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Rivers and Erosion – How Water Carves the World · Talking Science

• 0:00
  I want you to picture something for a second.
• 0:01
  Just imagine you were standing right at the edge
• 0:05
  of this massive, breathtaking canyon. Oh, yeah,
• 0:08
  like a really immense drop. Exactly. You're looking
• 0:11
  out over this immense chasm, right? just staring
• 0:15
  down at these sheer rock walls that plunge, you
• 0:19
  know, so far into the earth. They look like they
• 0:21
  could just swallow entire mountains whole. Right.
• 0:24
  It's a very humbling kind of view. It really
• 0:26
  is. And as you're standing there, just like taking
• 0:29
  in the sheer scale of it, you're faced with this
• 0:31
  really jarring reality. Yeah. That massive void
• 0:34
  was carved by a river. Yeah. The water did all
• 0:37
  that work. Which is crazy to think about. It's
• 0:39
  the exact same unhurried flow of water you could
• 0:42
  happily... like paddle the canoe on. Yeah. So,
• 0:45
  okay, let's unpack this because staring at a
• 0:47
  mile of empty space that used to be solid rock,
• 0:50
  I am really struggling with the physics of it.
• 0:52
  I mean, that's a very natural reaction. Right.
• 0:54
  Because water is yielding. It's soft. Rock is
• 0:57
  hard. How does a peaceful stream physically crack
• 1:00
  open the earth? Well, the mistake we make is
• 1:02
  looking at water through the lens of human time.
• 1:04
  If you only observe a river for, you know, a
• 1:07
  few decades, water is just water. It seems gentle.
• 1:10
  But to understand how it cuts a canyon, you have
• 1:13
  to introduce deep time. Deep time, meaning like
• 1:17
  millions of years. Millions upon millions of
• 1:19
  years, yeah. When you apply deep time to the
• 1:21
  equation, rivers cease to be just bodies of water.
• 1:25
  They become these relentless geological engines.
• 1:28
  Even with millions of years, though, I mean,
• 1:30
  water flowing over solid granite feels like it
• 1:34
  should just polish the rock, maybe. Like make
• 1:37
  it slippery? Yeah, you'd think it would just
• 1:38
  kind of glide over it. Right. It doesn't feel
• 1:40
  like it should be able to literally saw a mile
• 1:42
  down into the crust. So what is the actual physical
• 1:45
  mechanism doing the cutting? So it comes down
• 1:47
  to kinetic energy, basically, and what that energy
• 1:50
  carries. If we travel up to the factory floor,
• 1:52
  so to speak, the birthplace of the river high
• 1:55
  up in the mountains. The water isn't just flowing,
• 1:57
  it's falling. Because it's so steep up there.
• 2:00
  Exactly. Gravity is pulling it down incredibly
• 2:03
  steep gradients, and that steepness converts
• 2:06
  the water's potential energy into raw kinetic
• 2:09
  energy. And that energy fuels this physical process
• 2:12
  called hydraulic action. Okay, hydraulic action.
• 2:15
  I've heard that term, but what does it actually
• 2:17
  look like on a microscopic level? Is the water
• 2:21
  just hitting the rock with a lot of force? It
• 2:23
  goes much deeper than just surface impact, actually.
• 2:26
  Oh, really? Yeah. So as water surges turbulently
• 2:30
  over a riverbed, the sheer pressure of the current
• 2:33
  forces water and bear bubbles deep into tiny
• 2:36
  microscopic fissures in the bedrock. Oh, like
• 2:39
  into the tiny cracks in the stone itself. Right.
• 2:41
  And then when the water rushes past and recedes
• 2:43
  for just a split second, the pressure suddenly
• 2:45
  drops. So those trapped air bubbles rapidly expand
• 2:48
  and they literally explode outward. Wait, wait.
• 2:50
  So the river is essentially using high pressure
• 2:53
  air pockets to blast the rock apart from the
• 2:55
  inside? Exactly. It is a continuous series of
• 2:58
  micro explosions happening like millions of times
• 3:00
  a day. That is wild. Yeah. And over time, that
• 3:04
  hydraulic action pries the bedrock apart. But
• 3:07
  that's really only the first phase. Did more.
