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)

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.

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]

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]

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]

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.

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.