A huge basin of rainwater and snowmelt dammed by Alaska’s Mendenhall Glacier has started to release, and officials on Tuesday urged residents in some parts of Juneau to evacuate ahead of what could be a record surge of floodwater downstream.

Officials in recent days have been warning people in the flood zone to be ready to evacuate. On Tuesday morning they confirmed water had started escaping the ice dam and flowing downstream, with flooding expected late Tuesday into Wednesday.

Flooding from the basin has become an annual concern , and in recent years has swept away houses and swamped hundreds of homes. Government agencies installed a temporary levy this year in hopes of guarding against widespread damage.

“This will be a new record, based on all of the information that we have,” Nic

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