Glacial Lake Outburst Floods

A glacial lake outburst flood (GLOF) is the sudden release of water stored in a lake associated with a glacier. It occurs when the lake's natural ice or moraine barrier is overtopped or breached, producing a short-lived, high-discharge flood that may carry large quantities of sediment and debris.

Formation of the lake and natural dam

As a glacier erodes its valley and subsequently thins or retreats, meltwater may collect in an overdeepened basin. Such water can be impounded by glacier ice or by a moraine dam composed of rock fragments and sediment deposited by the glacier.

• Continued melting and glacier retreat can enlarge the lake and increase the volume of stored water.
• Moraine dams may contain loose sediment and buried ice, making them susceptible to seepage, erosion and subsidence.

Triggers and modes of dam failure

An outburst begins when water crosses, penetrates or destabilizes the natural barrier. Failure may occur suddenly or develop progressively as a small outlet enlarges.

• An avalanche, rockfall, landslide or ice calving can create a displacement wave that overtops and erodes the dam.
• Intense rainfall or rapid snow and ice melt can raise the lake level and reduce the dam's available freeboard.
• Seepage and piping can progressively remove material from within a moraine dam, while melting of buried ice can weaken it.
• An ice dam can fail when subglacial drainage passages open or enlarge, allowing rapid lake drainage.
• Earthquakes can destabilize surrounding slopes or damage an already weak natural dam.

Transformation into a destructive flood

Once water begins cutting through the barrier, erosion enlarges the breach and rapidly increases discharge, creating a self-reinforcing failure. In steep, confined mountain valleys, the flood can entrain boulders and sediment, becoming a debris-laden flow with greater erosive power.

• Downstream landslides, temporary blockages or failure of another lake can produce cascading floods and secondary surges.
• The severity depends on stored water volume, breach speed, valley gradient, sediment availability and downstream exposure.