Solve me a river

Summary

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Unexplainable – “Solve Me a River”

Date: October 29, 2025
Podcast: Unexplainable (Vox Media)
Host(s): Noam Hassenfeld (A), with Bird Pinkerton (D), Neil Dinesha (E), guest Sally Helm (C)

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Brief Overview

This engaging episode of Unexplainable takes the form of a science-filled game show, “Unexplainable or Not,” exploring spectacular mysteries about rivers: Why are US freshwater mussels dying? Why do rivers sometimes radically change course? And which river is actually the longest in the world? Newcomer Sally Helm steps into the hot seat, working through the clues alongside the Unexplainable team. Notably, the show spotlights recent scientific breakthroughs and reflects on how our knowledge—and our rivers—are constantly evolving.

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Key Discussion Points and Insights

1. Introducing the Game: “Unexplainable or Not”

• The game: Sally is presented with three river-related mysteries. Only one has been recently solved by science. Can she guess which?
• Purpose: To showcase the river-themed podcast artwork and illustrate the ongoing, winding nature of scientific discovery.
  [01:05–02:45]

2. Mystery #1: Why Are Freshwater Mussels Dying? (Bird’s Segment)

• Biodiversity: The US is a hotspot for freshwater mussel diversity, boasting species like the heel splitter, the purple wartyback, and the evocative “turgid blossom pearly mussel”.
  • “We have one called the heel splitter... it has this, like, really sharp edge. So the name is sort of ouch.” — Bird (D) [04:07]
• Ecosystem role: Mussels are crucial for river health—naturally filtering water, stabilizing beds, providing habitat after death.
• Crisis: Several species have gone extinct or are endangered. Recent years have seen massive, rapid die-offs in specific habitats.
  • “He calls it like an ecological emergency.” — Bird (D) [07:05]
• Investigation: A multi-institution “Delta Strike Force” investigates if pathogens are to blame, but so far, the fundamental cause remains unidentified.
  • “In order to keep these incredible animals... researchers... are still hunting for the culprit here... they have not found it yet.” — Bird (D) [08:57]
• Sally's reaction: Wistful hope for an answer, but skepticism prevails.
  • “I'm gonna say maybe as emotional self protection that we don't. We don't know.” — Sally (C) [09:49]

3. Mystery #2: Why Do Rivers Change Course So Dramatically? (Neil’s Segment)

• “Rivers are steady—until they’re not”: While urban rivers like the Hudson or the East River feel permanent, elsewhere rivers can abruptly shift channels (called avulsions), sometimes by hundreds of miles.
  • “Sometimes rivers can dramatically change course by as much as hundreds of miles... it like fully moves and it leaves a dry channel behind where it was before.” — Neil (E) [11:06]
• Devastating impacts: Cites the 2010 Indus River avulsion in Pakistan, affecting 20 million people and causing nearly 2,000 deaths.
• Historical context: References Old River Control in Louisiana—a massive US Army project to prevent the Mississippi from jumping channel.
• What causes avulsions?: Long mysterious—even with high stakes, scientists haven’t fully understood the triggers.
  • “But it's still kind of a mystery. Like, we don't really know what causes these avulsions to happen. Unless we do.” — Neil (E) [14:03]
• Sally’s reasoning: Believes this mystery has likely been solved given its importance—but is reminded that recency matters.
  • “I'm gonna say we figured this one out. I'm gonna say someone knows why.” — Sally (C) [14:36]

4. Mystery #3: Which Is the Longest River in the World? (Noam’s Segment)

• “What is the longest river in the world?”
  • Most say the Nile, but some scientists argue for the Amazon.
• Measurement Paradoxes:
  • The coastline paradox: The more closely you measure, the longer a winding river seems—almost fractal in nature.
    • “The further you zoom in, the longer the river gets.” — Noam (A) [17:54]
  • Sources and ends are ambiguous. Different upstream sources can extend a river's “official” length, and even endpoints can be debated.
• Subjectivity and Philosophy:
  • The answer varies by method and data; perhaps there is no single longest river.
  • “You've convinced me that the lengths, infinity, it's all an approximation anyway... My answer is there's no longest river.” — Sally (C) [20:24]

5. Game Show Deliberation and Guess

• Sally’s Final Guess: She shifts, betting that the mystery solved is the cause of the freshwater mussel die-offs.
  • “Okay, I'm gonna say that we have found why the muscles are dying. We know the answer to that.” — Sally (C) [26:59]

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The Reveal: Solved River Mysteries

[27:16–31:37]

The Recent Breakthrough: Why Rivers Change Course (Avulsions)

