,19. The time-lapse video below (provided by Joe Mallalieu) gives an example of glacial lake drainage at Russell Glacier, Greenland, due to temporary glacier flotation¹⁹.

Ice dam flotation

Ice-dam floatation occurs due to the difference in density between ice (~0.9 g/cm³) and water (~1.0 g/cm³)^13,18,19. As shown in the diagram below, when lake level reaches 90% of the glacier dam height the ice will lift from the bed, and water can escape through subglacial conduits. However, as a lake empties, and its level falls below 90% of dam height, the glacier drops to the bed and subglacial conduits close up. This stops lake drainage and allows a lake to refill.

Overspilling

Lake drainage can also occur by simple overspilling^13,17, for example, during a large rainfall or snowmelt event that causes lake level to rise. This is most common at cold-based glaciers, which are frozen to the bed and less permeable than warm-based glaciers².

Differences between moraine- and ice-dammed GLOFs

For a given potential energy (think of this as lake volume) moraine dam outbursts typically produce higher magnitude floods (see diagram below)¹². However, the threat of repeated GLOFs from a single lake is low because moraine dams are often destroyed during an outburst.

Ice-dammed lake drainage, by contrast, does not often result in dam destruction, meaning that ice-dammed lakes can drain and refill many times. A good example is Lago Catchet Dos in Patagonia (see image below), dammed by the Colonia glacier²⁰. This lake has drained in subglacial tunnels beneath the ice at least 21 times in the last 10 years²⁰.

So, while in the modern-day ice-dammed lake outbursts typically produce lower discharge, and less destructive, floods¹², they may have long-term impacts on downstream communities and infrastructure²⁰.

Will GLOF hazards increase or decrease in the future?

An important and interesting question! The threat from moraine-dammed GLOFs is typically greatest during periods of glacier retreat, whereas ice-dammed GLOFs are highest during periods of glacier growth.

Therefore, we might expect the number of moraine-dammed GLOFs to increase as mountain glaciers continue to shrink worldwide. However, because moraine dams are normally destroyed in lake outbursts, the number of GLOFs will likely start to decrease over time, as the capacity for storing glacial meltwater is gradually lost.

Further reading

• Explore this ArcGIS Story about Glacier Lake hazards in Alaska
• Jokulhlaups
• Glaciolacustrine landsystem
• Valley glacier landsystem
• Glacier lakes