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Juneau's Mendenhall River crests after glacial outburst; flooding considered minor

  • Author: Pat Forgey
  • Updated: September 28, 2016
  • Published July 11, 2014

JUNEAU -- A surge of water released from behind an ice dam flowed through Juneau this week but crested late in the day Friday and had emergency officials breathing a sigh of relief.

"We did see a few homes, in areas that we're used to, get flooded, but it looks like the event has crested and started to subside," said Tom Mattice, emergency manager for the City and Borough of Juneau.

"We still have resources in the field, but it looks like we're on the right side of the curve," he said.

Meltwater from Suicide Glacier, dammed up behind the Mendenhall Glacier in Suicide Basin began releasing this week in what's known as a "glacial outburst," an event that's rare above a populated area. That influx of water brought Mendenhall Lake to a record high level and swelled the Mendenhall River to within inches of a record.

Friday, as flooding river water lapped against the foundations of houses along the river, the city instituted "soft" closures of numerous streets and warned about the dangers of driving or wading through floodwaters. The U.S. Forest Service, without residents to contend with, simply barred access to dangerous areas.

But late Friday afternoon, even while heavy rain fell, the tide turned.

"It's over," said Eran Hood, University of Alaska Southeast professor of environmental sciences, who has been studying and monitoring glacial outburst floods.

"It crested at 4:45 p.m., which is pretty good -- we'd guessed 4-6 p.m.," he said.

Forecasters are now saying the river is not expected to inundate any new areas and will steadily decline. Friday's rain will extend that only slightly, they said.

The first outburst flood from the glacier in 2011 took Juneau by surprise, but a unique monitoring program led by the University of Alaska Southeast now provides warning data from sensors deep underwater in Suicide Basin.

Different agencies, including the U.S. Geological Survey and NOAA's River Watch Center in Anchorage, and local National Weather Service experts each developed models to tell Juneau what to expect.

"In general their models held up very well and allowed us to gear up response," Mattice said.

He contrasted that with 2011, when there were no sensors and no awareness that the shrinking Suicide Glacier now placed Juneau at risk of glacial outburst floods.

"I was woke up at 5:30 a.m. with news of the flooding, and it crested about noon," Mattice said.

That year, the ice dam had already burst before Mattice went to bed, but no one even knew it was there.

This year, the sensors allowed Mattice and others to watch the water building to dangerous levels, something it doesn't always do. When it got high enough, they knew they'd be facing a flood, but they didn't know when.

"We had multiple days to go and knock on doors in neighborhoods and let people know and let the community know," Mattice said.

Then, as the Suicide Basin sensors showed levels falling earlier this week, they knew the outburst had begun and to expect a flood within a day or two.

And monitoring Suicide Basin levels enabled the agencies to predict the size of the flood. Hood said the 2011 flood was estimated at 9.8 billion gallons, and even though Suicide Basin's level was slightly lower this year, he estimated this year the basin held more water and less ice.

"Our guess was probably 1 or 2 billion gallons more," he said.

The river flow went from about 8 feet earlier in the week to 13.52 feet Friday. Record river height was 13.75 feet. But the river flow was much more dramatic than its height, surging from 2,650 cubic feet per second before the outburst to 16,800 cubic feet per second at its peak Friday.

Mendenhall Lake, from which the river flows, peaked at 11.81 feet Friday, above its record 11.1 feet.

Mattice praised the university scientists and other agencies and companies that helped provide Juneau with the warning system.

"We bought them some equipment, but we could not do this without the partnerships with the University of Alaska Southeast, the USGS, the National Weather Service and the River Watch Center in Anchorage," he said.

While they contracted for some flights for installing and maintaining the sensors, helicopter tour companies were helpful in transporting personnel and equipment and in basin observations during deadhead flights, Mattice and Hood said.

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