A Glacial Lake Outburst Flood (GLOF) is a catastrophic natural concept defined as the sudden release of a large volume of water from a glacial lake. These lakes form when meltwater from a retreating glacier accumulates in a depression, often impounded by a natural dam of glacial ice, bedrock, or, most commonly, a moraine complex—a ridge of rock and debris left by the glacier. The concept is not new, but its threat has intensified in the Hindu Kush Himalaya (HKH) region, including India, due to accelerated glacial melt driven by climate change.
The mechanism of a GLOF involves the failure of the natural dam, which can be triggered by several factors. As glaciers melt faster due to rising temperatures, the volume of water in the lake increases, putting immense pressure on the dam. The dam's failure can be initiated by external events like an earthquake, a landslide, a rockfall, or an ice avalanche plunging into the lake, which creates a displacement wave that overtops the dam. Internal processes, such as water seeping through the moraine structure, can also compromise its stability. Once breached, the water is released rapidly, often lasting hours to days, causing massive downstream flooding with peak flows that can reach as high as 15,000 cubic meters per second.
The increasing frequency and risk of GLOFs are directly connected to climate change and glacial retreat. Between 1990 and 2015, satellite data documented a significant increase in both the number and average size of glacial lakes across the Himalayas. India's Himalayan region alone has over 5,000 identified glacial lakes. The concept connects directly to disaster management and infrastructure planning, as GLOFs pose a persistent threat to downstream communities and hydroelectric projects, as seen in the devastating October 2023 GLOF from the South Lhonak glacial lake in Sikkim. The risk is particularly high in the HKH, where a 2023 study found 15 million people are at risk from this hazard, mostly in China, India, Nepal, Pakistan, and Peru.