• Doug Edmonds (Indiana University) and colleagues have solved the mystery of river avulsions.
  • “So the person you just heard is Doug Edmonds, who’s… one of the co-authors of a paper about why avulsions happen.” — Neil (E) [28:02]
• Factors Involved:
  1. Gradient advantage: Water seeks a steeper slope, moving toward a channel if it offers a greater drop.
     • “The simple idea is that water likes to go downhill... Doug calls that gradient advantage.” — Doug Edmonds (H) [28:21]
  2. Superelevation: Rivers deposit sediment, building up their beds above the floodplain, making breakouts more likely.
     • “If you imagine just like you jack up a car... that sediment kind of brings the river up above its surrounding floodplain.” — Doug Edmonds (H) [28:50]
• How was it solved?: Using space lasers (NASA’s lidar satellites) to measure surface elevation and detect recent avulsions globally.
  • “They used lidar from these NASA satellites to do two things. First, they actually measured the surface of rivers... [and] found evidence of avulsions that happened over the past 30 years.” — Neil (E) [29:37]
• Conclusion: Both gradient advantage and superelevation together predict avulsions.
  • “We ended up realizing that both superelevation and gradient advantage actually work together to create the best predictor we now have of when a river avulsion will occur.” — Doug Edmonds (H) [29:57]
• Applications: The creation of hazard maps for at-risk regions, and the importance of engineered spillways to mitigate disasters, especially under climate change.
  • “These hazard maps could help people... build the most targeted kinds of infrastructure... Or at the very least, educate them on the risks.” — Neil (E) [31:10]

Reflection on the Cover Art

The episode’s cover art is a vintage “avulsion” map showing the ever-changing paths of the Mississippi, symbolizing the constant flux of science and nature.

• “Science, too, is this thing... that is constantly carving new channels for itself... both shaped by the environment around it and shaping the environment around it.” — Bird (D) [32:24]

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Notable Quotes and Memorable Moments

• On mussels:
  • “That is a poem in itself. That's a good novel.” — Sally (C) [04:59]
  • “These mussels, right, they are doing all this invisible work and then on top of that, they're helping stabilize the riverbed. They're feeding animals.” — Bird (D) [05:23]
• On avulsions:
  • “Sometimes rivers can dramatically change course by as much as hundreds of miles... it like fully moves and it leaves a dry channel behind where it was before.” — Neil (E) [11:06]
  • “It's called something like the Mississippi Meander map. It's basically the map of Neil's mystery.” — Noam (A) [32:01]
• On the fractal river paradox:
  • “All rivers are actually the same. Trick question.” — Noam (A) [18:00]
  • “My answer is there's no longest river. ...They're all Zeno's River.” — Sally (C) [20:26]
• Fun banter and playfulness:
  • “You guys are playing chess. I'm playing checkers. Birds playing Parcheesi.” — Sally (C) [26:52]
  • “I'm playing four dimensional Parcheesi.” — Bird (D) [26:57]
• On space lasers:
  • “They did it with space lasers. Yep.” — Neil (E) [29:28]
  • “I did not know space lasers were coming.” — Sally (C) [29:33]
• Original Song (Noam):
  • “When I was young it was paradise / Growing up right by the riverside... Maybe lasers from space / Will see the avulsions of yesterday” — Noam (I) [34:54–36:23]

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Important Timestamps

• [03:40–09:03] — Mystery #1: Mussels dying off, investigation, and ongoing mystery.
• [10:07–14:14] — Mystery #2: Rivers changing course, introduction to avulsions, and global impacts.
• [15:13–20:24] — Mystery #3: The longest river paradox, coastline paradox, measurement challenges.
• [24:40–27:09] — Game Show return, Sally’s thought process and final guess.
• [27:16–31:37] — The answer revealed: Solving river avulsions with science, space lasers, and new hazard maps.
• [34:54–36:23] — Listener treat: Noam’s original “river avulsion” song.

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Takeaways

• Solved mystery: The recent breakthrough is in understanding and predicting river avulsions—how, why, and when rivers abruptly change course.
• Still unexplainable: The causes of freshwater mussel die-offs are not yet fully understood. The question of the world’s “longest” river is mired in paradox and subjectivity.
• Big picture: Our understanding of rivers—and science in general—is ever-changing, shaped both by new tools (like lidar “space lasers”) and by the persistent mysteries of the natural world.
• Tone: Informative, playful, and wonder-filled, balancing the seriousness of ecological challenges with curiosity and humor.

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Transcript

[A]

Support for Unexplainable comes from Anthropic, the team behind Claude. You know those questions that keep you up at night, the ones that make you fall down rabbit holes until 2am Claude is an AI that can help you if you want to dive deeper instead of rushing to surface level answers. You can use Claude to help work through the complexity. You can upload documents, you can search the web and you can explore different angles on problems that genuinely interest you. You can try Claude for free at Claude. AI Unexplainable.

[B]

Support for the show comes from Charles Schwab. At Schwab, how you invest is your choice, not theirs. That's why when it comes to managing your wealth, Schwab gives you more choices. You can invest and trade on your own. Plus get advice and more comprehensive wealth solutions to help meet your unique needs. With award winning service, low costs and transparent advice, you can manage your wealth your way at Schwab. Visit schwab.com to learn more.

