This interview originally aired on "In the Moment" on SDPB Radio.
We take a deep dive into South Dakota hydrology with state geologist Tim Cowman. He explains the factors behind the devastating flooding throughout much of the southeast in June.
Cowman discusses our land, our rivers and the rainfall that inundated them. He shares how we got lucky in some ways this year and why we may not be so lucky next time.
Cowman is the state geologist with the South Dakota Department of Agriculture & Natural Resources.
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The following transcript was auto-generated and edited for clarity.
Lori Walsh:
I believe the last time we had you on with us live, there had been an earthquake that we were talking about, and this is a more unusual and more devastating event with the flooding, so thanks for your return.
I want to go back a little bit, and just seconds ago the National Weather Service posted a look back at what happened, a thread on X, as well. So, let's talk about what happened. Tell me a little bit about how the flooding event took shape. What's essential?
Tim Cowman:
Yeah, so essentially we had heavy rainfalls over the Big Sioux, Vermillion and James basins that took place over a three-day period about June 19, 20 and 21. The magnitude of the rainfall was very large. A lot of areas saw at least 10 inches to 15 inches of rain, some areas even saw up close to 18 inches of rain, and those heaviest of rainfalls occurred basically west of Canton and in the Worthing area.
They fell on both the Big Sioux basin as well as the Vermillion basin, but there was also large widespread areas of heavy rainfall that went as far west as Mitchell and Chamberlain. The Mitchell area one, that was contributing to the James River basin.
But before that, a day or two before that, these same types of rainfalls moved up through north central Nebraska and dumped these types of heavy rains in the Niobrara River basin, which plays into this whole flood event, because the Niobrara eventually empties into the Missouri River just upstream of Lewis and Clark Lake.
Lori Walsh:
All right, so a lot of complicated things are happening here and a lot of comparisons are being made to the 2011 or 2014 floods, but here in 2024, I'm wondering if you could give us an outline of what kind of science and computer modeling you have at your disposal now. Either that maybe didn't exist in 2011 or 2014.
Or just what is interesting about how you in your role are watching this?
Tim Cowman:
Yeah, so the precipitation events that brought on this flood were probably similar to the 2014 flood on the Big Sioux River, but the difference was they were much more widespread, and so instead of it just being the Big Sioux River, the Vermillion River, the James River, the Niobrara River and the Missouri River all came into play here.
So we had five rivers and a mainstem reservoir that were all contributing to the mechanics of this flood, and the timing on the flood pulses on each of these river systems was important to try to figure out because if they all synced up and the crests all happened at the same time, the magnitude and the impacts of the flood would've been much worse.
So, we have a lot of stream gauges out there. The US Geological Survey has a good network of stream gauges on these rivers, and the Department of Ag and Natural Resources also maintains a good network of stream gauges so we can track flows as they're coming down the river. The River Forecast Center of the National Weather Service then tries to predict what the flows will be downstream at certain times and when the crests will happen. So, we all work together with that information to try to figure out when the floods are going to be at the highest impact and just how serious those impacts will be.
Lori Walsh:
So when you mentioned stream gauges, to be clear, and you mentioned flood pulses, this isn't something you can control at all, or is it?
Tim Cowman:
No, the stream gauge is simply an instrument that tells us how high the river stage is, and then, with that information, we can calculate what the actual flow is going down the river. That's important to know, for example, on the Big Sioux, Vermillion and James Rivers, because they all empty into the Missouri River and so they add to the flow of the Missouri River. So, knowing what those flows are at their crest and what the timing is of those crests coming into the Missouri will allow us to determine how high the Missouri River's going to get and whether or not it's going to cause flooding problems as well.
Lori Walsh:
Can they sync up? Is there a scenario where they could have all had their flood pulses or their crests at the same time?
Tim Cowman:
Oh, certainly. We came close in this flood. We got lucky. We got lucky that they got out of sync, but the Missouri River was being controlled mainly by the discharge out of Gavins Point Dam before the flood hit, and the Corps was not able to reduce those flows significantly out of the Missouri because they had so much water coming into Lewis and Clark Lake through the Niobrara that the lake was getting too high and getting close to overtopping the spillway gates, so they had to keep releasing enough water to prevent that from happening.
