Glacier-dammed outburst flooding occurred in Alaska on Saturday leading to record-breaking flooding along the Mendenhall River. Aug. 2023.
JUNEAU – Many residents of Mendenhall Valley were unable to return home on Tuesday as the nearby Mendenhall River surged with glacial lake water and flooded their neighborhoods.
Located on the north side of Juneau, those neighborhoods experienced extensive property damage, displacing many of its residents, according to City and Borough of Juneau officials.
Photos of those Alaska neighborhoods show homes turn into islands, as driveways and roads became immersed, and vehicles with floodwater high enough to reach the windshield.
What caused the Mendenhall River flooding?
The flooding event was caused by a confluence of factors.
First, the Mendenhall River, which runs next to the flooded neighborhoods, rose to record levels when it crested at 15.99 feet at 3:15 a.m. local time. For reference, the major flood stage for the Mendenhall River is 14 feet.
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This water level rise happened due to a phenomenon known as a “glacial lake outburst” that occurred upstream, specifically in the Suicide Basin and Mendenhall Glacier.
Map showing locations of Mendenhall Valley, Mendenhall River, Mendenhall Lake, Mendenhall Glacier and Suicide Basin.
(Climate.gov / FOX Weather)
Suicide Basin, a depression in the earth created by a receding Suicide Glacier, collects rain and meltwater from the glacier over the year. That water was initially kept in place by the Mendenhall Glacier.
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Oftentimes, when the basin fills with water, a channel forms through a lower level of the Mendenhall Glacier to release the water downstream into Mendenhall Lake, which feeds the Mendenhall River.
What happened recently, however, is that the water that collected in the Suicide Basin overtopped or flowed over the Mendenhall Glacier, according to the NWS Juneau.
Mendenhall Glacier.
(City and Borough of Juneau / FOX Weather)
This rushing water is what caused the Mendenhall Lake to rise to nearly 10 ft and the Mendenhall River to rise to record levels and flood neighborhoods.
NWS Juneau officials said the river level will fall out of the major flood stage. As of late Tuesday morning, the river level was measured to be nearly 12 feet and continues to drop.
Record-setting glacial lake outbursts
Flooded neighborhood due to rising water level of Mendenhall River.
(City and Borough of Juneau / FOX Weather)
NWS Juneau officials said flooding along Mendenhall River and Mendenhall Lake due to glacial lake outbursts from Suicide Basin has happened multiple times each year since 2011.
In the summer of 2023, a glacial lake outburst set a record when it caused the Mendenhall River to crest near 14.82 feet at 12 a.m. on Aug. 6.
One year to the date by a few hours, the glacial lake outburst this week broke the record.