[A]

This is Unexplainable or Not, the game show where we finally get some answers. Today we wanted to introduce you to one of the newest members of our team, Sally Helm. Sally has worked on all kinds of shows you probably listened to. She's the host of History this week from from the History Channel and she's done tons of amazing reporting for Planet Money. Welcome, Sally.

[C]

Hello. I'm so happy to be here. It's a delight. It's an honor. And I'm scared. It's a game show, so I'm in the hot seat.

[A]

Okay. Unexplainable or not is a game show where you have to guess what we know and what we don't. You're gonna hear three scientific mysteries and you're gonna hear them from me, from my co host, Bird Pinkerton.

[D]

That's me.

[A]

And from longtime friend of the show, science reporter Neil Denisha.

[E]

Hello.

[A]

Two of these mysteries are still unexplainable, but one of them has recently been solved. After you hear all three mysteries, you're going to get a chance to guess which one you think scientists have actually recently explained. Okay. And this week I want to start with a question we got from a listener.

[D]

Hi, team. Love the podcast, but there's one thing.

[C]

That'S been bothering me for a while now. Where does your cover art come from?

[A]

I'm really excited we got this question because we do have new artwork and we've been wanting to talk about it for a while on the show.

[D]

New ish.

[A]

Yeah, new ish. Bird is right.

[C]

Okay. You're Talking about, like, the podcast tile, the thing that shows up next to unexplainable.

[A]

Yeah, and it's a river map from 1944. And we figured the best way to explain why we chose a river map was with an entire game show all about rivers.

[C]

Obviously the easiest possible thing.

[E]

What else would you do?

[A]

Talk about it the least amount of work possible. So first up, you're gonna hear a mystery from Bird.

[D]

Sally, hello.

[C]

Hi, Bird.

[D]

Hello, Sally. We've known each other for a while at this point, ever since then.

[C]

Hello.

[F]

These many years.

[D]

Yeah. Interns at npr. The same intern class. But I realized I've never asked you an important question all this time, which is, are you a fan of mussels?

[C]

Mussels. Okay. Since we're talking about rivers, I'm going to assume we mean the kind that has shells that come together.

[D]

We are indeed talking about freshwater mussels.

[C]

Okay.

[D]

So the unsung heroes of rivers, as some might say. Myself, I love an underdog.

[C]

I'm already on board.

[D]

So, basically, freshwater mussels are in all kinds of rivers in the US and this researcher professor at the University of Wisconsin, Madison, this guy named Tony Goldberg, he told me that the US Is actually a hotspot of mussel diversity. So we have all kinds of amazing species. We have one called the heel splitter that looks. It looks kind of like a clam, but then it has this, like, really sharp edge. So the name is sort of ouch.

[C]

So heel splitter are heels when we step on it? Yes.

[D]

And then there's. There's like, a kind of muscle that makes part of its little, like, fleshy bit essentially look like a tiny fish. And then, like, a bigger fish will come and try to eat the fake fish, and the mussel will explode eggs at the big fish, so it, like, carries the egg somewhere else. Pretty cool. And honestly, I just wanted to take a minute with some of the amazing names we've given to these muscles. So you have the purple wartyback. We got the pocketbook, the flat pigtoe, the mucket. My favorite, the golden riffle shell, the stirrup shell, and my favorite, the turgid blossom pearly mussel.

[C]

That is a poem in itself. That's a good novel.

[A]

I agree.

[D]

And I think, like, these mussels are just delightful, right? Like, in and of themselves. But Tony was telling me that they are also fairly vital for river ecology. So they essentially filter water like it is their job. They, like, suck in the water, right? They pull out particulates, so that can be food. But also pathogens like E. Coli, herbicides just like chemicals in the stream, and then whatever remains, they just kind of like poop out. Hmm.

[C]

And are they doing this to like eat and get nutrients for themselves, but then as a byproduct, they clean the water.

[D]

Yeah. And there's this, this video from like the, the Minnesota Department of Natural Resources. It's this like, little time lapse. And so they start with like two containers of like dirty kind of brownish orangish muck. One has mussels in it, one doesn't. And over the course of literally like a few hours, you watch the dirty tank, right. It remains filthy brown and the muscle tank just like cleans up. Essentially the water becomes see through and probably not like lovely, but certainly more lovely than it was before. So these mussels, right, they are doing all this invisible work and then on top of that, they're helping stabilize the riverbed. They're feeding animals. They're like dead shells provide habitats. So they're just this like, amazingly helpful, delightful creature that we have the privilege of having in our rivers. Except unfortunately, for a while now, some specific mussels have been struggling. So actually some of the species that I told you about, like the flat pigtoe, the stirrup shell, also unfortunately, the turgid blossom pearly mussel.

[C]

No bird.

