On the hunt for hidden dams

(Theme music plays)

Timmy Broderick: Hi there, welcome back to the Scienceline podcast. I’m Timmy Broderick, your host and the Multimedia Editor here at Scienceline.org. Make sure to subscribe to our show, whether you’re listening on Spotify, Apple Podcasts, Stitcher, or whatever platform you use.

Timmy Broderick: Okay, so today’s story is about the quest to find some hidden dams. Madison Goldberg will take it from here.

Madison Goldberg: A few weeks ago, I took a little field trip.

(Sound of car door closing)

Madison Goldberg: Okay! Off to find some dams.

(Sound of seat belt buckling and ignition)

Madison Goldberg: There was one stream I wanted to visit. So I drove along the Hudson to a little spot called Garrison, about 45 miles north of New York City.

Madison Goldberg: I just saw one of those signs that they put up when there’s road work or construction. Instead it said, ‘Caution: deer mating season.’ [laughs] I have definitely left Manhattan.

Madison Goldberg: Garrison is peaceful and pretty. It has winding roads and lots of trees that were just barely hanging onto their fall leaves when I visited. It also has Philips Brook, which is what I was looking for.

Madison Goldberg: I should be pretty close.

GPS voice: Turn right onto Snake Hill Road. Then the destination is on your left.

Madison Goldberg: The stretch of Philips Brook that I looked at was pretty narrow — only a few feet across in some places. It flowed between two mossy stone walls.

Madison Goldberg: It’s in the Hudson River watershed. It’s definitely kind of your quintessential babbling brook. You can hear it babbling.

(Sound of water flowing gently)

Madison Goldberg: And as much as I would’ve liked to stay there the rest of the day, I was on a mission.

Madison Goldberg: Oh, looks like I found a dam … or something.

Madison Goldberg: It was a little hard to tell at first. With the major caveat that I’m really bad at estimating heights, I’d say it was around ten feet. But it was made of stone, and some parts of it just looked like piles of rocks.

Madison Goldberg: I don’t know, it’s pretty inconspicuous.

Madison Goldberg: Later I found a second dam, then another one, a little ways up the stream.

(Sound of stream flowing)

Madison Goldberg: Okay, let me explain why I went hunting for dams.

(Music fades in)

Madison Goldberg: I wanted to get an up-close look at something I just learned about. It turns out rivers in the United States are full of little dams like these. And here’s the thing: Some of them are basically invisible. What I mean is, they aren’t on anyone’s radar at the state or national level. One researcher I talked to called them “ghost dams.”

Madison Goldberg: And scientists told me these dams matter, especially if we want to protect our rivers and fish. So on this podcast, let’s talk about ghost dams: what they are, why we need to think about them, and how one group of scientists is tracking them down.

Madison Goldberg: I’m Madison Goldberg with Scienceline. And I hope you stick around, because it’s going to be a dam adventure. Sorry.

(Music fades out)

Madison Goldberg: Okay, first question: How many dams do you think there are in the United States?

Frank Magilligan: Usually the answer is something like 500, or someone says, ‘I’m going to be crazy, I’m going to say 2000.’ So when I say the actual number, which is 90,000 dams in the United States — which basically says that we as a nation have been building a dam, on average, one dam per day since the signing of the Declaration of Independence.

Madison Goldberg: That’s Frank Magilligan.

Frank Magilligan: I’m a professor of geography at Dartmouth College.

Madison Goldberg: Magilligan’s technical title is kind of amazing. He’s what’s known as a fluvial geomorphologist. That means he studies how rivers flow, and how they change because of things like agriculture, floods and dams.

Madison Goldberg: That 90,000 number — right now it’s at 91,772 — comes from the National Inventory of Dams, which is run by the U.S. Army Corps of Engineers. But since the national inventory only lists dams that could be dangerous if they failed, or that meet certain size requirements, it’s missing a lot. You can see that when you compare it to the inventories that states keep.

Frank Magilligan: The National Inventory of Dams suggests that there are about, I don’t know, 4,400 dams within New England. But if you look at the state census of – the state inventory of all the dams, that number is about 14,000.

Madison Goldberg: So there are lots of dams that aren’t mapped on the national level. And some don’t even make it onto official state lists, either. A lot of these dams are small and don’t serve much of a purpose anymore. For instance, they might’ve been used centuries ago at places like sawmills and textile factories.

