Experts know it’s coming. They just don’t know exactly when, or how severe it will be.
A team including the U.S. Forest Service, the University of Alaska Southeast, the National Weather Service and the City and Borough of Juneau have been monitoring water levels at Suicide Basin, above Mendenhall Lake, for the last few years. They’ve been keeping an eye out for a jökulhlaup, the Icelandic term for a flood of glacial water caused by the bursting of a dam, since billions of gallons of water surprised Mendenhall Valley residents in 2011, flooding many Mendenhall River homes.
The team recently installed a seasonal sensor to monitor the water levels in the basin.
Weather Service Hydrologist Aaron Jacobs said the waters in the basin are rising at a steady pace of around two feet per day. During the heavy rains earlier this week, the water level rose at a quicker pace. Tuesday, it rose about three feet.
Its historical high is 52 feet, in 2012; levels rose to more than half that this week. But the dam could burst before its previous maximum.
“There are a lot of unknowns with this, due to ice lost,” Jacobs said.
Juneau Emergency Management Coordinator Tom Mattice compares the basin to a glass full of ice cubes and Coca Cola.
“You could have a glass of Coke, and you have a whole bunch of ice at the top, and the water at the bottom,” he said. “We’ve been draining water out the bottom, but every year there’s less ice in the Coke. It’s the same number on the gauge, but there’s less ice and more water. We have no way of correlating how much water is in the basin.”
This, Mattice said, leads him to believe the so-called “action stage” in the sensor is not accurate.
There is some good news, though: Suicide Basin’s levels have tended to begin decreasing on the gauge about a day before the Mendenhall Lake’s water levels begin to rise, Mattice said.
“If the monitoring system works, we should have a little bit of notice,” Mattice said.
Jacobs said the water level right now is more than last year, but not as much as a few years ago.
“It looks like an event (will happen); but it’s too early to tell how severe it will be,” he said.
Last year, the water “dribbled out” over the course of the summer, Jacobs said, a much less severe event than the outburst floods caused by the water bursting through the ice dam.
Jacobs expects water will begin leaving the basin, one way or another, at the end of June or beginning of July, though he said it may release earlier or later.
Mattice said he expects the water in the basin to release sometime before August 1.
“There’s not that much snow left up there,” Jacobs said. “This was not a normal snowpack year, and we did have an abnormally dry, warm spring that definitely melted a lot of the snowpack … We are on top of it, and monitoring it 24 hours a day.”
“It’s important for residents downriver to recognize that it’s building again,” Mattice said. “We’ll be getting out more public information as the situation continues to heighten.”
Monitor the water’s rise at http://water.weather.gov/ahps2/hydrograph.php?wfo=pajk&gage=jsba2.