With the summer heat rising and water levels climbing in Suicide Basin, emergency officials, city officials and the National Weather Service are closely monitoring the threat of another glacial lake outburst flood (GLOF) in the Mendenhall River Valley. This threat follows a record flood event on August 6, 2024, which swept through neighborhoods, damaged property, and reshaped how Juneau prepares for these recurring flooding events.
Suicide Basin, located above the Mendenhall Glacier, has released floodwaters nearly every year since 2011. These events occur when meltwater trapped behind the glacier breaks through ice dams, sending torrents into Mendenhall Lake and River below.
As of Monday, July 7, the U.S. Geological Survey (USGS) reports that the basin’s laser-monitored water elevation is approximately 1,224 feet, well below the 1,247 feet recorded on the same date last year, and more than 100 feet below 2023 levels. The current elevation is about 147 feet below the overflow channel, which begins spilling at 1,371 feet.
If the rate of rise in the basin remains around 4 feet per/day, this would result in a full basin in 37 days, though the rate of the rise could change.
These differences in water levels from 2024 and 2023 to current levels are due to the cold spring/early summer time temperatures. Freezing levels remained 3000 to 4000 feet through the spring and early summer time but are now around 7000 feet, meaning rapid melt could accelerate water accumulation.
Monitoring equipment, including two daily cameras and a USGS laser sensor, remains active. Officials caution that water level data may show occasional jumps or drops due to icebergs disrupting sensor readings.
City and Borough of Juneau (CBJ) officials, in partnership with state, federal, and tribal agencies, have implemented Phase 1 and Phase 1A of a near-term flood mitigation strategy. With guidance from the U.S. Army Corps of Engineers, CBJ has installed HESCO barriers along vulnerable stretches of the Mendenhall River. These modular barriers are designed to protect against floodwaters as high as 18 feet.
Given the growing threat and lack of a permanent solution, officials urge Juneau residents, especially those in the Mendenhall Valley, to take stay informed and prepared.