• 3:09
  Oh, yeah. Once those fragments break off, the
• 3:12
  mechanics get much more violent. The river captures
• 3:15
  those broken pieces of mountain and they become
• 3:17
  its physical tools. Okay. Which brings us to
• 3:20
  the next major mechanism, what we call abrasion.
• 3:24
  Okay, I always picture abrasion like heavy -duty
• 3:27
  sandpaper. Like the water is just dragging grit
• 3:31
  and gravel along the bottom to grind away the
• 3:33
  bed. What's fascinating here is that the sandpaper
• 3:35
  analogy is a good starting point, but it's really
• 3:38
  more like a sandblaster combined with a jackhammer.
• 3:40
  A jackhammer. Okay, that's a violent image. Well,
• 3:43
  geologists categorize a river's cargo. We call
• 3:46
  its load into different types. The heavy materials,
• 3:49
  the large stones and boulders, they make up the
• 3:51
  bed load. Right, because they stay on the bed
• 3:53
  of the river. Exactly. But because the water
• 3:54
  is moving so fast and turbulently in the mountains,
• 3:57
  these heavy rocks don't just slide nicely along.
• 4:00
  They get picked up by the current, carried forward,
• 4:02
  and slammed back down against the bedrock. So
• 4:05
  they are actively hammering the riverbed as they
• 4:07
  travel downstream. Yes. It's a process called
• 4:09
  saltation. They bounce and they smash. They smash
• 4:13
  into the bedrock and they smash into each other,
• 4:15
  shattering into smaller and smaller pieces as
• 4:17
  they go. Wow. A mountain river is literally sawing
• 4:20
  its way down into the earth using the earth's
• 4:23
  own fragmented stones as the teeth of the saw.
• 4:26
  The mountain provides the exact ammunition required
• 4:29
  for its own demolition. Okay, that completely
• 4:32
  changes how I look at a mountain stream now.
• 4:34
  Like, it's not just a peaceful brook. It's a
• 4:37
  high -speed, self -sustaining demolition site.
• 4:39
  That's a perfect way to describe it. But mountains
• 4:42
  don't go on forever, right? I mean, eventually
• 4:44
  the river descends out of the highlands. The
• 4:47
  steep slopes soften. The landscape flattens out
• 4:49
  into plains. Right, the gradient totally changes.
• 4:52
  So when that gradient drops, what happens to
• 4:54
  all that kinetic energy? Because, you know, physics
• 4:57
  says it can't just vanish. No, the energy doesn't
• 4:59
  vanish, but its application fundamentally shifts.
• 5:02
  Up in the steep mountains, gravity dictates that
• 5:05
  the river's main job is downward erosion. Cutting
• 5:08
  that deep V shape into the earth. Exactly. But
• 5:11
  once it hits the flatter plains, it loses that
• 5:14
  aggressive downward momentum. So its primary
• 5:17
  job shifts from eroding to transporting. Right.
• 5:20
  It has to move all the debris it just blasted
• 5:23
  out of the mountains. And moving that cargo is
• 5:25
  a monumental task. We mentioned the bouncing
• 5:28
  bed load, but the river is also carrying a massive
• 5:31
  suspended load. Suspended load. So that's the
• 5:33
  stuff floating in the water. Yeah. These are
• 5:35
  the fine particles of like silt and clay that
• 5:38
  swirl in the current. It's actually what gives
• 5:40
  lower rivers that cloudy kind of muddy look.
• 5:43
  Oh, okay. Like the Mississippi or something.
• 5:46
  Exactly. And beyond that, there is the dissolved
• 5:48
  load. Wait, dissolved? Yeah. Like salt and water.
• 5:51
  Right. minerals chemically leached from the rocks.
• 5:54
  Totally invisible to the naked eye, but it represents
• 5:57
  thousands of tons of material moving downstream.