[D]

Yes, I am very sorry to say they were all, all declared extinct actually in 2023 because I hadn't been seen for decades. Some other species are endangered, but also, Tony says sort of more recently, researchers have started seeing these massive mussel die offs. A whole bunch of mussels will just like die over the course of even just one season. So he calls it like an ecological emergency.

[E]

Hmm.

[D]

And Tony is an epidemiologist, so he studies like pathogens in animals, everything from monkeys, robins, fish, mussels. And he joined like a whole team of people that wanted to investigate this. He said they called themselves the delta strike force of muscle. Interested scientists, but in Tony's case, he's looking to see if the problem could be a pathogen or of some kind, a bacteria or a virus or a parasite.

[C]

Makes sense. Mussels dying. Maybe they're getting sick from something in the water.

[D]

Exactly. And so they kind of had to start from scratch, he told me. In some ways, like he said, they had to figure out how to identify even that a mussel was sick.

[C]

If you just look at its shell, you're like, oh, that's a mussel. And you'd have to see its.

[D]

Well, yeah, they kind of Just like open and close and, like, hang out. So they had to sort of figure out, like, how do you identify sickness in a musc? And there were a lot of viruses in these mussels, for example, but he had to figure out, like, were any of those a problem? Are there viruses or bacteria or parasites that are in the sick or dead mussels, but not in the healthy ones? And so, basically, in order to kind of preserve our rivers as we know them, right? In order to keep these incredible animals in them that literally keep them clean for us, researchers like Tony are still hunting for the culprit here, Right? The smoking gun that is killing off these muscles in these big die offs. And they have not found it yet.

[C]

Okay.

[D]

Unless they have.

[C]

Please tell me that we know what is after our deer muscles.

[A]

What do you think, Sally? Do you think we know or do you think it's still a mystery?

[C]

You know, I feel so much like I want us to know that I'm wondering if we don't, you know, like, I'm wondering if I just have optimism and hope in my heart that we have figured it out and it seems like we should be able to. But I'm intrigued that it's hard to even know what a sick muscle looks like. That makes me worry that maybe we don't know enough about muscles to really answer this question. So I'm gonna say maybe as emotional self protection that we don't. We don't know.

[A]

I love the way you're thinking about it. So no need to lock in your answer yet. Just have it float around in the back for now.

[C]

Great. As if on a river.

[A]

And Neil is up next. He's got another mystery for you about rivers.

[C]

All right.

[E]

Hello. We were not interns at npr and this is the first time I'm meeting you.

[C]

Never met you before.

[E]

Hi. Hello. Well, since I've never met you, I also don't know where you live. Do you live near a river?

[C]

You know, I live in Brooklyn, so kind of near the Hudson. And I grew up in Los Angeles, which is like, you know, ocean country. So rivers are a little less known to me overall.

[E]

So I also live in Brooklyn. And I think one thing I've taken for granted in Brooklyn is that the east river and the Hudson river stay where they are. Like the amount of water in them might change. Like there might be like a flood or a drought, and that could sort of affect how much water. Water is in the river. But the course for the most part is pretty stable. And they're so stable that we build entire cities around them. Right. Like New York exists, and these two waterways have sort of been a major part of the shape of the city. But sometimes rivers can dramatically change course by as much as hundreds of miles. And it's not like just a river meandering one way or the other a little bit and then coming back to where it was before, it like fully moves and it leaves a dry channel behind. Where it was before what? I know, right? So imagine if the Mississippi river, when it hit Louisiana, instead of going east and then down to Baton Rouge and New Orleans, it went west just like, zoink.

[C]

Different course.

[E]

Exactly. And, you know, it would completely transform these cities. Like, New Orleans would be in a lot of trouble. I mean, the entire state would. And this actually almost happened back in the 1960s. The Army Corps of Engineers built this huge floodgate on the Mississippi so that it wouldn't change course and join another river called the Atchafalaya.

[C]

Okay. It was trying to change course, and the Army Corps of Engineers was like, we can't allow this. And they were kept apart by the Army.

[E]

They got kept apart by the Army Corps Engineers. This tale of river love was stopped.

[C]

Okay.

[A]

Yeah.

[C]

It's a star crossed love story. I'm hooked up.

[D]

Question. Would they have had to rename one or the other of the rivers if they'd merged?

[E]

That's a good question.

[D]

Do they hyphenate?

[C]

But that just kicks the can down the road.

[A]

What if that river meets another river? Bird. Come on.

[F]

Exactly, Right.

[C]

You know what?

[D]

This is unimportant.

[E]

So anyway, these dramatic changes in course are called avulsions, and they can be pretty catastrophic. I talked to Douglas Edmonds, who's a professor at Indiana University, and he told me avulsions have caused some of the biggest river floods in the history of our planet. There's a really recent tragic example of this when the Indus river in Pakistan moved west by 30 to 60 miles back in 2010, and that flooding affected about 20 million people and almost 2,000 people died.

