The National Weather Service in Juneau has issued a special weather statement indicating that water levels have started to drop in Suicide Basin, a glacial lake that’s dammed by the Mendenhall Glacier.
Hydrologists and meteorologists who are monitoring basin activity believe this drop was caused by water spilling over the top of the ice dam. Cameras and water gauges deployed to monitor the basin alerted them that water levels started dropping on Wednesday evening.
That doesn’t necessarily indicate that the basin is draining yet, but residents along the Mendenhall Lake and River should begin to prepare for flooding.
Last year’s glacial outburst flood in Juneau was catastrophic. Water broke the banks of the Mendenhall River and surged into Scott and Sarah Louise Griffith’s neighborhood on Meander Way. Like many residents in the area, they were caught off guard.
“We just hadn’t experienced anything like that, and then it was too late,” said Scott Griffith. “Like by the time it was four or five feet of water in the cul-des-sac we were stuck here.”
Glacial outburst flooding is an annual occurrence in Juneau. Suicide Basin, a glacial lake dammed by the Mendenhall Glacier, fills with rain and meltwater each year. Then it eventually drains into the Mendenhall Lake and River.
The National Water Service forecasts this flooding, but last year their predictions were off.
“That was one of the most frustrating parts for me last year, cause you would look at your phone and it was oh it is going to peak at 12. Then oh, it’s going to peak at 13,” Scott Griffith said. “The peak prediction just kept going up and up.”
Flood waters eventually peaked at 15 feet. In low-lying neighborhoods like Griffith’s, a lot of houses were damaged by standing water. Just upstream, severe erosion of the riverbank undermined many homes, causing two to collapse into the river.
This year, like last year, the basin is full. Hydrologist Aaron Jacobs will be keeping a close eye on it.
“Once we do see water hit Mendenhall Lake, with the USGS lake gauge, we will probably be issuing a flood warning,” said hydrologist Aaron Jacobs.
Forecasters have a good idea of how much water is in the basin. The problem is, they don’t know how much water will drain. Jacobs says the basin typically does not drain all the way.
“But last year, the basin wanted to throw a curveball at us and decided to release all the water,” Jacobs said.
That’s changed the way the National Weather Service does their forecasts. This year, they will include a worst case scenario, to show how high the river will rise if the basin drains all at once.
“Could that happen again this year? Yes,” Jacobs said.
But there will be a lot of uncertainty until the drainage starts. Last year, it took 6 days from the time the basin filled up for the flooding to begin.
This July was especially rainy, so the Mendenhall River is running slightly higher than it was this time last year. But unlike last year, large portions of the riverbank are now lined with riprap.
At the Riverside Condominiums, there’s about 50 feet of tightly packed gravel between the river and the condo buildings, along with a line of massive boulders on the bank. It’s a manmade replacement for the land that was washed away during the flood.
Homeowner’s Association Vice President Mark Kissel says residents believe it will hold up when the water rises again this year.
“We’re on terra firma at least as long as the glacier permits it,” he said. “We’ve, I think, probably done the best that anybody could under the circumstances. And we’re very hopeful.”
Back on Meander Way, the Griffiths have also armored their riverbank. Sarah Louise says they have an evacuation plan too.
“I think I’m a little more anxious about it,” she said. So as soon as the basin releases, for me I’ll be moving everything upstairs, up to a higher level, start moving some of our cars out. And luckily we have a place we can go with us, our two kids, our cats and our puppy.”
People can learn more about this year’s potential flood impacts on the National Weather Service webpage. Scrolling down to the “Gauge Location” section and selecting “Activate MNDA2 FIM Gauge” creates a map of areas in the Mendenhall Valley that might experience flooding.
