What Is Known About the Nepal-Tibet Floods

Massive landslides and flash floods along the Nepal-Tibet border have left hundreds dead or missing and caused widespread destruction, with search and rescue operations under way on both sides of the frontier.

Chinese state media has reported significant casualties. Nepalese officials have warned of loss on a "huge scale." Rescue teams and military helicopters have been searching the valleys below the Himalayan range, where debris and mud have buried roads, bridges and vehicles.

The confusion over the cause

Nepal's foreign minister initially said an earthquake had been reported on the border shortly before the landslide, suggesting a tremor triggered it. The US Geological Survey first recorded the event as a magnitude 4.4 earthquake on Wednesday.

The agency later corrected the record, confirming the event "was instead a glacial collapse and debris flow" that caused catastrophic impacts downstream, and that no earthquake had occurred. Nepali authorities had separately told the BBC that a large mass of ice may have broken from a mountain, pulling rock and sediment with it.

The terrain problem

The steep slopes and narrow valleys that make the region spectacular also make it lethal in an event like this. Water and debris accelerate through confined channels, and the settlements built on valley floors sit directly in the path.

Access is now the constraint on the response. Roads and bridges that rescue convoys would use were among the first structures destroyed, leaving helicopters as the primary route into the worst-hit districts and slowing the count of who is missing.