A glacial lake is a body of water found in a landscape shaped by ice, either currently or in the recent past. These lakes are directly fed by meltwater from a glacier or ice sheet, or they occupy a depression created by glacial processes. Glacial lakes are common, defining features across mountainous and high-latitude regions worldwide.
Defining Characteristics of Glacial Lakes
Many glacial lakes have an opaque, milky, or striking turquoise coloration. This distinctive appearance is caused by ultra-fine sediment known as “glacial flour.” This flour is composed of pulverized rock particles, typically ranging from two to 65 microns in diameter, which remain suspended in the water column after being carried by meltwater.
The suspended particles scatter sunlight, reflecting blue and green wavelengths, which gives the lake its vibrant hue and high turbidity. The heavy concentration of this mineral material significantly reduces light penetration, affecting the lake’s thermal structure and aquatic life. Glacial lakes are also characterized by very cold water temperatures due to the continuous inflow of ice melt, resulting in complex temperature patterns.
The Mechanics of Glacial Lake Formation
Glacial lakes form because glaciers are effective agents of both erosion and deposition, creating the deep basins and barriers necessary to hold water. The immense weight and slow movement of ice over bedrock carves out depressions through two main erosional processes: plucking, where the glacier pulls away fractured rock, and abrasion, where rock fragments embedded in the ice scour the underlying surface.
This action often over-deepens valleys into characteristic U-shapes and excavates bowl-like hollows called cirques high on mountain sides. The material scraped away by the ice, called till, is deposited as the glacier melts. These deposits form ridges known as moraines, which act as natural dams across the valley floor or at the glacier’s terminus. Meltwater from the retreating ice then fills the newly created or dammed basin.
Categorizing Different Types of Glacial Lakes
Glacial lakes are classified based on the nature of the basin and the material that dams the water flow. Proglacial lakes are a common type, located immediately in front of the active glacier terminus, often dammed by a terminal moraine left behind by the ice. These lakes expand as the glacier retreats, and their stability depends heavily on the integrity of the moraine dam.
Another type is the cirque lake, which occupies the deep, bowl-shaped rock basin carved by the glacier at the head of a mountain valley. These lakes form after the glacier has vanished, filling with precipitation and meltwater. A third category is the ice-dammed lake, which forms when a mass of ice blocks the drainage path of a tributary valley or a river. These ice barriers can be temporary, as meltwater may eventually escape beneath or around the ice.
Glacial Lake Outburst Floods (GLOFs)
The most significant hazard associated with glacial lakes is a Glacial Lake Outburst Flood (GLOF), which involves the sudden release of a large volume of water when the natural dam fails. The dams involved are often unstable moraine ridges or temporary ice barriers. Failure can be triggered by gradual erosion and weakening of the moraine dam.
A more immediate cause is the displacement of water by a large rockfall, landslide, or ice avalanche plunging into the lake, which creates a massive wave that overtops and breaches the dam. The resulting flood is characterized by extreme peak discharge and immense erosive power, capable of traveling great distances downstream and causing substantial damage.