When a Glacial Dam Burst, an Alaskan Town Was Hit With a Sudden Flood

Around 3:30 in the morning on August 6, the first arrivals made their way into the emergency shelter officials had set up in a school gym in Juneau, Alaska.

“We were getting people in that were covered in glacial silt and soaking wet in their pajamas,” says Robert Barr, Juneau’s deputy city manager. “It was folks who lived so far inland, so far away from the river, that they just weren’t expecting to get water.”

Over the previous 24 hours, the Mendenhall River, which runs through Alaska’s capital city, had risen to a peak of about 16 feet above normal levels. The flood inundated houses and apartments blocks away from the riverbank, surprising residents who lived so far away from the river they didn’t expect flooding—even after a warning. For the second year in a row, the city experienced a record-breaking flood in early August. But these floods didn’t follow heavy rains or snow. Instead, they came as a lake held back by the nearby Mendenhall Glacier released more than 14 billion gallons of water in a matter of hours.

This year, says Barr, “we were super lucky that we didn’t have any casualties or deaths as part of this.”

Before 2011, the glacial lake contained in Suicide Basin near Juneau had never drained catastrophically. Located on the flank of the Mendenhall Glacier, the lake now drains multiple times every year, sending a pulse of floodwater rushing down the river into Juneau with little warning. These floods can destroy homes, apartment buildings and roads. They’re part of a global phenomenon of catastrophic glacial lake outburst floods.

“They’re really dynamic systems we don’t know much about,” says Bri Rick, a glacial scientist at the Alaska Climate Adaptation Science Center who studies glacial lake outburst floods in coastal Alaska and published an inventory of past Alaskan floods in 2023. “There’s no hard and fast rule about how they’ll behave.”

Glacial lake outburst floods are a reality for the estimated 15 million people living downstream from glaciers worldwide. They occur anywhere glaciers exist, like the Andes in South America, the Himalayas in Asia, the European Alps, Iceland and Alaska. Outburst floods, just one of many natural disasters influenced by climate change, have killed thousands of people and caused millions of dollars in damages. A warming climate and receding glaciers are making it increasingly uncertain how flooding from these glacial lake systems will behave in the future.

These sudden releases occur when a dam holding back water adjacent to a glacier fails. These dams can be made either of the ice of the glacier itself or of rocky material deposited by a past glacial advance. In coastal Alaska, ice-dammed lakes are most common. They’re so potentially dangerous, Rick says, since the water melts a larger and larger channel in the glacial ice as it flows.

Because of this, during an outburst flood the contents of the lake swell downstream rivers suddenly—sometimes without warning for those in the deluge’s path. They’re more surprising than normal flooding caused by heavy rain or snow, Rick says, as precipitation-caused flooding is slower because the rain “has to move through the entire basin, whereas an entire lake moves down the channel at once” during an outburst flood.

Timelapse captures flooding from a glacial dam outburst in Alaska's capital | ABS-CBN News

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Studies like Rick’s, which inventoried glacial lakes in Alaska to reveal potential flooding hazards, are key to helping those living in the potential path of a glacial flood understand the risks they face. However, Rick says that before her work, the most recent attempt at cataloging potential outburst floods in Alaska was done in 1971. Even this 50-year-old inventory was incomplete. It was biased toward events with human impacts, since modern technologies like satellite imagery that could detect floods in remote locations weren’t yet available to scientists.

Rick is part of an effort by scientists around the world to create databases as an important tool in better understanding patterns of outburst floods. “These inventories are there to provide context,” says Simon Cook, a glaciologist at Scotland’s University of Dundee who has worked on similar databases for South American glacial lakes. But, until recently, inventories based on modern remote sensing techniques just didn’t exist, he says.

With studies like his and Rick’s, though, that’s changing. In the recent Alaska inventory, Rick was able to identify 60 percent more outburst floods in Alaska over the 35 years between 1985 and 2020 than had previously been documented over a 100-year timeframe. “We do now have quite a powerful bank of data to try to get a feel, really, for how frequent these events have been and whether there’s any sort of pattern to that,” says Cook.