[C]

Wow.

[E]

I know. It's really brutal, Right.

[C]

Because we don't expect rivers to change in this way. And like you said, we've built whole cities and whole neighborhoods, and people have their lives in a place where they didn't expect a river to be.

[A]

Yeah, exactly. And you want to be like, right up against the river. Right. To use it as best as you can.

[E]

Right. There have historically been so many benefits to being on a river. Right. It's great for trade. It's also a good water source. If it's not super polluted. It's also just kind of nice to be by a river. And so, you know, that particular instance in Pakistan, it happened during a really heavy monsoon storm. So it was especially quick. But most of all, Doug told me, are slower. They take decades or even centuries. So if he notices in time, we can build things like that floodgate in Louisiana, which is called Old River Control. And if anyone's interested in reading more about that, there's a great essay by John McPhee called Atchafalaya. I love it so much that I talk to people about it unbidden all the time. But the point is that, you know, in either case, whether it's the slow gradual avulsion or the fast moving one caused by a storm like in Pakistan, the stakes are really high. But it's still kind of a mystery. Like, we don't really know what causes these evulsions to happen. Unless we do. And we can do something about it.

[C]

On this one, I'm gonna say we do. I just feel like we know. I feel like rivers are so big, the stakes are so high. I mean, it is crazy that this happens. Like imagining the zoink. The turning left instead of right, or right instead of left. It does feel like a magical thing. Yeah. I'm gonna say we figured this one out. I'm gonna say someone knows why.

[A]

Okay.

[D]

But you do have to remember that this was unexplained for a long time.

[C]

No, it's so true. As I was saying that, I was like, we've had rivers. We've had rivers all time.

[E]

Like the Indus one was like not that long ago, you know?

[C]

Yeah.

[A]

Hold your thought for now. There might be another mystery coming. Or actually, there is another mystery coming from me right now. And mine starts with a seemingly simple question. What is the longest river in the world?

[C]

The Nile.

[A]

The Nile, Great answer. That is basically what everyone would say. It's what the US government says. It's what the Guinness Book of World Records says. It's what Wikipedia says. But scientists are starting to wonder whether the Amazon might actually be longer.

[C]

That was my number two.

[A]

You're very good on river length. Just intuitive river length.

[C]

I had a deep intuition of that.

[A]

So when I heard about this, I was sort of surprised that it was even a question.

[C]

Right. Like, can't we just measure?

[A]

Can't you just measure the river?

[C]

Big tape measure to the Amazon.

[A]

We see where the river is. Just measure it. So I called up this explorer, Yuri Sanada.

[D]

An explorer.

[A]

An explorer. I'm so excited to Just interview an explorer, especially because he has this expedition that he is planning to go the entire length of the Amazon on a boat.

[C]

Wow.

[E]

And it has a really big tape measure on it.

[A]

Yes. It's gonna just leave one end at the beginning of the Amazon, and then the snapback is just really dangerous. But he told me there's two things that make answering this question really hard. So one is deciding how to measure it, and two is deciding where it starts and ends. So let's just start with how to measure it.

[C]

Yeah. Do you mean, like, what tool to use? Or, like, do we go with meters or feet?

[A]

So it's related to something called the coastline paradox. Basically, like, rivers. Coastlines are really tough to measure. So if you look at Norway and compare it to Russia on a map, Russia looks like it has this huge coastline. Norway is just this little country hanging out at the top of Europe. But when you zoom in, you'll see that Norway has all of these things kind of etching into the land, these fjords. And when you follow every little nook and cranny into Norway, turns out Norway actually has the second longest coastline in the world. Much longer than Russia, longer than the entire European Union. And it's because when you zoom in, these squiggly lines get longer. And it's the same thing with rivers. So from above, the twists and turns, they get smoothed out. You see, like, okay, the river is just this kind of line, but when you zoom in, you see, it takes all of these little twists and turns, and. And the question is, how far should you keep zooming in? Should you zoom in to where you'll be measuring every twist and turn with a ruler or whether you'll be measuring every twist and turn with a microscope? The further you zoom in, the longer the river gets. It's honestly almost like a fractal. You could zoom in to the point that this question becomes absurd. And there's a situation where it could become, like, an infinitely long river that's probably beyond where we need to go.

[D]

All rivers are actually the same.

[A]

All rivers are the same. Trick question. And then also, like, rivers are changing. They're moving. It's tough to get a consistent measurement. Just like Neil was saying, it's like.

[C]

You'Re gonna have to approximate, like, what level are you approximating exactly?

