Jökulhlaup is an Icelandic term that means a glacial outburst flood. Due to global warming, the melt from alpine glaciers accumulates under the ice stream and is occasionally discharged. Once the meltwater builds, the ice above is lifted, and the water either moves outwards from pressure or becomes an underwater lake. Thinning ice rivers are repeatedly raised first and may result in the meltwater moving up the glacier until a channel forms and will burst with the release of water flowing downstream in raging torrents. This phenomenon most recently occurred at the Mendenhall Glacier, which sits high above sea level elevation in the City of Juneau.
Flooding from the glacier has been happening every summer since 2011, but this weekend’s overflow smashed previous water-level records by nearly three feet. “It really exceeded our expectations,” said National Weather Service meteorologist Andrew Park, adding that it is “pretty devastating for the community.”
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Violent flooding from the basin above the glacier began on a perfectly dry day in 2011. Park said meteorologists and hydrologists were at first baffled as to why the river was rushing when there had been no rain. But then they found that a basin above the Mendenhall Glacier was creating enough pressure to push its water through the glacier and into Mendenhall Lake, Park said, causing an entirely new flooding threat to the community.
Every year, the basin fills until the pressure causes the water to look for a path out.
“Water finds a way,” Park said. “It finds the weakest point.”
The meltwater from the Mendenhall Glacier and Mendenhall Lake collects in what is named the Suicide Basin,
The National Weather Service writes an update on the basin levels:
Current lake level as of 5:15 PM Sunday evening is 7.22ft with levels receding.
Waters continue to recede. Although flooding is no longer expected to pose a threat, hazards persist in and near the Mendenhall River. Please continue to heed remaining road closures, stay out of the river and away from unstable banks and give plenty of space for resources to respond and provide assistance.
The Mendenhall Lake gauge crested at 11:15 PM Saturday night with a max level of 14.97 feet which is well above the previous record stage of 11.99 feet set in July 2016.
Significant flooding was reported with water in areas that previously have not seen flooding. Significant bank erosion has been reported as well with a few structures lost to the river. Tree-fall and debris are in the river. The US Coast Guard recommend mariners use caution in the affected areas.
Please check back for updates as this event unfolds.
Digital Journal on the Economic hit to tourism for Juneau residents.
The Mendenhall Glacier is melting quickly in response to a warming climate. The Mendenhall Glacier has retreated approximately 2.5 miles since its most recent maxima during the Little Ice Age in the mid-1700s, according to the U.S. Forest Service.
Today, the glacier is receding so quickly that by 2050, it might no longer be visible from the visitor center it once loomed outside.
Juneau is the second-largest city in the United States by area. As of the 2020 census, the City and Borough had a population of 32,255. And between the months of May and September, the city experiences an influx of about 6,000 people from cruise ships every single day.
Juneau sits on the mainland of Alaska at sea level, with tides averaging 16 feet (5 m), right below steep mountains about 3,500 to 4,000 feet (1,100 to 1,200 m) high. On top of these mountains is what is known as the Juneau Icefield.
The Juneau Icefield is a large ice mass from which about 30 glaciers flow; two of these, the Mendenhall Glacier and the Lemon Creek Glacier, are visible from the local road system in Juneau.
From the Alaska Coastal Rainforest Center
Rappelling down rock cliffs, rafting across glacial lakes, and traversing icy crevasses, all in the presence of a massive, ever-changing glacier – it's all part of the job for glaciologists monitoring glacial outburst floods (also known as jökulhlaups) in Alaska.
In July of 2018, Suicide Basin, a dead ice branch off of the Mendenhall Glacier in Juneau, unleashed more than one billion cubic feet of water (the equivalent of over ten thousand olympic-sized swimming pools) into the Mendenhall River, which flows through the highly populated Mendenhall valley. Such outburst flood events have occurred almost every year since 2011, when the glacier-dammed lake first began forming during the summer months.
Glacier lake outburst floods, which occur when water dammed by a glacier suddenly releases and floods downstream areas, happen across Alaska and in glacial landscapes around the world. Outburst floods can impact infrastructure, such as bridges and roads, and population centers. These floods often catch communities off guard because they often occur in remote locations that are difficult to monitor. In Juneau, AK CASC and ACRC scientists Eran Hood, Christian Kienholz, and Gabriel Wolken are collaborating with teams from the National Oceanic and Atmospheric Administration (NOAA) and the U.S. Geological Survey (USGS) who have been monitoring Suicide Basin for several years. The goal of their work is to better understand and predict the outburst floods that are released from Suicide Basin. "Combining resources and personnel across agencies allowed us to dramatically expand our campaign of geophysical measurements and flood monitoring in the basin this season," said Kienholz, who visited the basin more than ten times, mostly with Jamie Pierce from the USGS.
Check out the erosion.