KATHMANDU, Aug. 29 -- Rescue workers searching for more than 1,500 people still missing across Nepal's shattered northern valleys confronted a new emergency Friday: the debris lake that formed at the Nepal-China border after Wednesday's catastrophic glacial collapse has started to overflow, raising the prospect of a second surge of water through communities already buried under mud, rock, and grief.

Officials in Nepal's Rasuwa district confirmed that water from the barrier lake, which accumulated behind the mass of ice and sediment that initially dammed the Lhende River, was spilling over its edge as of Friday. The lake's exact dimensions remain unknown. Roads to the site have been destroyed, limiting monitoring to helicopter overflights...