JUNEAU, Alaska — Suicide Basin, the source of Juneau’s annual glacial outburst floods, was full as of 9 a.m. Tuesday, putting the Mendenhall Valley on standby.

With the basin full, the 2026 flood is expected to occur within one to six days. The flood begins when water from Suicide Basin, which is dammed by the side of the Mendenhall Glacier, lifts part of the glacier, creating a channel underneath the ice that sends billions of gallons of water into the Mendenhall River. The river runs through the Mendenhall Valley, Juneau’s largest residential area.

The City and Borough of Juneau has alerted residents in the Mendenhall Valley flood zone to be prepared for an evacuation advisory and have a go-bag ready. The evacuation zone this year encompasses approximately 1,900 residences.

Residents can monitor the situation through the city’s Emergency Flood Response page and the National Weather Service’s Suicide Basin monitoring page. Residents can also sign up for text alerts and enter their address to see whether they are in the flood advisory evacuation zone.

The Mendenhall River’s current water level is about 4 feet, below typical levels for early August.

For comparison, in 2025, an atmospheric river pushed the Mendenhall River to about 9 feet before the outburst flood began. Last year, Suicide Basin filled on Aug. 10. After the glacial dam burst, the Mendenhall River reached a record peak water level of 16.65 feet on Aug. 13, 2025.

The National Weather Service in Juneau is forecasting that the river will peak at about 16.3 feet, according to senior meteorologist Dan Slagle. The weather service is also forecasting a full release from Suicide Basin, similar to last year, but the exact volume remains unclear.

“We are in a little better starting point now because of the drier weather we’ve seen in July and August. But I think overall, that doesn’t contribute as much as how much (water) releases from the basin itself,” Slagle said.

The 2025 flood damaged about 50 homes. Flood impacts were mitigated in part by HESCO barriers — wire and fabric flood walls filled with sand — installed along parts of the river. This year, the city expanded its HESCO barrier system by about 30% and deployed pumps to evacuate water that breaches the barriers.

In 2024, the river’s water level reached 15.99 feet and damaged nearly 300 homes. The city did not have HESCO barriers in place that year.

Officials have also warned residents to stay back from HESCO barriers during the flood. A video from last year shared by the City and Borough of Juneau depicts a tree swept downstream in the Mendenhall River striking and breaching a barrier.

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