Then the Big Sioux, Vermillion and James, all three had these pulses heading down to the Missouri as well. As it turned out, the Big Sioux crested at North Sioux City on Sunday, which would've been the 23rd of June, the Vermillion River crested the next day on Monday, and then the James River crested the day after that on Tuesday. So they were about a day out of sync, which meant that flows in the Missouri could start dropping off because the other rivers had already crested and were able to take on then the additional flows by the later cresting rivers.
Lori Walsh:
So tell me how fast you're getting that information and sharing it with other people. Take me into the room and the computer monitors and the phone calls. What does the communication look like? How quickly is the information coming into you? How quickly can you make sense of it? Help me understand that process, if you would.
Tim Cowman:
Right. So there are a lot of agencies involved both at the local government level, state government level, and the federal government level. And some of the key players are, for example, the county emergency management at the local level. Then at the state level we have the Ag and Natural Resources, which is where I'm from, and we have the DOT because of roads that are being closed like the interstate, and then we have the governor's office being represented as well.
We also have the Office of Emergency Management, which is in the Department of Public Safety, all interacting together with the local government and with primarily the Corps of Engineers and the National Weather Service at the federal government level.
All of us have information to contribute and responsibilities to analyze this information, and so there was a lot of communication going on amongst all three levels of government sharing the information that we had available at the time.
Lori Walsh:
So, do you have a lot of understanding of what the worst-case scenario might be? So when you say we got a little lucky with when the rivers crested, do you know what the impacts would possibly be if the situation unfolded slightly differently?
Tim Cowman:
Yeah, the flooding impact would've been even worse down in the North Sioux City, Dakota Dunes, McCook Lake area if it had all crested at the same time because the Missouri would've been flooding more and then it also would've created basically a backwater effect for the Big Sioux. In other words, with the Missouri being higher, the floodwaters of the Big Sioux would've been slowed down much more and sat there much longer and got higher.
With the Missouri being lower, those floodwaters can move through faster and it keeps the crest from getting any higher.
So that's on the Missouri side, but on the Big Sioux side, I mean, that was by far and away the worst damage that happened, and that was because of the record flow that we had coming down the Big Sioux with a high volume of water. It was hard to predict the timing of the crest and that timing kept getting moved up, moved up, moved up, and before we knew it, the crest was there about a day, day and a half earlier than it originally was predicted to get there. So, that created a situation where all of a sudden you're out of time and the crest of the flood is here.
Lori Walsh:
Yeah, say more about that, 'cause I'm trying to picture when you say it kept getting moved up and moved up. You mean your analysis kept shifting with the information or more rain kept falling and so it kept changing, or the river behaved in a way that we didn't know? So help me understand what you mean by that, 'cause my mind's going 10 different directions.
Tim Cowman:
Yeah, so the timing of the crest is put out by the River Forecast Center for the Missouri River basin, and they have models that they use that look at the water coming down the river, and then they predict when it's going to crest at specific points along the river. The times that the crests were being predicted over the weekend sort of in that Friday, Saturday timeframe were out further than when it really happened. So in other words, the crest came faster than their models were predicting it would come, and I think that has to do with the fact that this was a flood that was off the charts.
So the models are calibrated really up to what the highest flood had been previously, and that was in 2014 on the Big Sioux River, but once you start getting beyond that, and this one was way beyond that, then the models can have some trouble accurately predicting what's really going to happen. I think that plays into part of why the timing kept getting shifted is that the models just weren't able to accurately predict it.
Lori Walsh:
Yeah, so I want to talk about impacts in a moment, particularly some impacts that maybe we haven't considered fully and a little bit about the future. But from what you just said, before we get to that part, then are the models recalibrated? What are we learning right now about this from the severity of this event that is going to help us prepare better in the future?
Tim Cowman:
Right. So that's exactly what'll happen is during the post-flood analysis, this event will be added into the National Weather Service models and they'll be able to handle floods that are up to this magnitude more accurately because our models will contain that information.
On the state side, we have models too, but our models are used for predicting what's going to be inundated with flood water. Given the flows that the National Weather Service predicts are going to come through, we can predict what's going to be flooded. Our model actually did pretty good. It simulated this flood event pretty well, but we'll still be able to tweak it and make it even better now that we're able to put this information into it for future flood events.
The magnitude of this flood was, like I said, much worse than the previous bad one, which was in 2014 on the Big Sioux. The flood in 2014 we think was somewhere between a 100- and 500-year flood, so this one will be even greater, probably somewhere between a 500- and a 1,000-year flood.
