New Delhi, Aug. 29 -- What happened along the Nepal-China border on August 26 is difficult to comprehend because it began with something that appears almost immovable: a glacier. A vast mass of ice and underlying rock near Langtang Lirung suddenly collapsed, producing a seismic shock initially mistaken for an earthquake. Satellite observations subsequently confirmed that the tremor was generated by an enormous ice and rock avalanche.

A huge section of ice and bedrock plunged from an altitude of around 5,200 metres, setting off a chain reaction that sent ice, boulders, mud and water into the Lhende Khola and subsequently the Bhote Koshi and Trishuli river systems.

The consequences were devastating. Water levels reportedly surged by eight...