Glacial outburst floods aren't a new phenomenon in Alaska, but climate change is scrambling the geography of risk.

A huge basin of rainwater and snowmelt dammed by Alaska’s Mendenhall Glacier has started to release, sending a potentially record surge of flooding toward parts of Juneau.

Residents of Alaska capital Juneau are being urged to evacuate amid an “imminent threat of catastrophic flooding from a glacier lake outburst flood.”