Chapter 16: Running Water

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Welcome to the Deep Dive, your shortcut to being well informed.

Today we're diving into something familiar but maybe surprisingly powerful.

Running water and its incredible impact on our planet.

It really is profound.

We see rivers all the time, but their geological role is immense.

Okay, let's unpack this a bit.

Rivers, they're vital, right?

For economies farming, power, fun.

They're lifelines.

Absolutely.

Couldn't live without them in many places.

But here's the twist we want to explore.

They are the dominant force changing Earth's surface.

They carve, they carry sediment, and sometimes, well, they unleash devastating floods.

What's truly fascinating here is seeing water as this dynamic link in the whole Earth system.

It's constantly cycling, constantly sculpting.

That's the core idea we want you to grasp today, how water shapes our world.

Right.

So our mission is to take all this geological information which can feel dense and make it clear, engaging, hopefully relevant.

I want you to look at a river, maybe even a landscape, differently after this.

We'll cover where the water is, how rivers form, how they flow in a road, where that material goes.

And importantly,

our relationship with floods, how we manage that power.

Let's get started.

So the big picture first, where is all this water and how does it move around?

Earth's kind of special in the solar system, isn't it?

Perfect spot for water in all three states, ice, liquid, gas.

Exactly.

That allows for our oceans and this constant global water movement, the hydrologic cycle.

And then the numbers are pretty wild when you break it down.

Oceans hold what, over 96 %?

Over 96 .5%, yeah.

So freshwater is just a tiny sliver, about 2 .5%.

And most of that is locked up in glaciers and ice sheets.

That's right, over two -thirds of the freshwater.

Groundwater is the next biggest chunk, around 30%.

So the surface water we actually see, lakes, rivers, swamps, it's a fraction of a fraction.

Rivers themselves are less than half a percent of freshwater.

Less than half a percent of surface freshwater, which is already only a tiny part of the total freshwater.

It's vinescule.

It makes you realize how critical that moving water is.

Okay, so this tiny amount is constantly moving.

Tell us about the hydrologic cycle.

It's this unending circulation.

Water moves between the oceans, atmosphere, land, even living things.

The hydrosphere, atmosphere, geosphere, biosphere.

Think of it like earth's plumbing.

Driven by the sun and gravity.

Exactly.

Key processes.

Water soaking into the ground that's infiltration.

Water flowing over the surface that's runoff.

Okay.

Then water turning to vapor from oceans, lakes, soil evaporation, and plants releasing water vapor transpiration.

Right.

Plants sweating.

Sort of.

Yeah.

We often combine those last two into evapotranspiration.

There's a balance overall.

More water evaporates from the oceans than falls back as rain.

But the runoff from land flowing back into the oceans makes up the difference.

Keeps sea levels pretty stable, generally.

But the key insight here.

The key insight is that despite being such a tiny percentage, this running water, this runoff is the single most important agent of erosion shaping our planet's land surface.

Period.

That's huge.

And you mentioned groundwater.

Yeah.

Crucial hidden player.

It seeps out slowly into streams, keeping them flowing between rainstorms.

It's the buffer.

Got it.

So we know water cycles.

How does it collect into, well, streams and rivers?

It rains.

Then what?

Well, that rainwater either soaks in, infiltrates, or it runs off.

Several things decide which path it takes.

Like how hard it's raining.

Exactly.

Intensity and duration of rain.

Also how wet the soil already is.

The type of surface matter is hugely concrete versus forest floor, obviously.

Makes sense.

More runoff in cities.

Definitely.

And the slope of the land and vegetation cover plants slow down runoff, help infiltration.

So runoff starts small.

Yeah.

Begins a sheet flow, just a thin layer moving downslope.

But gravity quickly concentrates it into tiny channels called rills.

Like tiny little ditches?

Pretty much.

Rills merge into bigger gullies than those feeding to brooks or creeks what geologists broadly call streams.

So stream is the general term.

Technically, yes.

Any water flowing in a channel.

We usually save the word river for the larger streams that have tributaries feeding into them.

Okay.

And all these streams collect water from a specific area.

Right.

That's the drainage basin or watershed.