[A]

Any way you measure it is an approximation. Right? You just have to decide what approximation you're comfortable with. But that's only one of the big issues. The next one is, where are you measuring from? So you might think, okay, just Go to the beginning of the Amazon. Right. But there's no one beginning. There's a bunch of little rivers that come out of the Andes and feed into the Amazon. You know, people used to think the furthest was the Maranon river in northern Peru. But then a few years ago, explorers followed another tiny river from the Amazon and they realized it went further away. This was the Montaro River. And Yuri says that at the source of the Montaro river, it just looks like some water dripping from a rock in the mountain.

[C]

That feels like the beginning.

[A]

I mean, it could be, but this is just the furthest source they've got so far. There could be another river that they haven't found that's just really tiny and twisty that could actually go longer and start even further away. But it's so remote, it's so high up. We're talking like 15,000ft up in the Andes. It's so high up that Yuri said that for the entire first month of his expedition, he's going to be whitewater rafting down the mountain for a month.

[C]

This is why we get to have explorers.

[A]

Yes. This is why we still, in 2025, we need explorers. And then the same issue happens at the end of the Amazon. It's less complicated, but when the Amazon hits the Atlantic, it runs through a series of islands right at the Atlantic where the land kind of starts to break up. And there's this question, does the river follow along the side of the islands, or does the river end when the islands start? And so we're left with this situation where right now most people think the Nile is the longest. One African scientist got in touch with Yuri because he heard about his expedition, was like, please, please, don't take the record from us. But Yuri is plowing ahead on his expedition, unless he's already done his expedition. And we do know what is the longest river. And I am pulling a fast one on you.

[C]

I'm gonna say, and I don't think Yuri would agree with me, but I'm gonna say this is not an answerable question at all. You've convinced me that the lengths, infinity, it's all an approximation anyway.

[A]

Oh, so your answer is there's no longest river?

[C]

My answer is there's no longest river. Yeah. I mean, one of my high school teachers once told me the difference between physics and math is if you're walking towards a wall in math, you never get there. Like, if you're going halfway towards the wall, you just always go halfway again. But, you know, in physics, eventually you get There.

[D]

Zeno's River.

[C]

Zeno's river. Exactly. They're all Zeno's River.

[A]

I mean, it's a creative explanation. You will have to ask, did we choose to answer this question with math or with physics?

[C]

Yeah, exactly. I mean, I think we don't know. I think we don't know this one.

[A]

Okay, you got three mysteries. You got the mystery of mussels, you got the mystery of rivers changing course, and then you got the mystery of the longest river. They're all unexplainable, or at least they all were unexplainable at one point. One of these mysteries has recently been figured out, and you're going to have a chance to make your final guess after the break. Support for Unexplainable comes from Anthropic, the team behind Claude. Claude is an AI that can help you explore different angles on all kinds of questions. Like the other day I was watching the Red Sox and they played this sound after a strikeout and I was like, huh. I don't remember when that became a thing. But it also wasn't something that I could just search for easily. So I asked Claude. He gave me some sources. I tracked him down, and before I knew it, I had the definitive history of the Woohoo. I also use Claude when I'm curious about genuinely complicated things. I'll ask it something big. We'll go back and forth to hone exactly what I'm looking for. And yeah, it does get things wrong sometimes, but when I double check or ask for a citation, it'll give me a better source. Now, it's important to know how to use Claude well, like ask it follow up questions. Actually check out the citations it gives you. But if you do, Claude can be a powerful tool. It helps me find articles and primary documents that I never would have come across. And it helps me dig wider and deeper, all while staying in control of of the digging. You can try Claude for free at Claude AI Unexplainable and see why the world's best problem solvers choose Claude as their thinking partner.

[G]

We all have moments where we could have done better. Like cutting your own hair.

[A]

Yikes.

[G]

Or forgetting sunscreen. So now you look like a tomato.

[E]

Ouch.

[G]

Coulda done better. Same goes for where you invest. Level up and invest smarter with Schwab. Get market insights, education and human help when you need it. Learn [e-mail].

[F]

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[D]

Unexplainable.

[A]

Okay. It's unexplainable. Or not. Sally, welcome back. Hello. Businesslike.

[E]

One word.

[D]

We're so happy to be here.

[A]

Hello. No, Sal, I feel like Sally's got her game face on. You know, she's ready. She's ready to go.

[D]

She is the person who you're playing Parcheesi, and she's like, this is actually not a joke, and I am mad.

[E]

At you that your go to game there was Parcheesi.

[A]

I don't even think I've ever played Parcheesi. What is Parcheesi? No, it was like, one of the most.

[D]

If you go. I went to, like, a board game exhibit, and like, half the board games.

[C]

Were Parcheesi is the most classic game. And also, I've never seen it. That's not.

[A]

This is what I'm saying, Sally.

[D]

Right. Well, later, as team bonding, we're all playing Parcheesi.

[C]

Okay.

[D]

And it will probably start a fight, presumably with Sally, because she takes games very seriously, apparently.