Madison Goldberg: But if they’re small, why are they a problem? Well, for one thing, there are the fish.

Frank Magilligan: Those little dinky dams have tremendous ecological ramifications as well: for example, for blocking migratory runs of fish, right? It doesn’t matter if it’s 100 feet high or if it’s 10 feet high, those fish can’t pass that dam.

Madison Goldberg: Researchers told me dams are especially bad for fish like herring. They live in the ocean but migrate back into rivers to spawn. Dams affect things like water flow and temperature, so what was once a good spawning habitat might not be anymore. Plus, they can block fish from reaching that spawning habitat at all.

Madison Goldberg: To be clear, researchers also told me that every dam and every species is different. So we can’t automatically say that all ghost dams are hurting fish migration or spawning. But Magilligan says it’s important to know where the dams are, so we can make sure restoration projects like dam removals do as much good as possible.

Frank Magilligan: If you take a dam out, if there’s another dam immediately upstream, you don’t achieve the success that you were hoping to gain. We try to identify those dams that, once removed, give us the greatest bang for the buck ecologically, environmentally. But if it turns out there’s a dam that you didn’t know about, that kind of undermines your initial effort.

(Music fades in)

Madison Goldberg: Different groups, like environmental nonprofits, have been removing dams to let rivers flow freely — and it’s a really big topic that deserves its own podcast. For now, let’s talk about how we might track down some of these ghosts. Turns out it’s not so easy to do on foot, like I tried to do.

(Music fades out)

Suresh Sethi: It’s just hugely labor-intensive to walk every stream network. I mean, a small watershed is going to have literally hundreds of kilometers of streams.

Madison Goldberg: Suresh Sethi is an associate professor in the Department of Natural Resources and the Environment at Cornell.

Suresh Sethi: I am a quantitative ecologist, and I work predominantly on fisheries management systems, in both freshwater and marine environments.

Madison Goldberg: He and a team of scientists, led by Brian Buchanan, came up with a different way to hunt the ghosts: using computers. They wrote about their method in 2021.

Suresh Sethi: And what our algorithm does is searches along the network and looks for gradient breaks. So, like, when you see a big dip in elevation, you know that that was probably either a waterfall or a dam.

Madison Goldberg: They tested out the program in a couple spots in the Hudson Valley. Turns out it was pretty good at finding ghosts.

Suresh Sethi: And we’re actually able to predict the presence of the small dams with 90 to 95% accuracy using this technique.

Madison Goldberg: This is how I decided to visit Philips Brook: They’d found a bunch of unmapped dams in that area. And in a lot of other areas, too.

Suresh Sethi: In these two test watersheds, we found four and 15 times as many ghost dams as there were actual dams in the inventory. So, think of that scaling up over the Hudson. That’s an absolutely massive impact to migratory species.

Madison Goldberg: They also measured something else: what’s known as the “connectivity” of the streams. In this case, that meant how many kilometers there were from the mouth of the river to the first dam. That calculation looked really different when they didn’t include the ghost dams.

Suresh Sethi: And it gives us a connectivity picture that is much rosier than it is in reality.

Madison Goldberg: Next, he hopes to scale the program up so it gets good at finding ghosts in lots of different rivers.

Suresh Sethi: We still need to know, where should we use our limited precious restoration resources to maximize benefit? And having the ghost dams in the inventory, of course, is critical for making sound decisions.

(Music fades in)

Madison Goldberg: The goal is to focus on the projects that will do the most good for ecosystems.

Madison Goldberg: For Scienceline, I’m Madison Goldberg. Have a great dam day.

(Music fades out)

(Outro music fades in)

Timmy Broderick: And that’s the show; thanks for listening! Feel free to share this with family or friends or whomever you think might be interested. The Scienceline podcast is available on every major platform. Be sure to subscribe wherever you listen. Oh, and don’t forget to give us a rating and review on Apple Podcasts. It helps other people to find our show. 

Timmy Broderick: Scienceline is a production of NYU’s Science, Health and Environmental Reporting Program. Our theme music is by Jahzzar at the Free Music Archive. And for more information, please visit us at Scienceline.org or send us an email at scienceline@gmail.com. See you next time!

(Outro music fades out)