It's all the land that drains into one particular stream system.

Think of it like a funnel.

They're separated by high ground called a divide.

Like the Continental Divide.

That's a massive example, yeah.

But divides exist between tiny adjacent streams, too.

The Mississippi basin is enormous drains, nearly 40 % of the continental U .S.

Wow.

And streams grow.

They do.

Through headward erosion, they actually lengthen their courses upslope, chewing away at the divide, capturing more drainage area over time.

Interesting.

And rivers have different zones.

Yeah.

A useful way to think about them is in three zones based on the main process happening.

Up in the headwaters, it's the zone of sediment production.

Erosion is king weathering, landslides, channel scouring.

Okay.

Where the material comes from.

Then you have the main zone of sediment transport.

The trunk streams just moving that material downstream.

Not much net erosion or deposition.

Just a conveyor belt.

Basically.

And finally, at the mouth of the river where it meets a lake or the ocean, that's the zone of sediment deposition.

The water slows, drops its load, builds features like deltas.

Production transport deposition.

Got it.

Now, you mentioned geology influences this.

Oh, absolutely.

The underlying rock structure leaves a huge imprint.

Here's where it gets really interesting.

The drainage patterns.

The network of streams itself tells a story.

How so?

Well, a dendritic pattern looks just like a branching tree.

That usually means the underlying rock is pretty uniform, flat lying.

The water just flows downhill wherever gravity takes it.

Okay, simple enough.

But then you see a trellis pattern.

It looks like a garden trellis, right?

Parallel streams flowing in valleys joined by short tributaries coming down the sides at right angles.

Oh, I can picture that.

That's a dead giveaway for areas with folded rock layers like the Appalachians.

Alternating bands of resistant ridges and easily eroded valleys force that pattern.

So the pattern reveals the hidden structure.

Exactly.

There are others too, like radial patterns flowing off a central high point like a volcano, or rectangular patterns where streams follow faults and joints in the rock, making lots of right angle turns.

Fascinating.

It's like reading a map hidden in the landscape.

What about those rivers cutting straight through mountains?

Water gaps.

Yeah, those are cool.

Two main ways they form.

An antecedent stream might have been there before the mountain started rising.

It just kept pace, cutting down as the land lifted.

Wow, persistent.

Or it could be a superposed stream.

It established its course on softer, overlying rock layers first, then as it cut down, it just kept going, sawing right through the harder, folded rocks underneath, maintaining its original path, like it was laid on top, then cut down.

Like the river didn't even notice the mountain rising beneath it.

Pretty much.

It reveals a complex history.

Okay, let's get into the mechanics.

What makes the water flow so powerfully?

Is it smooth?

Rarely smooth.

That's laminar flow.

Straight paths.

Very uncommon in nature.

Almost all stream flow is turbulent flow.

Chaotic.

Swirling.

Mixing.

Think eddies.

Rapids.

And that turbulence is important.

Crucial.

It keeps sediment suspended and helps erode the channel bed and banks much more effectively.

What controlled how fast it flows?

The velocity.

Several factors.

The gradient, the slope, is a big one.

Steep mountain streams flow faster than rivers on flat plains, obviously.

Makes sense.

Then the channel shape.

A narrow, deep channel actually flows faster than a wide, shallow one for the same amount of water.

Less friction along the edges and bottom what we call the wetted perimeter.

Huh.

Okay.

Less drag.

Right.

Channel size and roughness also play a role.

Bigger, smoother channels flow faster.

Boulders, logs, rough beds, they slow things down.

Got it.

And the amount of water.

Absolutely critical.

That's discharge the volume of water passing a point per unit time.

Calculated as channel cross -sectional area times velocity.

So more water generally means faster flow too.

Often, yes.

And discharge varies hugely.

The Mississippi is massive, but the Amazon River disturbs something like 13 times more water.

13 times.

Unbelievable power.

So if you look at a river profile from start to finish.

The longitudinal profile.

It's usually contaves deeper near the headwaters, gentler near the mouth.

So the slope decreases downstream, but you said velocity increases.

That seems weird.

It does seem counterintuitive, but velocity generally does increase downstream.