[A]

Anyway, so we've got three mysteries for you to choose from. Only one of them has been solved. Mystery one, what is killing freshwater mussels? Mystery two, why do rivers change course, sometimes by hundreds of miles? And mystery three, what is the longest river in the world? So, Sally, without making your final guess just yet, what are you thinking? What's going on in your head?

[C]

I mean, as we know, my initial impression was, we know about the rivers. We don't know about the mussels. We know about the evulsions. We don't know about the mussel killer and the Question of the longest river has become a philosophical one in my mind. That is unanswerable. I think what I'm feeling now is that maybe actually we know about the muscles because Bird is right. The river question has been unsolved for so long that maybe it's still unsolved. Maybe it's still just like a giant geological mystery.

[A]

I would say that when you're playing a game show, you might not want to take advice from the person trying to trick you.

[C]

That's true. That's a great point.

[E]

This is high level parteesi.

[C]

You guys are playing chess. I'm playing checkers. Birds playing Parcheesi.

[D]

I'm playing four dimensional Parcheesi.

[C]

Okay, I'm gonna say that we have found why the muscles are dying. We know the answer to that.

[A]

Okay. Muscles dying. Final answer.

[C]

Yes.

[A]

Okay, here's the answer.

[H]

It's almost as if you live in New Orleans, but over the course of a month, the river has disappeared. It's run dry. The river channel is still there, but the water is gone.

[D]

Okay.

[E]

Bird.

[D]

Oh, I'm sorry.

[A]

I wish people could see Sally's face looking like just. And bird just dancing like, ho, ho, ho, ho, ho. Wow.

[C]

Okay, so we've solved this long running geologic mystery that we've been struggling with for centuries. Neil, what do we know?

[E]

Yeah.

[C]

Why does this happen?

[E]

So the person you just heard is Doug Edmonds, who's that professor at Indiana University I mentioned earlier. He's one of the co authors of a paper about why evulgence happen.

[H]

So there are really two competing ideas for why river evulsions occur. One is simple and one is a little more complex.

[E]

The simple idea is that water likes to go downhill. And so if the water finds a steeper slope than the one it's currently on, it'll go that way instead. Doug calls that gradient advantage.

[H]

So the more complex explanation has to do with the fact that especially near the coastlines, where a lot of evulsions tend to occur, rivers are depositing sediment on their beds.

[E]

They're full of sediment, they're carrying it downstream. Some of it settles to the bottom of the river as it goes along.

[H]

If you imagine just like you jack up a car with a jack, that sediment kind of brings the river up above its surrounding floodplain.

[E]

That is what Doug calls superelevation, which is just one of those fantastic scientific names that everything should have. And we've known for a while that sediment can cause superelevation. We've known that since I think the 1940s or so. But it's been historically really hard to measure superelevation. Kind of related to what Noam was talking about earlier. There's, like, all kinds of factors that makes it hard to measure what the surface of a river actually is like. But that is exactly what Doug and his team did. And they did it with space lasers. Yep.

[H]

Space lasers.

[C]

No. Yes. I did not know space lasers were coming.

[E]

I. You know, I didn't either. Yeah. They used lidar from these NASA satellites to do two things. First, they actually measured the surface of rivers, as they are now all over the world. But they also use the satellites to find evidence of evulgence that happened over the past 30 years. And they found a bunch that they had no idea existed before.

[H]

We ended up realizing that both superelevation and gradient advantage actually work together to create the best predictor we now have of when a river evulsion will occur.

[C]

Wow.

[E]

And, you know, I think, like, in a lot of the US And Europe, our rivers are really engineered. We have things like dams and levees and that floodgate I mentioned earlier. But we also have seen how they fail. You know, 20 years ago, very famously, the levees on the Mississippi failed when Hurricane Katrina hit New Orleans. And similarly in the Indus river in Pakistan. Part of the reason that the evulsion happened is because a levy failed. So what Doug says, actually, is that the best sort of solve for evulsions, if you're going to go down the engineering route, is making spillways which essentially divert the flow of the river when it's in flood and sort of send some of that water and sediment into, like, a floodplain or somewhere else that, like, you know, you can safely sort of just dissipate some of the force. But, you know, that kind of infrastructure is expensive. And for most of the rest of the world, which might not have the capacity to build that infrastructure or the time, frankly, these hazard maps could help people figure out where the most urgent places to build the most targeted kinds of infrastructure could be, or at the very least, educate them on the risks, which is especially relevant now, because climate change is making flooding worse. There's more water in the atmosphere. When storms happen, they dump more water on things like rivers that then could flood in the way that the Indus river did. And so that data could just save a whole lot of lives.

[C]

Wow. So we know why rivers change course, and we can do something about it. This is good news, Neil.

[A]

So just to. Just to circle back to that initial question we got at the beginning, we started the show with this question from a listener about why did we pick our cover art? And our cover art is a map from 1944 of all the courses that the Mississippi river has taken over thousands of years.

[D]

Wow.

[C]

It's an avulsion map.

