A melting Alaska glacier keeps inundating Juneau with floods

The torrent of frigid meltwater that burst from an Alaskan glacier Tuesday and flooded at least 100 homes, swamped cars and forced residents to wade to safety has become a summertime scourge for the residents of Juneau who live in its path – and one with no easy solutions.

A year ago almost to the day, another glacial outburst flood scoured away homes and undercut riverfront condominiums as then-record amounts of water eroded vast swaths of the bank along the Mendenhall River. The floodwaters originate in a hemmed-in basin above the city that fills up with rain and glacial meltwater in the summer until the pressure becomes so great that billions of gallons of water suddenly force their way underneath the Mendenhall Glacier and down into Juneau.

Last year’s damage prompted residents and local officials to discuss a wide range of possible defenses: from giant siphons to suck out the basin to underground tunnels to riverbank barriers to bombing the glacier. But such costly and complicated ideas didn’t progress far, and scientists at the time were unsure whether a flood of that size would happen again soon or at all.

This week’s flood turned out to be the biggest yet.

The water level in Mendenhall Lake – at the foot of the glacier – reached nearly 16 feet at 3:15 a.m. on Tuesday, a foot higher than last year’s record, according to the National Weather Service. While the prior outburst proved destructive to homes right along the river, this year flooding was far more extensive and inundated neighborhoods farther away.

“There was a tremendous amount of water that came out at one time,” said Aaron Jacobs, senior service hydrologist with the National Weather Service in Juneau.

Glacial outburst floods have poured out of Suicide Basin more than 30 times since 2011.

It is challenging to predict exactly how large they will be, since conditions change each year. The jumble of icebergs in the basin keep melting – adding more liquid water to the pool – and the glacier that acts as a dam keeps thinning and retreating as the atmosphere warms, so scientists don’t know exactly when the pent-up water might release.

But when the basin fills to the point it overtops the glacier, as it did over the past week, authorities know flooding is coming soon.

On Sunday, the National Weather Service said that flooding was “imminent” and predicted “near record” lake levels of 15 feet. The office updated that the next day to 15.7 feet.

But in describing how different streets in Juneau might be affected by flooding as lake levels rose, the forecasts did not say what might happen to residents beyond 15 feet.

There was “significant inundation in neighborhoods that were not anticipating inundation,” Juneau City Manager Katie Koester said during a city meeting Tuesday, according to the Anchorage Daily News.

Jacobs said the service promoted online flood maps to help people understand the potential impacts.

“We were hoping that information would get out there,” he said.

Eran Hood, a hydrologist with the University of Alaska Southeast in Juneau who studies the Mendenhall Glacier’s dynamics, has been involved in using drones to map the size of Suicide Basin to estimate the volume of water it could hold. After seeing imagery taken from a colleague’s helicopter flight Tuesday, after the basin had drained, Hood said it appears to be widening out into the main glacier and deepening as more ice melts. At the same time, the glacier holding back that water is thinning.

“The water level when it cut loose this year was lower than it was last year,” Hood said. “But the basin was larger.”

“We’re chasing a moving target in terms of being able to really lock down the flood volume,” he added.

Eventually, once the Mendenhall Glacier recedes past Suicide Basin, these particular floods will no longer be a problem. But that could be decades from now, Hood said, and in the meantime larger-magnitude floods like those of the past two years “could become relatively common.”

“What we saw yesterday can happen again and likely will happen again in the future,” he said. “And it could be bigger.”