Major flood outburst from Mendenhall Glacier in Alaska not possible without climate change

An outburst flood from a glacial meltwater lake sent record amounts of water into Mendenhall Lake and down Mendenhall River toward Juneau, Alaska, between August 4 and August 6, 2023.

Significant flooding was reported in areas along the river that have not seen flooding from these types of events in the past, the National Weather Service (NWS) office in Juneau said. Flood waters swept trees and buildings into the river as significant erosion occurred.

The Mendenhall Glacier fills a large valley north of Juneau, Alaska, and creates 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 on a yearly basis. NOAA Climate.gov image, based on Landsat satellite data from May 20, 2019.

“Decades worth of erosion happened in one weekend,” Rick Thoman, Alaska Climate Specialist at the Alaska Center for Climate Assessment and Policy told Climate.gov. “The buildings that fell into the river and those that are uninhabitable now, were not right next to the river on Friday afternoon.”

The Mendenhall Lake gauge crested at 14.97 feet August 5 around 11:15 pm AKDT, smashing the previous record by nearly 3 feet (11.99 feet in July 2016) and exceeding the initial forecast of 12.3 feet from the Alaska-Pacific River Forecast Center in coordination with NWS Juneau.

At the height of this event, the United States Geological Survey (USGS) gauge at Mendenhall Lake recorded a streamflow of 25,200 cubic feet per second (cfs), which is “historic,” Andrew Park, a Meteorologist at NWS Juneau told Climate.gov. According to USGS data the previous maximum streamflow only reached 16,300 cfs in 2016.

Streamflow at Mendenhall Lake during the previous record-high outburst flood, in 2016 (blue line), and during the much larger event on August 4-6, 2023 (orange line). In 2016, the lake in Suicide Basin drained several times, though later events were smaller. NOAA Climate.gov image, based on data from the U.S. Geological Survey.

Park went on to put the numbers into perspective, saying that a streamflow of 17,100 cfs has 1% chance of occurring in a given year and a streamflow of 26,000 cfs has a 0.2% chance of occurring in a given year, based off of data from a 2020 FEMA flood insurance study.

A flood so extreme that it has only a 0.2 percent chance of happening each year is sometimes called a “1-in-500-year flood.” But a 1-in-500-year event does not mean that these events can only happen once every 500 years, just that over long periods of time, the average return frequency would be every 500 years. The flooding following this outburst was equivalent to a 500-year event through the Mendenhall Valley. *

This event released a torrent of water downstream from the Suicide Basin, into the Mendenhall Valley, which is Juneau’s most heavily populated neighborhood, according to the Alaska Coastal Rainforest Center (ACRC).

Over two days in early August, a glacial lake on a tributary of the Mendenhall Glacier burst through its ice dam and drained into the Mendenhall River. Time-lapse images from a remote camera captured the dramatic drop in lake levels. NOAA Climate.gov movie, based on images from the National Weather Service forecast office in Juneau, Alaska.

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