-Ravi Prakash
The recent floods in Nepal have laid bare the terrifying danger behind the sudden surge of water that rushed into the valley. The boulders, ice and mud swept along with the floodwaters left people in the affected areas with virtually no time to escape. However, the central question remains what exactly triggered the disaster that day in Nepal. Was it simply a cloudburst caused by dense cloud formations? Did an earthquake play a role? Was the disaster caused by a melting glacier? Or did some other major event occur at extreme altitude in the Himalayas, out of view and undetected at the time? As scientists began examining the event, the picture that emerged proved far more complex — suggesting this may not have been a single, isolated disaster, but rather a chain reaction of events unfolding one after another.
The origin of the disaster has been traced to the northern side of Langtang Lirung, in the Langtang mountain range, at an altitude of nearly 5,200 metres above sea level. An examination of satellite images taken before and after the disaster revealed a startling development: a massive glacier on the mountain, along with the section of rock beneath it, appears to have suddenly broken away and slid downward. According to scientists’ estimates, the section that broke off may have measured between 1 and 1.3 kilometres in width, and came hurtling down at tremendous speed from an extremely steep mountain slope.
The sudden collapse of the glacier and the accompanying mass of rock triggered a severe avalanche. The resulting debris of ice and rock entered the Lende Khola river, temporarily blocking its flow and forming what functioned as a natural dam. As water rapidly accumulated behind this barrier, the water level in downstream areas reportedly rose by as much as 9 metres within just 30 minutes.
On the question of whether this qualifies as a classic Glacial Lake Outburst Flood (GLOF), current information suggests it does not fit that description precisely. A GLOF typically refers to the release of water following the breach of an ice-and-rock barrier holding back a lake formed by melting glacial ice. In this case, however, there is no evidence that a lake existed at the site prior to the disaster. Instead, the collapse of the mountain and the resulting debris blocking the river created a new accumulation of water — indicating that this was a compound, or chain-reaction, disaster, with one event triggering the next.
Scientists are now closely monitoring the new lakes that have formed behind the accumulated ice and rock. Nepali authorities have warned that the flood threat will persist until these natural barriers are fully and safely cleared. It will take further time to determine the precise role climate change played in the disaster. However, it is already established that the Himalayas are warming, and that permafrost — the frozen ground that helps hold mountain slopes together — is losing its stability across the region.
Experts note that most existing early warning systems are designed to monitor rainfall patterns or known glacial lakes, but are ill-equipped to detect rapid hazard cascades of this kind, which unfold far faster than conventional monitoring systems can track. The key lesson from this disaster, experts say, is that preparedness cannot be limited to individual hazards such as earthquakes, cloudbursts, or GLOFs in isolation — systems must also account for scenarios in which one disaster triggers another in rapid succession.
Given that the Himalayas serve as the source of many of Asia’s most vital river systems, experts stress that what unfolded in Nepal is not merely a story confined to that country alone, but a significant warning for the entire Himalayan region, including India.