As Suicide Basin Fills, This Scientist Keeps an Eye on It

Suicide Basin, a glacial lake behind the Mendenhall Glacier, poses a major flood risk to Juneau residents this time of year.

Glacial outburst floods, or jökulhlaups, have occurred every year for the past decade on the Mendenhall River. The basin fills with rain and snowmelt during the summer until it drains downstream.

Last year’s flooding was catastrophic. As the basin refills, scientists at the University of Alaska Southeast are keeping a close eye on it. KTOO’s Anna Canny spoke with one of them.

This interview has been edited for length and clarity.

Anna Canny: I guess you could first introduce yourself and tell me a little bit about your background and what brought you to Juneau this summer.

David Polashenski: Absolutely. My name is David Polashenski and I’m a postdoctoral researcher here at the University of Alaska Southeast. I started this position a few months ago after finishing my PhD in Fairbanks at the University of Alaska. And yes, my background is in ice dynamics and glaciology. So Eran Hood hired me to help with the Suicide Basin monitoring project this summer. I would say I’ve been primarily continuing the monitoring work that UAS, the USGS, and the National Weather Service have been doing together as a partnership for the past few years. So this summer we’ve gone up to the basin three times now and we’ve done three drone surveys. So when we go up there, we just have a little quadcopter drone with a camera on it. And we fly it a normal flight path, so it’s basically like mowing the grass back and forth on the glacier, taking pictures from below. And each photo overlaps the previous one, so you can stitch together all 2,000 or so photos from a given survey into one big image of the entire basin. And then, using digital image processing, you can create a contour map so you can track how the shape of the basin changes over time and how the water level rises and falls.

Anna Canny: So you were talking about basically figuring out the shape of the pool. How much does that change from study to study?

David Polashenski: Yeah, so for this summer it doesn’t change all that much, but year to year it changes slowly but surely. There’s sort of two competing feedbacks on how much water the basin can hold. The Mendenhall Glacier itself acts as a dam, and as the Mendenhall Glacier drops, the height of the dam gets lower, and that means you can hold less water behind the dam as the ice melts, essentially. But then there’s the competing feedback of as the Mendenhall Glacier retreats further up its main branch, then the area of ​​this basin increases over time. So I looked at it this morning, I think the basin has dropped on the order of tens of feet over the last few years, in terms of the height of the ice dam, but it’s also expanding up the main branch, so you have these competing feedbacks. And it’s not clear which one wins right now.

Anna Canny: Is it true that it becomes shallower in some areas, but also wider?

David Polashenski: Precisely.

Anna Canny: By mapping out those competing factors, you can determine how much water it can hold and how much water it can release.

David Polashenski: Right. So the monitoring efforts that we’re trying to keep up with, there’s really two things that we’re concerned about, how much water is in the basin? And then the million dollar question, which is actually much harder, is how fast is it draining? So how big is your bathtub and what size pipe is coming out of the bottom, essentially, determine the dynamics of this flood. And we’re getting better estimates of the total volume over the years, especially after the big flood last year, we were able to do a drone survey when the water level was really, really low. So that gave us the basic topography of the basin, which we just hadn’t seen before because the water level had never dropped that low. The two really big time periods that are really important to get a contour map of are now, when we’re approaching the maximum water level. And then after the flood, once we get the water back up, we want to re-map the entire basin. That way we can get the ratio of the highest to the lowest water level and estimate the total volume.

Anna Canny: Well, you said we’re almost at that maximum volume. When was the last time you were there and what did you see?

David Polashenski: Yeah, we were there exactly a week ago, last Thursday. And I would have to check again, but I believe the water level was about 402 meters. And so the water level is lower than last summer, but it is still filling up fast. This morning, when I checked the monitoring website, it was about 410 meters. So we still have about 10 or 15 meters to go to get to a similar water level as last summer.

Anna Canny: Okay. And I mean, it’s not always like it fills up and then it just empties like a clock. There have been years where it empties before it’s full, right?

David Polashenski: Yeah, exactly. So, this has been going on since 2011. And sometimes, like last year, it drains catastrophically and almost completely, but then other years it only drains, you know, a partial discharge. So 20%, 40% of the basin. And so that’s really the million dollar question: How much of the total volume is going to come out when it starts to drain? Weather permitting and helicopter logistics permitting, we’re hoping to get back up sometime in the next few days. And if the basin does drain later, you know, in a couple of days or weeks, I would encourage everyone to keep an eye on the website to see if and when the discharge actually starts. The National Weather Service will be issuing a flood forecast.