Alaska declares catastrophic flooding declaration threat from glacial lake outburst

Alaska Gov. Mike Dunleavy issued a disaster declaration Sunday in response to an “imminent threat of catastrophic flooding” from a glacial lake outburst flood (GLOF) linked to a side basin of the Mendenhall Glacier.

That declaration comes after a joint local disaster declaration by the City and Borough of Juneau, and the Central Council of the Tlingit and Haida Indian Tribes of Alaska.

The declarations were made in anticipation of a potentially imminent and record GLOF from Suicide Basin, which sits above the Mendenhall Glacier.

“Hydrologic monitoring by the National Weather Service (NWS) and US Geological Survey (USGS) confirms that the volume of water currently impounded in Suicide Basin has reached or exceeded levels observed during prior flood-of-record events,” officials said in a statement. “A release is expected at any time. Flooding is likely to affect the Mendenhall River and surrounding neighborhoods in the Mendenhall Valley.”

Officials said that this year’s conditions follow two consecutive years of severe flooding.

The August 2024 GLOF caused widespread damage to homes, public infrastructure and utilities, resulting in a state and federal disaster declaration.

“By issuing this declaration before the flood occurs, we can position state resources and personnel in advance to support local and tribal governments in their efforts to protect lives, homes, and essential services,” Dunleavy said in the statement. “Our goal is to act early to reduce impacts and preserve community safety.”

Officials said that the Alaska State Emergency Operations Center has been activated to support local and tribal efforts, which include interagency coordination, emergency permitting, public messaging and preparations for protective actions and potential response.

“This year is different. We’re not just responding – we’re ready,” City and Borough of Juneau City Manager Katie Koester said in a statement. “The HESCO barriers are in place, and we believe they will perform as designed, but our job is to plan for every scenario – especially in the face of what could be another historic flood event. This proactive emergency declaration gives us the tools to act swiftly and decisively.”

The NWS said that the USGS web camera and laser gauge in Suicide Basin showed indications of over-topping the ice dam with icebergs that are being pulled toward the overflow channel and slow lowering of water levels.

“This means the basin is at full capacity and in the past it has taken up to six days from the start of over-topping, Sunday, August 10, for the outburst flooding event to begin,” the NWS said.

The NWS went on to say that “now is the time to finish up flood preparation and planning.”

“Suicide Basin is full and beginning to overtop,” a message read at the top of the City and Borough of Juneau website. “Flood expected. Follow emergency alerts and use the resources on this page to stay safe.”

City officials said that following the overtop notification from the NWS, an alert was sent out to all cell phones in the Juneau area via the Wireless Emergency Alert and Emergency Alert Systems.

A second alert will be sent when the basin begins to release.

Printed evacuation preparation notices are also being distributed starting Tuesday afternoon to all homes within the 17-foot inundation zone.

What is a glacial lake outburst flood?

A glacial lake outburst flood (GLOF) isn’t like the typical type of flooding that occurs from torrential rain and thunderstorms.

According to the USGS, glacial lakes form in small side valleys that have lost ice and then become dammed by the glacier in the main valley or dammed by rocks and debris.

Over time, as the ice-free basins fill with water, the USGS said, lakes reach a breaking point where water pressure opens up a drain underneath the glacier that releases the water downstream, which can result in a GLOF.

“These events are unpredictable and have caused significant loss of life and infrastructure worldwide,” the USGS said. “Furthermore, it is unclear how these lake hazards may change as temperatures rise and glaciers continue to melt.”