Suicide Basin levels drop, signaling a possible new release above Mendenhall

Water levels inside Suicide Basin dropped sharply Tuesday morning, and the National Weather Service says that may mean another glacial outburst release has begun above Mendenhall Lake.

Cameras and water level sensors in the basin showed it falling nearly three feet in two hours. Forecasters, hydrologists and their partner agencies are assessing what that means.

Nothing is flooding. Mendenhall Lake sat at 2.8 feet Tuesday morning, far below its 9-foot flood stage, and the river gauge near Auke Bay was under 3 feet with a forecast peak of about 3.2 feet Thursday — well short of the 8-foot action stage.

A secondary release could come at any time. The basin has filled fast, rising 37 feet in a week. But secondary releases have historically been far smaller than the main summer events: the last two, in October 2024 and mid-September of last year, crested at 10.77 and 9.63 feet — above flood stage, but nowhere near what Juneau saw in August.

Those main releases are why the valley pays attention. Suicide Basin, held back by glacier ice beside the Mendenhall Glacier, has produced outburst floods nearly every year since 2011, and they have been getting worse. The 2024 release set a record at 15.99 feet and damaged about 290 homes. This August's crested at 14.7 feet after Unified Command asked residents of roughly 2,000 Mendenhall Valley homes to evacuate — though it came in below the 16-foot-plus crest forecasters had feared.

The Army Corps of Engineers says the retreat of Suicide Glacier and changes to the Mendenhall have made valley flooding both more likely and more severe.

Readings and updates are posted at weather.gov/ajk/suicidebasin.