Lori Walsh:
Right. But I'll say the same thing that the roofer said when they fixed my shingles after a hailstorm, I said, "Oh, this should be good for another 25 years." He said, "Or tomorrow." So, I guess this gets to this idea of how often we anticipate having severe weather events. I was just looking at the book "Ecology of Dakota Landscapes" this morning as I prepped for this conversation, and Carter Johnson and Dennis Knight say that we need comprehensive plans to mitigate adverse effects of climate change.
So my question to you is, as we look at more severe events, how are we developing plans to mitigate some of those adverse effects of climate change?
Tim Cowman:
Well, I think we have to adjust our thinking on how we prepare for these types of future events, because one thing we are seeing is that these 100-year plus floods have been coming every five years in the Big Sioux basin. So there was the 2011 flood more on the Missouri side, but that had the same type of origin of it. Then there was the 2014 flood, the 2019 flood and now the 2024 flood.
So the precipitation patterns that generate those types of floods seem to be coming together more and more frequently, and so I think we have to assume that these things aren't going to happen every 10, 20, 30 years. They're going to be coming more often than that.
Lori Walsh:
What happens to the watershed? That's some of the impacts that we don't think of. We focus at first, I think rightfully so, on the loss of people's livelihoods and lives and property, but it's also time to consider what happens to trees and river banks and erosion and fish and the ecosystem. What do you know about the impacts of an event like this that maybe people have not been considering?
Tim Cowman:
Well, certainly erosion is one of the big ones. A lot of sediment gets moved into the river channel and then deposited elsewhere, so that shows up after the floodwaters have receded. There are, in some cases, like on the Vermillion River especially, we've seen some huge erosion of the bank where the river has cut a large channel quite a ways outside of where it normally would flow, and that's taken farmland. It's also threatened homes.
Those are really, really some of the big impacts that happen, but it does have an impact on vegetation as well. There could be some trees that will die because of it or at least be set back, but those kinds of things are some examples of some of the impacts that we see.
Lori Walsh:
Yeah, pollution, just the things that are not moving down, the increase in pollution, sewage line breakages, ag runoff. What do we see about water quality and how long does it take a watershed to recover?
Tim Cowman:
Well, the immediate concern is in the floodwaters that are remaining behind, they've got all kinds of nasty things in them, so people need to stay out of those floodwaters until they recede. But also, there's the example at Vermillion where the city's wastewater discharge lines ran underneath the river on their way to the wastewater treatment plant, and because of erosion on the bank as well as erosion of the bed of the river, all three of those lines were severed, which meant that raw sewage is now dumping into the Vermillion River.
So there's work in progress now to put a temporary line in place to handle that waste, and then eventually a permanent line will go back underneath the river, but those are certainly impacts that create water quality problems.
Lori Walsh:
If I just moved here, if I'm a new mayor or a new senator, I just got elected to the state House or something, I'm going to be making policy. What sorts of things do you want me to know? What would be some of your advice to somebody who is moving into community leadership, whether it's in a township or statewide, that they need to understand about South Dakota hydrology and these kinds of events?
Tim Cowman:
Yeah, I think one of the biggest things, and this really applies statewide, is that people are attracted to water bodies to build housing developments on. They want to be around lakes, they want to be next to rivers, they want to be next to creeks. And we got to remember that, especially with the rivers and creeks, those are the floodplains and those are meant to take floodwaters. So there's only so much you can do to control those floods. They're going to happen anyway.
I think that's the big thing that planners need to be aware of is that building in close proximity to flowing boundaries of water in their floodplains is not necessarily a good idea.
Lori Walsh:
Any final thoughts? Anything I didn't ask or a lot of questions that you're getting frequently from the public that you want to make sure you take this time to address to a statewide audience?
Tim Cowman:
I think a couple of things to be aware of is just the fact that we are starting to see these flood events happen more frequently, so I think we need to be aware as people and as a state that this is something that is a threat, and we need to take a look at where we live and what kind of danger we may be in and be prepared well in advance for any flood that may come our way.
Lori Walsh:
I won't ask you how much you slept during the five days at the end.
Tim Cowman:
Some nights not very much, no.
Lori Walsh:
Well, thank you for that, and thank you for coming here today. Anytime that you have information to share, we will be here for you. Tim Cowman, state geologist with the South Dakota Department of Agriculture and Natural Resources. Thank you so much.
Tim Cowman:
Thank you, Lori.
