Flood — Juneau Borough, Alaska

2023-08-05 to 2023-08-06 · near Douglas, Juneau Borough, Alaska

Event narrative

The level of Mendenhall Lake was near normal levels of 5.5 feet before this event started, late Thursday August 3rd, and by Saturday morning water levels were over moderate flood stage of 10 feet. By this time areas near Skaters Cabin Rd, View Dr, and the Mendenhall Glacier Visitor Center were experiencing moderate flooding impacts with the Mendenhall Lake campground being evacuated. By midday on Saturday, the water levels exceeded the previous record from 2016 of 11.99 feet and impacts were increasing along the Mendenhall River. Homes along View Dr were cut off due to the significant amount of flooding over the road along with flood waters from 1 to 3 feet deep. Areas below the Mendenhall Back Loop Road bridge started to see flooding of yards and increasing bank erosion in the afternoon of August 5th.

The river continued to rise at a fast pace through the day into the evening hours and the City of Juneau then set up their Emergency Operations Center. In the evening hours, many areas (roads and homes) below Mendenhall Back Loop Road bridge along the Mendenhall River were experiencing moderate flooding along with significant bank erosion of 50 to 200 feet. From that bank erosion one home fell into the river with another large 3 story condominium building was evacuated as it became compromised and could also fall into the river. Going into the late evening hours a portion of another house fell into the river with many other homes along the river being undermined and at risk of falling into the river. Homes and roads around Meander Way, Marion Dr, River Court and Killewich Rd flooded with moderate impacts in the late evening and the local electric company cut power to those residences due to flood waters being 2 to 4 feet deep.

The river crested near 14.97 feet (preliminary crest from the USGS) around 11:30 PM AKDT on August 5th. At the peak of the event there was 3 to 7 feet of water in and around homes along View Dr. The Mendenhall Lake and River were back below minor flood stage of 9.0 feet by 9:45 AM AKDT on Sunday, August 6th.

While other locations in the Mendenhall Valley, above Mendenhall Back Loop Road bridge, have experienced flooding impacts from these types of events in the past, they saw major damage this time around. This event was 3 feet higher than the previous record from 2016 and the Mendenhall Lake and River rose approximately 9.42 feet.

In the end there were about 48 homes affected with 13 destroyed, 8 with major damage and 7 with minor. This historic event saw the largest amount of water released from Suicide Basin in a single event and approximately 14 billion gallons of water came out of the basin and into the Mendenhall Lake and River system.

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 3 other years (2018, 2019 and 2020), was full to the top of the Mendenhall Glacier (top of the ice dam) and started to flow down a side drainage channel. After about 7 days, Suicide Basin began to drain sub-glacially. Mendenhall Lake started to rise from the additional water from the basin Thursday evening into the early morning of Friday, August 4th. In the morning hours of Friday, August 4th visual reports from the USGS Mendenhall Lake gage and webcam confirmed that the basin had started to drain. The Mendenhall Lake gage steadily rose through Friday, then the rate of rise increased 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 an impressive nine and a half feet over less than two days, breaking all previous records, and causing significant impacts. This flooding event was historic due to areas seeing flooding that usually do not flood along with the severity of the flooding and the rate of riverbank erosion. There were moderate to major flooding impacts in the Mendenhall Valley, where a majority of Juneau's population resides, and historically does not flood.

View location on OpenStreetMap → (58.4453, -134.5638)

Source: NOAA Storm Events Database, event_id 1127010. Narrative written by the NWS forecast office that issued the report.