Alaska’s capital city of Juneau is urging many residents to evacuate, bracing for the arrival of what could be record floodwaters flowing downstream from a basin dammed by the area’s Mendenhall Glacier, with the event being driven by climate change amid glacial retreat, according to a federal agency.
Summer glacial flooding, known as a glacial lake outburst flood, or GLOF, threatens parts of the city due to a combination of rainwater and snowmelt.
Authorities say flooding from the Mendenhall River will probably crest around 4pm Alaska time, or 8pm ET on Wednesday. They hope that recently installed emergency flood barriers will hold back the waters and protect Mendenhall valley, where a majority of Juneau’s 32,000 full-time residents live.
On Tuesday morning, authorities confirmed water had started escaping the ice dam, with flooding expected into Wednesday. Some Juneau residents in the flood zone have already evacuated as officials intensified their warnings on Tuesday, saying “Don’t wait, Evacuate TONIGHT.”
The National Weather Service (NWS) office in Juneau said in an X post late on Tuesday that local hydrologists had adjusted their Mendenhall flooding forecast, anticipating that the river would crest at over 16ft on Wednesday morning.
Nicole Ferrin, with the NWS, said during a briefing on Tuesday that the flood warning was issued after “a lot of analysis” but the calculations were complicated by rainfall causing significant rising of the lake and river and confirmed that a sub-glacial release had occurred.
“This will be a new record based on all of the information we have,” Ferrin said, according to the Juneau Empire.
The Mendenhall Glacier fills a large valley north of Juneau, creating an ice dam for a meltwater lake that fills Suicide Basin. Since 2011, outburst floods from the depression have been pouring into Mendenhall Lake and rushing down the river toward Juneau each year.
But the annual Mendenhall glacial lake outburst flood is judged to be intensifying as a result of climate change.
The National Oceanic and Atmospheric Association (Noaa), said in a statement that Alaska had warmed twice as fast as the rest of the US over the last several decades.
Over the last century Alaska’s average annual temperature has risen 3.1 degrees fahrenheit and the overall trend continues to increase, according to data from Noaa’s National Centers for Environmental Information.
Scientists have attributed the retreat, melting and thinning of glaciers over the last century to Earth’s warming climate. Alaska’s glaciers are among the fastest-melting glaciers on Earth and have been in steep decline since the late 1980s, according to the Alaska Climate Science Center.
On the record amounts of water now threatening Juneau, Rick Thoman, Alaska climate specialist at the Alaska Center for Climate Assessment and Policy, told Climate.gov, Noaa reported: “Without climate change, there is no reason to think that this would be happening on the Mendenhall Glacier, then in the lake, and downriver.”
An outburst in August 2023 sent record amounts of water into Mendenhall Lake and down Mendenhall River toward Juneau, inundating areas that had not experienced flooding before, the NWS office in Juneau said at the time, causing significant erosion.
The Mendenhall Glacier is about 12 miles from Juneau and considered a popular tourist attraction. Juneau lies 800 miles from Anchorage, where Donald Trump and the Russian president, Vladimir Putin, are due to meet on Friday to discuss the war in Ukraine.
Flooding from the glacier has become an annual concern for almost 15 years, and in recent years has swept away houses and swamped hundreds of homes.
It is blamed on the retreat of a smaller glacier near Mendenhall Glacier – a casualty of the heating 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 blue ice Mendenhall glacier.
The Mendenhall was originally named Sitaantaagu (“the Glacier Behind the Town”) or Aak’wtaaksit (“the Glacier Behind the Little Lake”) by the Tlingit Indians, but later called Auke (Auk) Glacier, for the Auk Kwaan band of Tlingit Indians, by naturalist John Muir. It was renamed in 1892 for Thomas Corwin Mendenhall, who had helped determine the boundary between Canada and Alaska. It is considered a relic of the little ice age that lasted until the mid-18th century and is now receding at about 100 to 150 feet a year.
The Associated Press contributed reporting
