Nepal plans to drain four dangerous glacial lakes after floods killed over 1,400 people. The lakes threaten downstream towns with a glacial lake outburst flood. What does that mean?

A glacial lake outburst flood, or GLOF, happens when a natural dam holding a glacial lake fails. The dams are made of loose rock, ice and sediment. An avalanche, earthquake or rising water pressure can break them.

When the dam fails, millions of litres of water rush downstream in minutes. The flood destroys roads, bridges and villages in its path. Nepal has reported about 26 such floods in four decades.

As glaciers retreat, meltwater collects behind these weak dams and the lakes grow. Draining lowers the water level and reduces pressure on the dam. Nepal will use canals and gated structures at Thulagi, Lower Barun, Lumding Tsho and Hongu II. Sensors and early-warning systems will protect downstream communities. Nepal lowered the Imja lake near Everest ten years ago in a similar operation.