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As residents of southeast Alaska welcomed the return of sunny, warm weather this week after two months of heavy rains, they probably did not expect floods to be in the forecast. But Juneau’s Taku River reached a new record flood level Wednesday morning, even with blue skies overhead. 

The flooding was not from a storm. Instead, water from a glacial lake caused the river to rise after it burst through the ice dam keeping it in place.

Scientists predict sunny day floods like this one will be more common in glaciated landscapes as climate change not only leads to more extreme weather events but also reshapes the dynamics of glacier meltwater after ice retreats. In Juneau, researchers from the University of Alaska Southeast, the U.S. Geological Survey (USGS) and the National Weather Service are working to determine where the newest glacier lake floods will surge, and what threats they pose to communities downstream of the ice flow.

The aptly-named “Lake No Lake,” because of its yearly pattern of appearing and disappearing, sits at the toe of the Tulsequah Glacier. Lake No Lake drains annually into the Taku River when water pressure and warm temperatures wear a hole in the surrounding barrier of ice. What made the flow so strong this time, however, was the combination of the ice dam break with large amounts of meltwater from the winter’s deep snowpack and unseasonably heavy rains.

Aaron Jacobs, senior service hydrologist and meteorologist at the National Weather Service in Juneau, said the cause of Wednesday’s flood really began in the winter, when snow accumulation exceeded what the region normally gets. In addition, the snowpack, 175 percent to 200 percent deeper than usual, was slow to melt during Alaska’s cooler spring this year. But abnormally hot temperatures in June triggered rapid snowmelt high in the glaciated mountains above the coastal towns, causing rivers and creeks to overflow.

Then, an intense rainstorm known as an atmospheric river dumped even more water into the system. The extra boost from Lake No Lake’s outpouring pushed river levels over the edge.

“It was just a perfect storm in hydrology where all the right conditions were right there at the right time to produce these record stages,” said Jacobs. He also said climate change may increase occurrences like these as southeast Alaska gets warmer and wetter.

Sunny day flooding is often associated with places like Miami, Florida, where sea level rise attributed to warming can send several feet of water into the streets on days with elevated high tides. But places like Alaska, the Himalayas and the Andes may also begin to see more sunny day floods over the next century, as a result of glacier melt.

“With a warming environment or climate change, we’ve seen new glacial dams form and then release,” Jacobs said.

Emerging Patterns

In 2011, Suicide Basin, the site of another glacier lake near Juneau, emptied, causing a large flood on the nearby Mendenhall Lake and River and putting Juneau hydrologists on alert before sunny-day floods were even on their radar.

“When that event started, we didn’t know where the water was coming from,” said Jacobs.

After that flood, an interagency team of scientists formed to monitor and forecast future floods from the Mendenhall Glacier.

Hydrologist Eran Hood, a professor at the University of Alaska Southeast, was one of the first scientists on the scene in 2011. He’s been leading a study over the past 10 years to better understand when the Suicide Basin dam will break every summer, and how intensely it might flood Juneau’s neighborhoods. He and the other scientists on the team set up timelapse cameras and sensors that measure water levels in Suicide Basin. They also use drones to take photos and elevation data for mapping.

Predicting the timing, location and severity of floods remains a challenge, partly because some lakes, like Suicide Basin, are concealed underneath the ice.

Hood said, “The dynamics are all subglacial, so they’re not easy to study. We don’t have a good idea of what sets up the system to either be a big flood year or a multiple smaller flood year.”

Known as “jökulhlaups,” events like this one are a widespread occurrence across glaciated landscapes. But the outpouring floods in Juneau are of particular concern because of their proximity to residential areas. About 50 to 100 residents live along the Taku River in semi-remote cabins. One picture shared on the Juneau National Weather Service Twitter feed showed the water from a flood in 2004 carrying away a one-room cabin. That was the previous record high, when river levels reached 45.1 feet following Lake No Lake’s release. This year, the flood level peaked at 45.5 feet.

“There’s so many outburst floods that happen in the world, but a lot of them are in extremely remote locations,” Hood said. “To have one that I can pretty much see where the flood starts from my office and that has impact in [Juneau residents’] lives and their yards, it makes it more gratifying to study because there’s just so much community interest.”

Transient Trends

It’s not just lake outbursts that may lead to higher sunny day floods. As northern latitudes warm two to three times faster than the global average, the resulting physical changes to the types of ice on a glacier present another source of future floods.

Glaciers get larger when snowfall lasts from one winter to the next. The snow that sticks around for over a year is called “firn”—think of it as an intermediate state between the fluffy stuff that falls in the winter and the hard-packed ice of ice cubes. Firn is relatively porous, which means meltwater can percolate down into it and be stored rather than flowing into glacial lakes and rivers.

But as glaciers begin to retreat, there’s less firn to hold water. As a result, more meltwater runs off the older, impervious glacial ice and into the watershed.

Seth Campbell, director of academics and research of the Juneau Icefield Research Program, is studying this mechanism of increased meltwater runoff. He said he thinks of glaciers as long-term banks that store water in the form of ice. “You might have a period of years to decades that you do have greater amounts of water flushing down,” he said.

The predicted increase in glacier floods as a result of climate change is only temporary, though. Eventually, glaciers will disappear. “As glaciers retreat, you might swap back in the other direction where there’s less water over time,” Campbell said. 

The compounding factors that caused the Taku flood and the uncertainty of when and where future flooding and drying will occur all highlight a key theme of global warming: heightened instability.

“It always throws curve balls at us,” said Jacobs, “We have an idea of how it’s going to work and feel really confident of the forecast. And then all of a sudden, something else happens.”