A glacial lake outburst flood (GLOF) is a sudden and catastrophic release of water from a glacial lake, which is a body of water dammed by a glacier or moraine. The dam that holds back the water can be composed of glacier ice, glacial sediments, moraine, or bedrock. When this natural dam fails or is breached—due to erosion, buildup of water pressure, avalanches, earthquakes, volcanic activity, or glacier collapse—the water is released rapidly and can cause devastating floods downstream. Key characteristics of a GLOF include:

• Sudden and sometimes cyclic releases of water
• Rapid events lasting from hours to days
• Extremely high peak flows that can be many times greater than typical river flows
• Massive downstream flooding with potential to destroy infrastructure, cause landslides, and threaten lives

GLOFs are increasingly a concern due to climate change, which accelerates glacier melting, increases glacial lake formation, and raises the risk of dam failures. They have historically caused thousands of deaths worldwide and continue to pose a significant hazard, especially in mountainous regions like the Himalayas, Andes, European Alps, and Alaska. In summary, a glacial lake outburst flood is a sudden flood caused by the failure of a natural dam containing a glacier-formed lake, releasing vast quantities of water in a brief period with destructive potential downstream. This phenomenon is also related to terms like jökulhlaup, which refers to sub-glacial lake outbursts caused by melting or volcanic activity beneath glaciers.