To create the recent Alaskan catalog, Rick spent hours poring over satellite imagery to find glacial lakes that could send water rushing downstream. After identifying their location, Rick generated a time-lapse for each lake using Google Earth Engine, and then clicked through the images one-by-one looking for signs of a flood event. For each of the 121 lakes in her inventory, this took about 30 minutes. After spending so many hours looking at these images, Rick says, “I’d close my eyes and just see lakes draining.”

In the Alaska study, Rick and her fellow researchers found that the number of outburst floods seems to be holding steady in Alaska since 1985, and the average flood size is decreasing. But this regional-scale pattern doesn’t mean individual downstream communities are necessarily safer. Like the recent onset of climate-caused damaging floods from Suicide Basin in Juneau, Rick cautions, “any individual lake can have very different trends or experiences.”

The risk to human life and property at any given location downstream of a glacier is dependent on a multitude of physical and social factors unique to the locale, says Caroline Taylor, a glacial lake outburst floods researcher at Newcastle University in England. Researchers like Taylor calculate the risk to any given downstream location by considering the physical conditions of the lake and surrounding terrain, the infrastructure and people in the path of a potential flood, and how prepared a community is to predict and deal with an outburst.

For example, an outburst flood that flows into a large lake basin that’s been depleted during a drought year will have far less impact on a downstream community than a flood draining into a narrow riverbed already swollen with snowmelt. Additionally, a large city with no flood warning system would fare worse than a small town that closely monitors suspect glacial lakes upstream.

Communities like Juneau have some important advantages when it comes to providing citizens with adequate warning of an impending flood. After each flood, they learn from their experience and work to improve their advance detection systems. In recent years, the city started placing monitoring cameras and sensors above Suicide Basin so scientists and emergency managers could tell when a drainage event was underway. This year, they also installed a laser-based monitoring system to understand even more precisely when a flood is beginning.

The effort is paying off, says Barr, and the city had a full 24 hours to prepare for the flood earlier this month. This gave Juneau officials time to send out warnings and evacuation notices before the emergency began.

“Every year has been the best year, every year is a little bit better,” he says. “Last year we had some notice … it certainly didn’t feel like a full 24 hours. This year felt generous.”

Not every downstream community has the luxury of advanced technologies that give them time to prepare, though. Remote locations in the Himalayas or Peruvian Andes, says Cook, might not have adequate cellphone reception to effectively send out warnings.

One of the deadliest glacial lake outburst floods occurred in 1941 in the Peruvian city of Huaraz. On December 13 of that year, glacial Lake Palcacocha drained suddenly. Cook says the water “then cascaded down the valley, a long, straight valley, and picked up loads of boulders—some the size of the room that you’re in now—and then dumped [them] on the city of Huaraz.” Death estimates range from about 1,800 up to about 6,000 people.

To prevent future disasters, officials in high-risk areas like Alaska, the Himalayas, the South American Andes and the European Alps need to prepare the type of monitoring and warning systems like those being deployed in Juneau. But because each glacial lake and downstream community is unique, there’s no one-size-fits-all solution to mitigate the risk factors.

The next step for researchers is to help communities from around the world prone to glacial lake outburst floods create ways to manage those risks in their unique settings. Taylor hopes such future solutions can help humans coexist with glacial lakes, even if the lakes occasionally let loose.

Even in Juneau, more solutions are needed since major outburst floods seem to be becoming an annual occurrence. “We know once we get out of recovery phase, we’re going to need to start talking about mitigation and prevention and whether or not there’s anything we can do on that front,” Barr says. “And it’s really hard to think about, because [the] magnitude of the problem is so significant.”

Rudy Molinek | READ MORE

Rudy Molinek is Smithsonian magazine's 2024 AAAS Mass Media Fellow.