Flood — Juneau Borough, Alaska

2025-08-12 to 2025-08-13 · near Mendenhall Valley, Juneau Borough, Alaska

Event narrative

The level of Mendenhall Lake was already above the minor flood stage of 9 feet due to an atmospheric river and heavy rain in the days leading up to the event when the outburst flood started on the morning of Tuesday August 12th. By 1200 AKST the water levels had risen 1 foot to be over the moderate flood stage of 10 feet. By this time, areas near Skaters Cabin Rd and the Mendenhall Glacier Visitor Center were experiencing moderate flooding impacts with the USFS Mendenhall Lake Campground being evacuated. As water levels continued to rise, homes along View Drive were cut off with flood waters from 1 to 3 feet deep. The rate of rise continued to increase through the evening hours with Mendenhall Lake going over the major flood stage of 14 feet at 2325 AKST. The rate of rise was higher than last year and the highest observed with an over 3000 CFS increase per hour. For context, the average flow for this time of year for the Mendenhall Lake is around 3500 CFS. The river continued to rise and went over the previous flood stage record of 16 feet around 0420 AKST on August 13th and crested at a new record of 16.65 feet (preliminary crest from the USGS) at 0615 AKST (1515 UTC). The Mendenhall Lake gauge was back below minor flood stage of 9.0 feet at 1440 AKST on Wednesday August 13th. The river system was above flood stage for approximately 38 hours.

According to the City and Borough of Juneau (CBJ), this major flooding event produced significant impacts upstream of the Back Loop Road Bridge, along with minor impacts downstream along the Mendenhall River. These impacts below back loop bridge are different from last years impacts and not as severe due to flood protection. Based on FEMA damage definitions, CBJ identified 35 homes as affected, including 11 with minor damage and 6 with major damage. The most severe flooding occurred in the View Drive area, where CBJ reported 5 homes sustained major damage after 4'5 FT of water entered living spaces. Additional homes along Meander Way (15 total) were also impacted, including 2 with minor damage and 1 with major damage caused by water seeping under or through HESCO barriers (flood protection measures). Homes near River Road were also affected, and storm drains near the Safeway parking lot at Brotherhood Bridge backed up, contributing to localized flooding. CBJ also documented severe erosion along the left bank (river left) near the Back Loop Road Bridge at the event peak. This erosion damaged the bridge approach, which was already closed, causing part of the sidewalk to fall into the river and severed telecommunication and power lines. The loss of a utility pole into the river cut power and internet connectivity to nearby homes and businesses.

Wider weather episode

Since 2011, Suicide Basin, a side valley that is dammed by the Mendenhall Glacier, fills with water from snowmelt and rainfall then drains during the summer as a glacial outburst flood. This year, the glacier dammed lake in Suicide Basin, like in 5 other years (2018, 2019, 2020, 2023 and 2024), was full to the top of the Mendenhall Glacier (top of the ice dam) and started to flow down a side drainage channel on August 10th. In past years it has take from 2 to 4 days to begin to drain sub-glacially, but this year the detection of the sub-glacier release was complicated by the concurrence of heavy rainfall from an atmospheric river. This atmospheric river produced five to nearly eight inches of rain from August 7-12. A series of weather fronts moved across the region which tapped into sub-tropical moisture and transported that moisture poleward out of the North Pacific into the Gulf of Alaska and over Southeast Alaska. Not only was there heavy rainfall but very warm temperatures increased ice melt from the Mendenhall Glacier into the Mendenhall Lake and River system which increased the stream flows above minor flood stage leading up to the release.

The USGS laser sensor within the basin started to indicate a faster water level drop by the morning of Tuesday August 12th. USGS Mendenhall Lake gauge, which was already above the minor flood stage of 9.0ft, started to rise steadily from the additional water from the basin. Visual reports from aerial reconnaissance of the basin confirmed that the basin had started to drain sub-glacially. The Mendenhall Lake gauge steadily rose through Tuesday with the rate of rise increasing exponentially as the inflow to the lake from Suicide Basin was getting larger. Over the course of this glacier dam outburst flooding event, Mendenhall Lake and River rose eight feet over less than two days and it crested at 16.6 ft on Wednesday morning August 13th. This event broke all previous records, even last years record event by just over one half foot. This flooding event caused significant impacts but it was not as widespread as last years event due to flood mitigation actions. There were moderate to major flooding impacts in the Mendenhall Valley, mainly near the View Drive area.

View location on OpenStreetMap → (58.3616, -134.5942)

Source: NOAA Storm Events Database, event_id 1292221. Narrative written by the NWS forecast office that issued the report.