Juneau, Alaska’s capital, is bracing for what authorities warn could be record-breaking floodwaters, as a glacial outburst from the Mendenhall Glacier threatens sections of the city.
Residents in affected areas have begun evacuating, heeding urgent warnings.
The deluge, caused by rainwater and snowmelt accumulating behind a basin dammed by the Mendenhall Glacier, began escaping the ice dam Tuesday morning.
Flooding is anticipated to continue into Wednesday, impacting homes situated along the Mendenhall River and near Mendenhall Lake.
The Mendenhall Glacier, a popular tourist attraction, lies about 12 miles (19 kilometers) from Juneau, a city of 30,000 people.
Its proximity means that many properties on the city’s outskirts are directly in the path of the glacial outburst.
The National Weather Service predicts the flooding will peak between 8am and noon Wednesday.
Nicole Ferrin, a meteorologist with the service, said: “This will be a new record, based on all of the information that we have.”
Flooding from the basin has become an annual concern since 2011, and in recent years has swept away houses and swamped hundreds of homes. Government agencies installed temporary barriers this year in hopes of protecting several hundred homes in the inundation area from widespread damage.
The flooding happens because a smaller glacier near Mendenhall Glacier retreated — a casualty of the warming climate — and left a basin that fills with rainwater and snowmelt each spring and summer.
When the water creates enough pressure, it forces its way under or around the ice dam created by the Mendenhall Glacier, enters Mendenhall Lake and eventually flows down the Mendenhall River, as it did Tuesday.
Before the basin began overtopping, the water level was rising rapidly — as much as 4 feet (1.22 meters) per day during especially sunny or rainy days, according to the National Weather Service.
The city saw successive years of record flooding in 2023 and 2024 — with the river last August cresting at 15.99 feet (4.9 meters), about 1 foot (30 centimeters) over the prior record set a year earlier — and flooding extending farther into the Mendenhall Valley. This year’s flooding was predicted to crest at between 16.3 and 16.8 feet (4.96 to 5.12 meters).
In 2024, nearly 300 homes were damaged.
A large outburst can release some 15 billion gallons of water, according to the University of Alaska Southeast and Alaska Climate Adaptation Science Center. That is the equivalent of nearly 23,000 Olympic-size swimming pools.
During the 2024 flood, the flow rate in the rushing Mendenhall River was about half that of Niagara Falls, the researchers say.
City officials responded to concerns from property owners this year by working with state, federal and tribal entities to install a temporary levee along roughly 2.5 miles (4 kilometers) of riverbank in an attempt to guard against widespread flooding.
The 10,000 “Hesco” barriers are essentially giant sandbags intended to protect more than 460 properties completely during an 18-foot (5.5-meter) flood event, said emergency manager Ryan O’Shaughnessy.
The U.S. Army Corps of Engineers is at the start of what is expected to be a years-long process of studying conditions in the region and examining options for a more permanent solution, such as a levee.
The timeline has angered some residents, who say it is unreasonable.
Outburst floods are expected to continue as long as the Mendenhall Glacier acts as an ice dam to seal off the basin, which could span another 25 to 60 years, according to the university and science center researchers.
