A large section of a Himalayan glacier broke loose high in the mountains, near the Langtang region. A tremendous mass of ice and rock plunged roughly 1,200 meters (about 4,000 feet) into the Lhende Khola river valley. That set off a chain reaction: The falling glacier produced a huge ice-and-rock avalanche/landslide, picking up boulders, sediment and additional debris. The debris appears to have temporarily dammed the river. Water accumulated behind that blockage, and when it failed, an enormous surge of water, mud, rock and ice blasted downstream. River levels reportedly rose by roughly 9 meters — nearly 30 feet — in about 30 minutes in places. Interestingly, instruments initially registered what looked like an earthquake. The USGS subsequently determined that the glacier collapse itself generated the seismic signal — essentially, the landslide was so enormous that it registered like a magnitude-5-ish seismic event. As for why the glacier broke, that's still being investigated. Scientists are being careful not to say climate change definitively caused this particular collapse. But Himalayan glaciers are rapidly retreating, and warming can increase meltwater and thaw frozen ground while removing ice that helps support steep rock faces. All of those processes can make these high mountain slopes less stable. So, in shorthand, it looks like: Glacier collapse → massive ice/rock landslide → river blockage/debris flow → blockage fails → catastrophic flash flood.
