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The threat of so-called glacier outburst flooding in parts of Juneau since 2011. The Mendenhall Glacier — a thinning, retreating glacier that is a in southeast Alaska — acts as a dam for a basin that fills each spring and summer with rainwater and snowmelt. The basin itself was left behind when a smaller, nearby glacier retreated.

When the water in the basin creates enough pressure, it forces its way under or around the ice dam created by the Mendenhall Glacier, entering Mendenhall Lake and eventually the Mendenhall River.

The water level in the basin as of Friday stood at 1,353 feet (412 meters) and continues to rise, said Nicole Ferrin with the National Weather Service. It's just 15 feet (1.5 meters) from topping the ice dam.

The basin has been rising at an unpredictable rate, so there is no way to pinpoint when water would go over the top, she said.

“Some days when there's no rain, it only rises by a foot (0.30 meters),� she said. “Other days when there is heavy rain or a lot of sunshine, it rises by 4 feet (1.22 meters), so it's a variable.�

In some years, there has been limited flooding of streets or properties near the lake or river. But and marked successive years of record flooding, with the river last August cresting at 15.99 feet (4.9 meters), about 1 foot (0.3 meters) over the prior record set a year earlier, and flooding extending farther into the Mendenhall Valley.

Last year, were damaged in the flooding.

Capital City Fire/Rescue Chief Rich Etheridge warned people to stay away during flood stages because logs and other debris in the river can put would-be rescuers “at extreme peril.�

Once the water tops the dam, the city will set up a hotline where people can get information.

A large flood event can release up to 15 billion gallons of water, according to the University of Alaska Southeast and Alaska Climate Adaptation Science Center. That’s the equivalent of nearly 23,000 Olympic-size swimming pools. During last year's flood, the flow rate in the rushing Mendenhall River was about half that of Niagara Falls, the researchers say.

City officials responded to concerns from property owners this year by working with state, federal and tribal entities to install a temporary levee along roughly 2.5 miles of riverbank in an attempt to guard against widespread flooding. The installation of about 10,000, four-foot (1.2-meter) tall barriers is intended to protect more than 460 properties from flood levels similar to last year, said Nate Rumsey, deputy director with the city's engineering and public works department.

The U.S. Army Corps of Engineers is at the start of what's expected to be a yearslong process of studying conditions in the region and examining options for a more permanent solution. The timeline has angered some residents, who say it's unreasonable.

Outburst floods are expected to continue as long as the Mendenhall Glacier acts as an ice dam to seal off the basin, which could span another 25 to 60 years, according to the university and science center researchers.

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Associated Press writer Becky Bohrer in Juneau contributed to this report.