Why?

Because discharge usually increases as tributaries join.

The channel gets bigger and often smoother, making flow more efficient, overcoming the gentler slope.

Ah, okay.

Efficiency wins out.

Largely, yes.

And agencies like the USGS constantly measure discharge and river stage the water level using stream gauges, vital data for flood forecasting, water management, even bridge design.

Okay, so this turbulent, powerful water,

what exactly is it doing to solid rock?

How does it erode?

Good question.

Three main ways in bedrock channels.

First, quarrying.

The sheer force of the water can lift and pluck out blocks of rock that are already loosened by weathering or fractures.

Just ripping chunks out.

Pretty much.

Second, abrasion.

This is like sandpaper.

The particles the water carries sand, gravel constantly grind against the channel bed and banks.

This smooths pebbles and can carve out circular holes called potholes or swirling eddies concentrate the grinding.

I've seen those.

Yeah.

The classic abrasion features.

And third, corrosion.

This is chemical weathering.

The water itself can slowly dissolve certain types of rock like limestone.

Quarrying, abrasion, corrosion.

Okay, so the rivers picked up all this material.

How does it carry it?

It transports sediment in three ways.

First, the dissolved load.

This is mineral matter dissolved in the water.

Totally invisible.

Comes from chemical weathering.

Velocity doesn't really affect how much is dissolved.

Okay, invisible cargo.

Then the suspended load.

These are the fine particles, silt, clay, fine sand held up within the water by turbulence.

This is what makes rivers look muddy.

Most sediment is carried this way.

Like the Yellow River in China.

Perfect example.

And finally, the bed load.

This is the coarser stuff, sand, gravel, even boulders that moves along the channel bottom by rolling, sliding, or sort of hopping along.

Hopping.

Yeah, it's called saltation.

Particles get lifted by the current, travel a short distance, then settle back down, bumping other particles.

It's slower, more localized movement.

Dissolved, suspended, and bed load.

Got it.

And floods change this.

Dramatically.

We talk about a stream's capacity, the maximum amount of solid sediment it can carry, and its competence, the maximum size of particles it can move.

During a flood,

discharge and velocity skyrocket, so both capacity and competence increase massively.

Rivers do most of their erosional and transport work during these high flow events.

So short bursts of intense activity cart the landscape over time.

Exactly.

It's not usually a slow, steady grind.

After all that erosion and transport, where does the material go?

When does the river drop it?

Deposition happens whenever the stream slows down.

As velocity decreases, competence drops.

The biggest particles settle out first, followed by smaller and smaller ones.

So it sorts the material by size.

It does that sorting.

You find gravels deposited in one place, sands in another, silts further on.

And the general term for all the stream deposited sediment is alluvium.

Alluvium.

Okay.

So streams build things too, not just carve.

Let's talk about the channels and valleys they create.

You mentioned bedrock versus alluvial channels.

Right.

Bedrock channels cut directly into solid rock, often steep.

Think rapids, waterfalls, canyons.

Alluvial channels form in the stream's own sediment, the alluvium.

They're much more free to change shape.

And alluvial channels have different styles.

Two main types.

Meandering channels have those sweeping, s -shaped curves.

They mostly carry finer sediment and suspension.

The fastest water flows on the outside of the bend, eroding the bank, that's the cut bank.

Water slows on the inside of the bend, depositing sediment to form a point bar.

So the meander actually migrates sideways and downstream over time.

And sometimes they take a shortcut.

Exactly.

The river cuts across the narrow neck of a loop, abandoning the old meander, which becomes a crescent -shaped oxbow lake.

Seen those too.

What's the other type?

Braided channels.

These look like a complex network of dividing and rejoining channels separated by sand or gravel bars.

They form when a stream has a lot of coarse sediment, more than it can easily carry, and often variable discharge.

Think meltwater streams coming off glaciers.

Like the Nick River in Alaska you mentioned.

Okay, now broadening out from the channel to the whole valley.

The stream valley includes the channel and the terrain sloping down towards it.

Early on, streams focus on cutting down, creating narrow, v -shaped valleys.

Like Yellowstone River.

