Alaska Gov. Mike Dunleavy declared a state disaster due to the "imminent threat of catastrophic flooding from a glacier lake outburst flood" in the Juneau area.

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

This year, a giant wall of reinforced sandbags erected with the help of the U.S. Army Corps of Engineers held back the worst of the flooding in Juneau, to residents' great relief.

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, with flooding expected late Tuesday and on Wednesday.

The National Weather Service has issued a warning as glacial water flows into the Mendenhall River.