Flood — Juneau Borough, Alaska

2020-08-01 · near Douglas, Juneau Borough, Alaska

Event narrative

This event spanned the months of July and August. In the morning hours of July 30th the Mendenhall Lake and River began to slowly rise, the lake had been on a slow up tick as water from Suicide Basin was over topping the ice dam of the Mendenhall Glacier. Air reconnaissance during the afternoon hours of July 30th over Suicide Basin, which is about 2 miles from the terminus, in the afternoon of July 30th indicated that water had begun to drain from the basin subglacially. Mendenhall Lake and River continue to rise for the next day and a half as the glacier dammed lake emptied. The water levels reach minor flood stage of nine feet by the afternoon of July 31st morning and by 245am AKDT on August 1st Mendenhall Lake crested at eleven point fifty-five, one and half feet above moderate flood stage of ten feet. This was the third highest peak crest on record for this site to date. After the lake and river crested the water level receded slowly which is atypical for these types of events. Water levels went below minor flood stage of nine feet around 445pm AKDT on August 1st. There was minor to moderate flooding around Mendenhall Lake and the upper portions of the river above Back Loop Road. The Mendenhall campground was evacuated due to flooding of up to three feet at the entrance and within the campground with some minor damage done to the road system. The West Glacier Spur Road and the area around Skaters Cabin was flooded with three feet water. View Drive was flooded with two feet of water and the road cut off. There was also more flooding along the Dredge Lakes trail area and and few other trails around the Mendenhall Glacier Visitor Center run by the USDA Forest Service.

Wider weather episode

Throughout spring and the first part of the summer Suicide Basin, a side valley that is dammed by the Mendenhall Glacier, filled with water from snowmelt and rainfall. In the afternoon hours of July 30th visual reports showed signs of the basin beginning to drain sub-glacially as the dam was starting to fail. This is after a few days of water overtopping the ice dam. The Mendenhall Lake gage started to show this inflow of water from the basin through the evening hours and by the morning of the 31st the event was well on its way. Over the course of event Mendenhall Lake and River rose about six feet and crested at 11.55ft in early morning of August 1st. This was the 3rd highest crest on record for Mendenhall Lake.

View location on OpenStreetMap → (58.4285, -134.5855)

Source: NOAA Storm Events Database, event_id 915695. Narrative written by the NWS forecast office that issued the report.