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Scientists wary as water and ice build behind glacier dam above Juneau

Pat Forgey
A Mendenhall Glacier dam is holding back rainwater, snowmelt and ice filling Suicide Basin -- water that may soon surge toward town. Photo by Aron Stubbins, Skidaway Institute of Oceanography

JUNEAU -- It's been pouring rain in Juneau lately, which has some people bracing for a new and unusual kind of flooding precipitated by climate change.

High above Juneau's populous Mendenhall Valley, Suicide Basin is filling with rainwater, snowmelt and ice, which is being held back by a dam of the Mendenhall Glacier.

Sometime soon, the water may come surging toward town.

Scientists studying the glacier say there are indications the dam will soon release and send billions of gallons of water surging toward the lake at the terminus of the glacier, then into the Mendenhall River.

Eran Hood of the University of Alaska Southeast is part of a team watching the basin and says a release appears to be imminent.

"It should be substantial this year," he said.

A depth gauge in the basin appeared to show the water plateauing at about 88 feet followed by a few inches of decline, which might indicate an outburst is beginning, Hood said.

The first such release of water from Suicide Basin came in 2011. It's called a glacial outburst flood, or sometimes a "jökulhlaup," an Icelandic word for the event from a country that's been dealing with them for many years. They're less common in the United States, and much less common near populated areas.

The surprise 2011 outburst caused minor flooding in some low-lying Mendenhall Valley neighborhoods, resulting in cautionary evacuations, power outages and sandbagging but little damage.

In 2012 there was a small outburst, but in 2013 the basin didn't fill at all.

Scientists and emergency managers are trying to figure out how big such a flood could be. They think Suicide Basin may now hold as much water as it did in 2011, but even that doesn't provide much certainty about how big a flood would be.

First off, glacial outbursts don't so much "burst" the dam as they do slowly find a way under and through the glacier, while flowing faster and faster.

"It's like if you pulled the plug in your bathtub and as the water was draining the drain hole got bigger and bigger," Hood said.

 It may take many hours for the outburst to reach maximum flow, and a few days for the entire basin to drain.

Suicide Glacier once flowed perpendicularly into the main Mendenhall Glacier, but it and many other Juneau Ice Field glaciers are retreating. The basin that was once filled with ice is now holding water.

"The main glacier is acting as a dam for water that is trying to drain out of the basin. Basically what happens is you build up enough hydrostatic water pressure so it can lift the main glacier," Hood said.

Even with water pressure sensors installed after 2011, the uncertainties make planning a response difficult, said Tom Mattice, emergency manager for the City and Borough of Juneau.

"There are so many variables in how fast that hole will open up," he said.

They don't even know how high the water level in Suicide Basin was in 2011, let alone how much of what was in the basin was liquid and how much was ice. They do know from measuring downstream river flows that about 9 billion gallons of water were released.

The city helped fund the monitoring stations, which transmit data back to the city, university, National Weather Service and U.S. Geologic Survey.

"Most of Suicide Basin is under ice," Hood said. "But there are huge cracks, and you can see water coming up between the cracks, and around the margins there are some big leads of open water, which is where we have our sensor."

That likely means there's more water in the basin than the gauge would suggest, Mattice said.

"As the ice thins, at the same gauge number there's less ice and more water," he said.

Hood said Wednesday that his guess is the basin now holds about as much water as it did in 2011, when there were no measurements.

"We don't know how much loss of ice there has been," he said. "Its been thinning, and potentially that leaves more room for water to fill in the basin, but we don't have an exact measurement on that."

Another variable that may directly affect the size of the flood is the levels of Mendenhall Lake at the glacier terminus and the Mendenhall River that flows from it.

In 2011, the river was flowing below normal. Now, following a record rainy June and more rain since, it's above normal.

In 2011, the Mendenhall River was flowing at just over 2,000 cubic feet per second, while now it is at just more than 4,000 cubic feet per second, he said.

"The baseline flow is already much closer to flood stage," Hood said.

Glacier outburst floods happen elsewhere in Alaska, including on the Tulsequah River, also flowing out of the Juneau Ice Field, and on the Kenai Peninsula, but are much farther from residences.

Contact Pat Forgey at pat@alaskadispatch.com.