Kathmandu, Oct. 12 -- When Suna Gurung's phone rang at around 4:45 am on Sunday, he was fast asleep in Jagatbagar, a riverside settlement in ward 7 of Chumnubri Rural Municipality in Gorkha. He had no idea that a flash flood was surging down the Budigandaki river. A familiar caller from Ghattekhola warned him that a landslide-dammed river upstream had burst and urged him to flee immediately.

"I had driven up to drop off tourists heading for the Manaslu trek and was fast asleep," said Gurung. "When the call came, I initially couldn't believe it. But when he mentioned a barrier lake flood, I rushed outside. Within ten minutes, the river swelled dramatically in the dark. We raised an alarm, woke everyone up, and fled into the forest."

At 4...