• 6:00
  So out on the plains, the river... basically
• 6:02
  transforms into this massive geological conveyor
• 6:05
  belt. Exactly, a giant conveyor belt. But this
• 6:08
  is also where the shape of the river completely
• 6:10
  changes, right? Like it stops running in a straight
• 6:12
  line and starts carving these broad, sweeping
• 6:14
  loops that geologists call meanders. Yep, meandering
• 6:18
  is a classic feature of flatland rivers. I've
• 6:21
  always wondered about this, actually. If the
• 6:23
  ground is totally flat, why doesn't the water
• 6:26
  just take the shortest, straightest pass to the
• 6:28
  ocean? It seems like it should, right. But when
• 6:31
  you look at fluid dynamics, a straight line is
• 6:33
  actually incredibly unstable in nature. Really?
• 6:36
  Unstable? Yeah. When a river hits flat ground,
• 6:40
  any minor obstacle, like a slightly harder patch
• 6:43
  of clay, a fallen tree, an uneven bank, it will
• 6:46
  deflect the flow of water just a fraction of
• 6:48
  a degree toward one side. Huh. So a tiny nudge
• 6:51
  creates a slight curve. But how does a slight
• 6:54
  curve turn into a massive... miles wide loop
• 6:58
  it comes down to centrifugal force and sheer
• 7:00
  stress think about being in a car taking a sharp
• 7:03
  turn at high speed okay you get pressed hard
• 7:05
  against the outside door right water does the
• 7:07
  exact same thing as the river enters that slight
• 7:09
  curve the centrifugal force pushes the fastest
• 7:12
  most energetic water against the outside bank
• 7:15
  of the bend which means that outside bank is
• 7:17
  taking the absolute brunt of the river's energy
• 7:19
  It takes massive shear stress, accelerating erosion
• 7:22
  on the outer edge. But the physics on the inside
• 7:26
  of the curve are totally different. Because the
• 7:28
  water is slower there. Exactly. Because the fastest
• 7:30
  water is pushed outside, the water on the inside
• 7:33
  of the bin slows down dramatically. And when
• 7:36
  water loses velocity, it loses its ability to
• 7:39
  carry sediment. Oh, I see. So it starts dropping
• 7:41
  its suspended load right there on the inner bank.
• 7:44
  Got it. So it's aggressively chewing away the
• 7:46
  outside bank while simultaneously dumping new...
• 7:49
  land on the inside bank. Yes. And there is a
• 7:52
  fascinating three -dimensional dynamic driving
• 7:54
  this too. It's called helicoidal flow. Helicoidal
• 7:58
  flow. What does that mean? Well, as that fast
• 8:00
  surface water crashes into the outer bank, it
• 8:03
  gets forced downward toward the riverbed. It
• 8:06
  scrakes the bottom, sweeps across the channel,
• 8:08
  and spirals back up toward the inner bank. Wait,
• 8:11
  wait. So the water isn't just moving forward.
• 8:13
  It's moving in a continuous corkscrew shape down
• 8:16
  the river channel. A literal corkscrew, yes.
• 8:19
  This helicoidal flow acts like a vacuum. It sweeps
• 8:23
  the freshly eroded dirt from the outside bank,
• 8:26
  rolls it across the riverbed, and neatly deposits
• 8:29
  it on the inside bank. That is amazing. It creates
• 8:32
  a permanent feedback loop. The outside pushes
• 8:35
  further out, the inside fills further in. Over
• 8:38
  thousands of years, that tiny initial deflection
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  exaggerates into a massive, sweeping meander
• 8:45
  across the floodplain. And sometimes those loops
• 8:47
  get so extreme they actually cut themselves off.
• 8:49
  They do, yeah. As the meander loops wider and
• 8:52
  wider, the neck of land between the two curves
• 8:54
  gets narrower and narrower. Eventually, usually
• 8:57
  during a major flood, the river will just break
• 9:00
  straight through that narrow neck. It just takes
• 9:01
  the shortcut. Right. It takes the steep... more
• 9:04
  direct path and completely abandons the old loop.
• 9:07
  That abandoned curve of water is left behind
• 9:09
  as a crescent -shaped lake. An oxbow lake. Exactly,
• 9:12
  an oxbow lake. So by looking at a river valley
• 9:14
  from above, you know, seeing all the sweeping
• 9:17
  curves and the scattered oxbow lakes, we're essentially
• 9:20
  reading a physical record of fluid dynamics trying
• 9:22
  to balance energy across a flat landscape. Precisely.
