Bursting ice dam in Alaska highlights risks of glacial flooding around the globe

The gray, two-story home with white trim toppled and slid, crashing into the river below as rushing waters carried off a bobbing chunk of its roof. Next door, a condo building teetered on the edge of the bank, its foundation already having fallen away as erosion undercut it.

The destruction came over the weekend as a glacial dam burst in Alaska's capital, swelling the levels of the Mendenhall River to an unprecedented degree. The bursting of such snow-and-ice dams is a phenomenon called a jökuhlaup, and while it's relatively little-known in the U.S., researchers say such glacial floods could threaten about 15 million people around the world.

"We sat down there and were just watching it, and all of a sudden trees started to fall in," Amanda Arra, whose house continued hanging precariously over the river bank Monday, told the Juneau Empire. "And that's when I started to get concerned. Tree after tree after tree."

The flooding in Juneau came from a side basin of the awe-inspiring Mendenhall Glacier, which acts as a dam for the rain and melted snow that collect in the basin during the spring and summer. Eventually, the water gushed out from under the glacier and into Mendenhall Lake, from which it flowed down the Mendenhall River.

Water released from the basin has caused sporadic flooding since 2011. But typically, the water releases more slowly, over a number of days, said Eran Hood, a University of Alaska Southeast professor of environmental science.

Saturday's event was astonishing because the water gushed so quickly, raising the river's flows to about one-and-a-half times the highest previously recorded — so much that it washed away sensors that researchers had placed to study the glacial outburst phenomenon.

"The flows were just way beyond what anything in the river could withstand," Hood said.

Two homes were completely lost and a third partially so, Robert Barr, Juneau's deputy city manager, said Monday. There were no reports of injuries or fatalities.

Eight buildings, including those that fell into the water, have been condemned, but some might be able to be salvaged by substantial repairs or bank stabilization, he said. Others suffered lesser damage.

While climate change is melting the Mendenhall and other glaciers around the world, its relationship to such floods is complicated, scientists say.

The basin where the rain and meltwater collect was formerly covered by the Suicide Glacier, which used to flow into the Mendenhall Glacier, contributing ice to it. But the Suicide Glacier has retreated as the climate warms, leaving a lake in the basin dammed by the Mendenhall.

While that part can be linked to climate change, the unpredictable ways that those waters can burst through the ice dams and create floods downstream is not, they said.

"Climate change caused the phenomenon, but not the individual floods," Hood said.

The variability in the timing and volume of such floods makes it hard to prepare for them, said Celeste Labedz, an environmental seismologist at the University of Calgary.

More than half of the people at risk from glacial outburst floods are in just four countries — India, Pakistan, Peru and China, according to a study published this year in Nature Communications.

One of the more devastating of such events killed up to 6,000 people in Peru in 1941. A 2020 glacial lake outburst flood in British Columbia, Canada, caused a surge of water about 100 metres high, but no one was hurt.

Because the ground along the Mendenhall River is largely made up of loose glacial deposits, it's especially susceptible to erosion, Hood said. The damage could have been much worse if the flood coincided with heavy rains, he said.

Chris and Bob Winter built their house about 15.2 metres off the Mendenhall River in 1981. It flooded for the first time in 2014, an event that prompted them to raise their house about a metre. It flooded again on Saturday with nearly eight centimetres of standing water, enough to soak the carpets, subflooring and drywall.

"You just got to rip it all out," Chris Winter said. "I just don't know what's going to happen, but we can't live in our house right now."   

She said her biggest concern is that they are both in their mid-70s and will probably have to move south at some point.

"We raised our family, and they're gone and nobody's in Juneau," she said. "And I don't know that we'll be able to sell it."