Juneau prepare for catastrophic flood as glacier outburst surpasses all records

Officials in Alaska’s capital are urging some residents to evacuate as an annual but increasingly destructive threat looms, a massive glacial outburst from the Mendenhall Glacier that could set a new flood record.

After days of warnings, the National Weather Service confirmed Tuesday(August 12) that rainwater and snowmelt from a vast ice-dammed basin had begun rushing into the Mendenhall River. The peak is expected by late Wednesday afternoon, with forecasts suggesting water levels could crest between 16.3 and 16.8 feet, higher than the devastating floods of 2023 and 2024.

“This will be a new record, based on all of the information that we have,” said meteorologist Nicole Ferrin at a Tuesday news conference.


City officials have advised people in the flood zone to leave as a precaution. Some are heeding the warning, while others are staying behind.

Hatch, who lifted his home by about a meter after floodwaters reached his floorboards in 2023, worries that temporary flood barriers could backfire if water overtops them. “If it gets around the barriers and flows down, we basically become a bathtub,” he said.

Climate-driven threat

The flooding stems from a phenomenon known as a glacial lake outburst flood. Since 2011, the retreat of a smaller glacier near Mendenhall has left behind a basin that fills each spring and summer. When the water pressure becomes too great, it bursts under or around the ice dam formed by the Mendenhall Glacier, surging into Mendenhall Lake and then the river.

These events have grown more destructive in recent years.The river crested at 15.99 feet last August, damaging nearly 300 homes. Researchers estimate a large outburst can release up to 15 billion gallons of water, enough to fill nearly 23,000 Olympic-size swimming pools.

During last year’s flood, the river’s flow rate was about half that of Niagara Falls.

In anticipation of this year’s flood season, local, state, federal, and tribal agencies installed a temporary levee system along 2.5 miles of riverbank. The 10,000 “Hesco” barriers, giant sand-filled containers, are designed to protect more than 460 properties during a flood event of up to 18 feet.

But some residents are skeptical. “If the water exceeds predictions or seeps through, it could trap us,” Hatch said, pointing out that property damage could be worse in that scenario.

Long-term solutions are still years away. The US Army Corps of Engineers is in the early stages of studying options such as permanent levees. Researchers from the University of Alaska Southeast and the Alaska Climate Adaptation Science Center warn that such floods could continue for another 25 to 60 years, as long as the Mendenhall Glacier remains an effective ice dam.

For Juneau, with its population of about 30,000, the Mendenhall Glacier is both an iconic tourist draw and a recurring danger. Homes in the Mendenhall Valley sit just miles from the glacier and lake, many along the riverbanks now under threat.

The city has endured successive years of record flooding, with damages in the tens of millions of dollars.