• 9:25
  The shape of a river is a readable history of
• 9:28
  its energy state. I love that way of framing
• 9:30
  it. Yeah, a straight, deep channel tells you
• 9:33
  gravity and downward cutting are dominating.
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  A wide, wandering channel surrounded by oxbow
• 9:38
  lakes tells you lateral fluid dynamics and sediment
• 9:41
  transport are running the show. Here's where
• 9:44
  it gets really interesting to me, though. All
• 9:46
  of this material... The bed load, the suspended
• 9:48
  silt, the dissolved minerals, it's all riding
• 9:51
  this conveyor belt. Unstoppably moving downstream.
• 9:54
  But this process can't go on infinitely, right?
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  Eventually the river reaches the ocean. The conveyor
• 10:00
  belt has to stop. And this introduces the river's
• 10:03
  third great geological job. We had erosion, we
• 10:06
  had transportation, and now we arrive at deposition.
• 10:10
  Drop in the cargo. Right. As the river meets
• 10:13
  the sea, the flat topography and the sheer standing
• 10:16
  mass of ocean water act as a massive hydraulic
• 10:18
  brake. The river's velocity plummets almost instantly.
• 10:22
  And because carrying capacity is tied to velocity,
• 10:24
  the river simply drops whatever it's holding.
• 10:26
  It drops it in a very specific, sorted order
• 10:29
  based on settling velocity. Oh, so it doesn't
• 10:31
  just dump everything all at once. No, it's very
• 10:34
  organized, purely by physics. The heaviest sands
• 10:37
  and gravels fall out of suspension first. Then
• 10:40
  the finer salts settle. Finally, the lightest
• 10:42
  clays are carried furthest out into the saltwater
• 10:44
  before they slowly, eventually, drift to the
• 10:48
  bottom. But because it's dumping so much solid
• 10:51
  material in one centralized place, the river
• 10:54
  actually starts choking on its own cargo, doesn't
• 10:57
  it? Like it builds a roadblock out of its own
• 10:59
  sediment. It does. It fills in its own channel
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  until the water is basically forced to spill
• 11:03
  over the banks and branch out into dozens of
• 11:06
  smaller diverging streams called distributaries.
• 11:09
  Right. Distributaries. And this continuous dumping
• 11:12
  and branching fans out into that iconic triangular
• 11:15
  shape we call a delta. The river is literally
• 11:18
  building entirely new land masses out into the
• 11:21
  ocean. I was thinking about this earlier and
• 11:23
  it is a total mind bender. Like if you follow
• 11:25
  the logic of everything we just assessed. The
• 11:27
  hydraulic action in the mountains, the transportation
• 11:29
  across the plains, the deposition at the coast
• 11:31
  of Delta isn't just a swampy pile of mud. You
• 11:34
  are looking at a dismantled mountain. That is
• 11:36
  the profound reality of deep time. It's just
• 11:39
  wild. You are looking at the pulverized remains
• 11:41
  of distant highlands, reassembled grain by grain
• 11:45
  at the edge of the sea. Wow. Given enough millions
• 11:48
  of years, rivers will not just carve deep canyons,
• 11:51
  they will erase entire mountain ranges down to
• 11:55
  sea level. But hold on, that brings up a massive
• 11:57
  contradiction, I feel like. Oh, how so? Well,
• 12:00
  the Earth is billions of years old. If rivers
• 12:03
  are constantly grinding the land down and washing
• 12:05
  it into the ocean, why aren't the continents
• 12:08
  completely flat by now? Why do we still have
• 12:11
  the Himalayas or the Rockies? Ah! That is because
• 12:13
  the Earth's surface is trapped in a permanent
• 12:16
  staring contest. A staring contest. Yeah. On
• 12:19
  one side, you have the exogenous forces, water,
• 12:21
  ice, gravity, relentlessly trying to flatten
• 12:24
  the world. On the other side, you have endogenous
• 12:26
  forces, plate tectonics. Ah, right. Deep inside
• 12:29
  the Earth, massive crustal plates crash together,
• 12:32
