An ice-snow avalanche triggered small glacial lake outburst flood in Birendra Lake, Nepal Himalaya

Glacial lake outburst floods (GLOFs) pose a significant threat in high mountains. Here, we report a recent GLOF event from the Birendra Lake, Nepal Himalaya that occurred on 21 April 2024. We analyze its long-term evolution using multi-source satellite observations and reveal that the lake was formed before 1967 and gradually expanded to 0.25 km2 in 2000, after which it largely remained stable. Further analysis and field evidence indicates that a small ice-snow avalanche from the Manaslu Glacier had already hit the lake on 20 April, but a larger one on 21 April ultimately produced a surge wave and replaced lake water, leading to water discharge and substantial rise in the water level of the Budhi Gandaki River. Geomorphologic analysis suggests that the lake likely had earlier outbursts with a large breach in the dam. This is a probable reason along with small volume of the ice-snow avalanche, likely limited the scale of the outburst and resulted in less damage. Nevertheless, this event is significant despite being small in scale as it has demonstrated that even without significant dam breach process, powerful avalanches can displace enough water to generate surge waves across the lake, resulting in repeated GLOFs that can travel far. Such processes are expected to increase under future climate change.