Residents in several Swiss Alpine communities remain on high alert on Friday as a local river swelled dangerously behind a natural dam of rubble, rocks and ice following a glacier collapse earlier this week, German Press Agency (dpa) reported.

“We ask residents to make personal preparations so that they can leave their homes as quickly as possible,” the municipalities of Steg-Hohtenn and Gampel-Bratsch in the Lotschental valley said on their website late Thursday.