  buckling the surface upward to forge new towering
• 12:35
  mountain ranges. So the tectonics push the crust
• 12:37
  up into the sky, and the rivers immediately start
• 12:40
  attacking it to bring it back down. It is a permanent
• 12:42
  state of tension. Every gentle rolling hill you
• 12:46
  drive past used to be a jagged peak, and every
• 12:49
  jagged mountain you see today is currently in
• 12:51
  the process of being leveled. That puts things
• 12:54
  in perspective. Yeah, the scenery outside your
• 12:56
  window is just a frozen snapshot of an endless,
• 12:59
  slow -motion war between tectonic uplift and
• 13:03
  river erosion. So what does this all mean for
• 13:06
  us? Because, you know, this endless geological
• 13:09
  war isn't just ancient abstract history. It directly
• 13:13
  created the environment human beings rely on
• 13:16
  to survive. Oh, absolutely. We basically built
• 13:18
  our entire civilization right in the crossfire
• 13:21
  of this process, particularly when you look at
• 13:23
  floodplains and deltas. Human civilization owes
• 13:26
  its existence to river deposition, frankly. Really,
• 13:29
  its existence. Without a doubt. Because the river
• 13:31
  is grinding up rocks from the mountains, the
• 13:33
  sediment it carries is packed with essential
• 13:35
  raw minerals. When a river meanders and naturally
• 13:38
  floods its banks on the plains, the water slows
• 13:40
  down and drops a fresh layer of this fine, mineral
• 13:43
  -rich silt across the land. It's essentially
• 13:45
  delivering premium fertilizer straight to the
• 13:47
  soil. Exactly. Which explains why the Nile, the
• 13:51
  Tigris, the Mississippi... Almost all great human
• 13:54
  civilizations and our most productive agricultural
• 13:57
  zones are located right on top of active floodplains.
• 14:01
  Because we desperately need that rich soil to
• 14:03
  feed ourselves. Yes. But this creates a massive
• 14:07
  paradox. Right. We need the soil that floods
• 14:09
  create, but we build our cities on these plains
• 14:11
  and human beings cannot tolerate floods. Exactly.
• 14:14
  And the human response to that paradox has always
• 14:17
  been engineering. We try to fight the river.
• 14:19
  We try to isolate it. We build massive concrete
• 14:22
  levees, dams and artificial embankments to lock
• 14:25
  the river into a single permanent predictable
• 14:27
  channel. We stop the water from spreading out
• 14:30
  so we can protect our homes and our crops. Let
• 14:32
  me push back on this strategy, though, because
• 14:34
  looking. at the physics we just talked about,
• 14:36
  it feels fundamentally flawed. Oh, it has massive
• 14:39
  unintended consequences. Right. Because if a
• 14:42
  river's natural mandate is to erode, transport,
• 14:44
  and deposit, and we build concrete walls so it
• 14:47
  can never deposit its load on the floodplain,
• 14:49
  where does all that sediment go? It doesn't just
• 14:51
  vanish. If we connect this to the bigger picture,
• 14:54
  it goes exactly where you'd fear, and the consequences
• 14:56
  are devastating. Oh, no. By walling the river
• 14:59
  in with levees, we force the water to stay deep
• 15:02
  and move faster. It acts like a fire hose. A
• 15:05
  fire hose of mud. Yes. Instead of naturally spreading
• 15:09
  its sediment across the delta over time, the
• 15:12
  river shoots all that sand and silt straight
• 15:14
  out the mouth into the deep ocean. So we are
• 15:16
  completely... starving the delta of new land
• 15:19
  we are starving it and deltas are incredibly
• 15:22
  heavy geological structures because of all the
• 15:25
  mud exactly and without fresh layers of sediment
• 15:28
  being constantly added by seasonal floods to
• 15:31
  build the land up the sheer weight of the existing
• 15:34
  mud causes the land to compact and physically
• 15:37
  sink wow geologists call this subsidence wait
• 15:40
  so by building levees to protect coastal cities
• 15:43
  on a delta we are actually causing the ground
• 15:45
  underneath those cities to sink below sea level.
