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A deadly flash flood in Nepal’s Rasuwa district on August 26, 2026 has renewed concern over Glacial Lake Outburst Floods across the Himalayas.
ICIMOD explains how moraine-dammed lakes formed by retreating glaciers can suddenly fail, unleashing destructive floods.
With climate change expanding glacial lakes, experts warn rising GLOF risks threaten downstream communities and critical infrastructure.
The deadly flash flood that hit Rasuwa district of Nepal bordering Tibet on August 26, 2026, has brought Glacial Lake Outburst Floods (GLOF) across the Himalayas back into focus.
So, what exactly is a GLOF? The International Centre for Integrated Mountain Development (ICIMOD) defines it as “a sudden release of water from a lake fed by glacier melt that has formed at the side, in front, within, beneath, or on the surface of a glacier.”
The catchword in the context of GLOFs is ‘moraine’. A moraine is an accumulation of unconsolidated debris, rocks, sand, and clay deposited by a moving glacier.
“In the Hindu Kush Himalaya (HKH), moraine-dammed glacial lakes are common and numerous GLOF events have been traced back to the failure of moraine dams. A moraine-dammed lake forms as a glacier retreats, and meltwater fills the space between the proglacial moraine (in front of the glacier) and the retreating glacier,” notes ICIMOD.
The organization adds that as the volume of the glacial lake grows, so does the pressure on the dam containing it. Fragmentation of the source glacier, landslides and other processes can trigger displacement waves in the lake, potentially compromising the stability of the dam and resulting in a GLOF. Failure of the dam can also occur due to internal processes, such as water seeping through the structure.
According to ICIMOD, although GLOFs are not a recent phenomenon in the HKH, they remain a persistent threat to downstream communities and infrastructure and must therefore be part of any planning in the lower reaches.
“The frequency of GLOFs and risk from potential GLOFs are expected to increase as the climate continues to change. As temperatures rise, new lakes form, existing ones expand and sometimes merge, increasing the potential flood volumes in the high mountains.”
The Kathmandu-based organisation had released a report in 2018 on the status of glacial lakes in the HKH that mapped and classified more than 25,000 glacial lakes in the five major river basins spread across the region. “8.3% of the glacial lakes are located below 4000 masl.”
India has witnessed a major GLOF event in the past five years.
In February 2021, the Rishi Ganga flood in Uttarakhand destroyed the 35MW Rishiganga and 520MW Tapovan Vishnugad projects and killed more than 200 people, mostly workers. Although initially categorised as a GLOF, later it was termed a massive rock and ice avalanche that triggered a sudden debris flow.
Then, in October 2023, the South Lhonak lake outburst caused catastrophic flash floods down the Teesta river basin, destroying infrastructure and the Chungthang Dam.
