Exploring Mendenhall’s Worst Flood Yet

Inundation along Mendenhall River causes one home to collapse after glacial outburst water eroded riverbanks. (Screenshot from video by Sam Nolan)

JUNEAU, Alaska — In early August, an outburst of glacial water from Suicide Basin flooded Juneau’s Mendenhall River, destroying one home and rendering several others uninhabitable. This was a historic flood for the Mendenhall, as the area has never experienced erosion this severe. Aaron Jacobs, Senior Service Hydrologist at the Juneau National Weather Service, said in a statement:

This flooding event was historic in an area that had not seen this type of flooding or rate of riverbank erosion in the past. There were flooding impacts in the Mendenhall Valley, where a majority of Juneau’s population resides, where historically it does not flood. This event was 3 feet higher than the previous record from 2016, and the Mendenhall Lake and River rose approximately 9.42 feet.

The flood prompted the Juneau Assembly to declare a local emergency, as riverbanks were destabilized and threatened further damage. Alaska Governor Mike Dunleavy issued an emergency declaration shortly after to assist the city in relief efforts.

Suicide Basin, located between the Suicide and Mendenhall glaciers, has been releasing water in annual flood cycles since 2011. Of the 35 release events since 2011, August’s outburst flood was by far the largest, with water levels in the basin plummeting to a record low. Amanda Arra, who lived near the river, was blindsided by the flood’s magnitude.

“This had happened for ten years or more, and so they were saying it was just going to be an average flood, like what we’d had before. I actually went off and had a bike ride, and I came back and saw the river was much higher and I thought, ‘that’s weird.'”

Eran Hood is a Professor of Environmental Science at UAS, with a PhD in Geography. Hood says the dynamic nature of these events makes it difficult to predict their severity.

“These glacier floods tend to start very slowly with a trickle of water under the glacier.

This year, about 14 billion gallons of water filled into the basin. Water has a buoyant pressure; ice wants to float. That buoyant pressure lifts the main glacier up and the water starts to flow under the glacier. Then as the water flows, it generates friction, and the heat from that widens out the drainage channel.

As more water moves through, it widens up the hole, and then everything rushes out. When the water is drained, that hole will close up again because it’s under a lot of pressure. We don’t understand the exact mechanism, but once it drains, there is a drainage conduit to some extent.

Hood says this year’s record flood calls for further monitoring of the basin.

“We’re really going to increase our efforts to monitor up there. We need to understand how the ice near the front of the basin that’s acting as the dam is changing.”

August’s flood was so massive, it exposed parts of the basin that had never been seen before, allowing Hood and others who study the basin to produce more precise models of its water volume.

However, the unpredictable nature of its drainage mechanism makes flood prediction difficult.

“We can’t study that drainage channel, because it’s buried underneath all this ice,” said Hood.

After the flood, Hood and other researchers used drones to create a new 3D map of the basin.

With this new understanding of the basin’s potential for severe outbursts, Hood says there’s a good chance of larger, more destructive floods in the future.

“I think the chance that we have floods that are similar in magnitude to what he had this year is much greater than it was before because we’ve seen the potential,” said Hood.

“The question will always be, can we get lucky?”

As Mendenhall glacier recedes, the shape of the basin changes with it. Three sides of the basin are bedrock, but one of them is the Mendenhall itself. Eventually, Hood says, the glacier won’t be able to act as a dam, and outburst floods will stop. The question is whether floods will get worse before they get better.

While floodwaters have receded to normal levels since the outburst, the banks of the Mendenhall River remain eroded and unstable. Some Juneau residents with homes built along the river have been displaced, as future floods threaten further damage.

Among these homeowners is Amanda Arra, the provider at the UAS student health clinic, whose house was condemned after rushing waters eroded the land under its foundation.

“It wasn’t something that we ever thought would affect our house,” said Arra.

“We were actually really far back from the river. We had quite a bit of property between us and the river.”

The record event caused riverbank erosion between 20 to 150 feet, according to a National Weather Service report.

As waters crept closer to Arra’s home, projections and estimates of when the flood would peak were repeatedly postponed. Eventually, it became clear that evacuation was not only necessary, but urgent, as a neighboring home began to fall into the river.

“I had this short window of time when I had to grab what’s really important to me,” Arra said.

“Somebody came into my house and said, ‘You’ve got to get out, the river’s under your house now.’ It was very traumatic.”

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The morning after the floodwaters crested, the house was still standing. She and several others gathered her belongings, leaving her home behind.

“People just showed up. I didn’t even call people. I just had friends, and then they called people. This whole team of people came the next day and moved all my stuff out of the house.”

Arra’s home was condemned soon after. Her husband Nathan, who has advanced Parkinson’s disease, has been living in an assisted care facility. The costs of this care were already high, and the loss of their home only presented more challenges.

On August 8th, Nancy and Doug Peel, friends of Arra’s, created a GoFundMe to aid Arra and her husband in the recovery process. In the following weeks, the campaign met its $40,000 dollar goal, with over 275 donations. An update posted by Peel on September 5 reads,

“Thank you everyone for your generous donations to Amanda and her family. I know that many if not all of you have donated to multiple folks who have been impacted by this unusual flood event. That just speaks to the incredible giving spirit this community of ours has, not to mention those of you who have donated from afar. Your donations so far have allowed Amanda to continue to cover the expenses of assisted care at Riverview for Nathan while she navigates the complicated process of house stabilization.”

Arra says working at UAS has helped her during these difficult times.

“I just feel so fortunate to have my job here,” she says.

“I love my work here. It’s very rewarding. And so I feel like I’m doing this meaningful work and it actually gives me something to do other than worry about my house and my husband.”

Amanda Arra’s GoFundMe is still accepting donations.

“I can’t really describe what it was like that day. For about 48 hours after the event, I was shaking, and I couldn’t stop shaking. It’s like being in a hurricane, it’s like being Dorothy in the Wizard of Oz and seeing your house go flying up in the air. I mean, I watched half of my neighbor’s house fall into the river.”

In the weeks since the flood, residents like Arra have begun taking preventative measures to protect the Mendenhall riverbanks from future erosion. Eran Hood says that while these efforts are worthwhile, total flood prevention is not a realistic possibility.

“From a practical standpoint, there’s really nothing we can do to stop the basin’s natural process of filling and draining,” he said.

“Even if you put some sort of pipe, there’s a huge waterfall coming down from the Glacier above. Unless you could siphon water faster, it’s not going to make any difference.”

As for protective measures along riversides, and other mitigation efforts to help minimize damage, Hood says that residents lining the river with rocks are taking the right steps.

“That can be done, and can be more or less effective, depending on how big the future floods are.”

Record-breaking events like this have been occurring more frequently as glaciers recede and the planet warms. According to NOAA, over 5,000 rain and heat records were broken in the US over the summer, with over 10,000 broken across the world.

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