[A]

It's called something like the Mississippi Meander map. It's basically the map of Neil's mystery. And the basic reason I wanted to do a game show, just to answer this question, is to show how many mysteries there are in rivers. These things that maybe we take for granted, how much we still don't know. But, Bird, you were the one who found this specific 1944 river map. And I wonder if you could say a bit about what got you so excited about it.

[D]

Yeah, I think we were looking at a lot of really beautiful old scientific imagery that is just, you know, at the crux of sort of science and art, which is where I would like to hope that our show is. But I think what I really loved about this, as we were all talking about it, was this idea of, like, science, too, is this thing that is constantly sort of carving new channels for itself and finding new directions and new spaces and is, like, both shaped by the environment around it and shaping the environment around it. And so this particular image just sort of felt really representative of just some of the. The themes of our show.

[A]

I don't know. For me, I guess it reminds me that all the stuff that we take for granted, all of these things that you just. You look in the world and you see a river there, and you think that that just must be where it's always been. And it's kind of like the scientific knowledge we tend to take for granted that it's just that way because we figured out the right thing. And when you see the map that the Mississippi river has taken over thousands of years and you see that it's sort of gone in different ways and traced different pattern, to me, it tells you how hard it is to get to where we are today in terms of scientific knowledge and also rivers. And it also tells me that, like, so many of the things that we take for granted are not written in stone and are kind of always evolving.

[D]

I also like that Joey, the graphic designer we were working with, changed the colors such that it sort of also looks like guts and worms, but that is perhaps a less. We do like worms, theoretically advanced take on the logo.

[A]

Okay, one last thing before we leave. Sally.

[C]

Yes.

[A]

Even though you didn't win, you were.

[C]

So close, though I was so close.

[A]

Until Bird sabotaged you. Even though you didn't win, we do have a Consolation prize for you. We have a tradition on our game shows that I write a song about the revealed mystery. In this case, rivers changing course. So I'm gonna play it for you.

[I]

When I was young it was paradise Growing up right by the riverside by the riverside but one day I looked for tilapia and I couldn't find where the river was where the river was Mm mm How'd my river abandoned me? Maybe lasers from space Will see the evulsions of yesterday how my river curved away When I was young it was paradise But I've made my tilapia sacrifice no tilapia like anymore.

[C]

What?

[E]

I love this so much.

[D]

Noam, were you like. Did you write this while waiting for dinner? What is this? What is the tilapia?

[A]

I call it no tilapia.

[C]

Into the canon of river songs it goes.

[A]

That's it for unexplainable or not. Thank you to our guest, Sally Helm. Thank you to our presenters, Bird Pinkerton.

[D]

That's me.

[A]

And Neil Dinesha.

[E]

That's me.

[A]

And thanks to our audience for joining us. If you have a mystery or a solved mystery you want us to tell on an upcoming game show, let us know. You can write us@ unexplainableox.com we read all the emails and that is it for unexplainable or not. Goodbye.

[C]

Woo hoo.

[A]

This episode was reported by Bird Pinkerton, Meredith Hoddanott and me. No, I'm Hasenfeld. I also wrote the music production from Meredith, who also runs the show. Editing from Joanna Solotaroff. Guessing from Sally Helm. Thank you, Sally. Mixing and sound design from Christian Ayala. Fact checking from Melissa Hirsch. And Jorge Just and Julia Longoria are our editorial directors. Special thanks this week to Kim Moss for turning Bird onto freshwater mussels. And to Joey Sendai, Diego for designing our artwork. Thanks as always to Brian Resnik for co creating the show along with me and Bird. And if any of you out there have thoughts about the show, send us an email. We're@ unexplainableox.com you can also leave us a review or a rating wherever you listen, which really helps us out. And if you're into supporting the show even more and all of Vox in general, you can join our membership program. You can go to vox.commembers to sign up. And right now there's a sale. 30% off on an annual membership. Check it out. Unexplainable is part of the Vox Media Podcast network. And we'll see you next time.

[I]

When I was young, it was paradise. But I've made my tilapia sacrifice. No tilapia anymore.

[G]

We all have moments where we could have done better. Like cutting your own hair.

[A]

Yikes.

[G]

Or forgetting sunscreen so now you look like a tomato.

[E]

Ouch.

[G]

Could have done better. Same goes for where you invest. Level up and invest smarter with Schwab. Get market insights, education, and human help when you need it. Learn [e-mail].

[A]

And Doug, here we have the Limu emu in its natural habitat, helping people customize their car insurance and save hundreds with Liberty Mutual. Fascinating. It's accompanied by his natural ally, Doug. Uh, limu is that guy with the binoculars watching us? Cut the camera. They see us. Only pay for what you [e-mail]. liberty.

[C]

Liberty. Liberty.

[A]

Liberty Savings Fairy. Underwritten by Liberty Mutual Insurance Company and affiliates. Excludes Massachusetts.