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

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.

The Mendenhall Glacier — a thinning, retreating glacier that is a major tourist attraction in southeast Alaska — acts as a dam for Suicide Basin, which fills each spring and summer with rainwater and snowmelt. The basin itself was left behind when a smaller glacier nearby retreated.

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.

The glacial flooding that sent residents of Alaska’s capital city scrambling this week has become an annual ordeal for those who live along the picturesque river that winds from the nearby Mendenhall Glacier.