• 15:48
  Look at New Orleans and the broader Mississippi
• 15:50
  River Delta. The Mississippi is one of the most
• 15:53
  heavily levied and engineered rivers on the planet.
• 15:56
  Right, they have levees everywhere down there.
• 15:58
  And because the delta is completely cut off from
• 16:00
  its natural resupply of sediment, the land is
• 16:03
  subsiding at an alarming rate. It is sinking
• 16:06
  into the Gulf of Mexico. That is tragic. It is.
• 16:10
  Ironically, our engineering to prevent localized
• 16:12
  river flooding has made the entire region infinitely
• 16:15
  more vulnerable to catastrophic oceanic flooding
• 16:19
  from hurricanes and sea level rise. It really
• 16:21
  sounds like we're just picking a fight with millions
• 16:23
  of years of geology. Can we ever really tame
• 16:27
  it? a river with concrete long term? Or is it
• 16:30
  a fight we are guaranteed to lose? When you measure
• 16:33
  time in centuries rather than, you know, fiscal
• 16:35
  quarters, geology always wins. Geology always
• 16:38
  wins. Always. Sensible land and water management
• 16:41
  requires understanding fluid dynamics and respecting
• 16:44
  the deep time mandate of the water. We cannot
• 16:46
  just plug the exhaust pipe. So what's the alternative?
• 16:48
  Managing flood risk sustainably means giving
• 16:51
  the river room to spread out. It means designing
• 16:54
  designated floodways where the river can safely
• 16:56
  deposit its load, and rebuild the land without
• 16:58
  destroying communities. We have to work in tandem
• 17:00
  with the engine. Exactly. You have to work with
• 17:02
  the natural process, not against it. That is
• 17:05
  a completely different paradigm for how we view
• 17:08
  our infrastructure. Like, we have to stop seeing
• 17:11
  the river as a pipe we can route and start treating
• 17:15
  it like a living geological force. That's beautifully
• 17:18
  put. Next time you cross a bridge over a river,
• 17:20
  I really want you to look down at the water.
• 17:23
  If it's got a brown kinge to it, if it's murky
• 17:26
  and turbulent, realize you aren't just looking
• 17:28
  at dirty water. You are watching the land itself.
• 17:32
  You are watching a piece of a mountain from hundreds
• 17:35
  of miles upstream traveling, grain by patient
• 17:38
  grain to the sea. This raises an important question,
• 17:41
  though, and it's a thought I'd love to leave
• 17:43
  you with today. Oh, please do. We talked earlier
• 17:45
  about that grand tension between the earth pushing
• 17:47
  up and the rivers tearing down. Plate tectonics
• 17:51
  act as the engine pushing mountains into the
• 17:53
  sky and rivers relentlessly wear them away. Right.
• 17:55
  The ultimate staring contest. Right. But what
• 17:58
  if the Earth's tectonic engine ever just stopped?
• 18:02
  Stopped, like completely? Yeah, I mean, the core
• 18:05
  of our planet is slowly cooling. If billions
• 18:08
  of years from now, the deep heat dissipated and
• 18:11
  the tectonic plates stopped colliding, there
• 18:14
  would be no new uplift. No new mountains would
• 18:16
  ever be born. Wow. But the sun would still shine
• 18:19
  and the rain would keep falling. The water would
• 18:21
  keep flowing. Oh, wow. Because the erosion engine
• 18:24
  is powered by solar energy and gravity, not the
• 18:27
  Earth's internal heat. Exactly. So if uplift
• 18:30
  permanently stopped, would these patient, gentle
• 18:33
  rivers just keep sawing and transporting? Would
• 18:36
  they eventually wear every single continent perfectly
• 18:38
  flat until the entire solid landmass of Earth
• 18:41
  slipped quietly beneath a shallow global ocean?
• 18:45
  A perfectly smooth underwater world, all because
• 18:48
  the rivers never, ever stopped working. That
• 18:51
  is a haunting and beautiful image. Definitely
• 18:53
  something to mull over the next time you're out
• 18:55
  paddling a canoe. Thanks for diving deep with
• 18:57
  us today.