Classic example.

Or slot canyons.

But as streams mature, they tend to widen their valleys.

How does that relate to base level you mentioned?

Base level is key.

It's the lowest point a stream can erode down to.

Ultimate base level is sea level.

But lakes, resistant rock layers, or even a larger river can act as local or temporary base level.

So base level changes.

The stream has to adjust.

If base level drops, like sea level falling, the stream cuts down more aggressively.

If base level rises, like building a dam, the stream deposits sediment upstream to try and build its bed back up to the new level.

It's always trying to reach equilibrium.

Exactly.

A stream in equilibrium is called a graded stream.

It has just the right slope and velocity to transport the sediment supplied to it without net erosion or deposition.

It's a dynamic balance.

So young streams focus on valley deepening, down cutting, making v -shapes, rapids, waterfalls like Niagara.

Which is actually retreating upstream as their resistant cap rock is undercut.

Right.

And then older streams focus on valley widening through that lateral erosion of meanders, creating broad floodplains.

Floodplains, exactly.

Those flat valley floors built up by the river's own sediment during floods.

Sometimes though, you see winding rivers in deep, steep canyons.

In -sized meanders.

Yes.

Those form when a river that was already meandering on a flatter surface experiences a drop in base level, maybe due to land uplift.

It keeps its winding pattern, but starts cutting straight down into the bedrock.

The Colorado Plateau is full of stunning examples.

And stream terraces.

Those are remnants of former floodplains, left behind as the river cuts down further.

They look like steps along the valley sides.

Each step marks an older, higher level of the valley floor.

Amazing history recorded there.

Okay, deposition creates more than just floodplains, right?

Deltas.

Oh yes.

Deposition builds some major landforms.

Besides temporary sand and gravel bars in the channel, you get deltas.

These form where a river enters standing water, a lake, or ocean.

Velocity drops suddenly.

And it just dumps its load.

Right.

Usually forming a fan or triangle shape.

The sediment layers build out in a specific structure inclined foreset beds, horizontal topset beds on top, and fine bottomset beds further out.

The main channel often splits into smaller distributaries that spread the water and sediment out.

Like the Mississippi Delta's bird foot shape.

Classic example.

Though as you noted, that delta is complex, built from multiple sub -deltas.

And it's in trouble partly because artificial levees prevent sediment from replenishing the wetlands.

A real -world consequence.

What about levees?

Natural levees are built by the river itself.

During floods, as water spills out of the channel, it slows immediately, dropping the coarsest sediment right next to the bank, building up low ridges over time.

So the banks are naturally slightly higher.

Often, yes.

Behind these levees, you get low -lying, poorly -drained areas called back swamps.

Sometimes smaller tributaries, called Yazoo tributaries, get trapped in the back swamp and flow parallel to the main river for miles before finding a place to join.

Wow.

And alluvial fans.

Those are like deltas on land.

They form where a stream rushes out of a steep, confined mountain canyon onto a flatter plain.

The sudden drop in gradient causes it to deposit its load in a fan shape.

Very common in arid regions.

We've covered carving and building, but we have to talk about floods.

Absolutely.

One of the most direct impacts on us.

A flood is simply when the stream's discharge exceeds the channel's capacity and water spills onto the floodplain.

Natural but often made worse by us.

Definitely.

Urbanization, deforestation, altering channels, all can increase flood magnitude and frequency.

What are the main types?

We can categorize them.

Regional floods are often seasonal, covering large areas, usually from prolonged rain or rapid snowmelt.

Think the big Mississippi floods.

Often predictable to some extent.

Okay.

Then flash floods.

Sudden, rapid rise in water, often from intense thunderstorms.

Extremely dangerous due to high velocities and lack of warning.

Common in mountain canyons and, increasingly,

urban areas.

Because the water can't soak in.

Exactly.

Impervious surfaces mean rapid runoff.

Then you have ice jam floods, mostly in colder climates where ice chunks block a river, causing water to back up.

Right.

And finally, the rare but catastrophic dam failure floods like Johnstown back in 1889.

Now that 100 -year flood term,

it's misleading, isn't it?

Very misleading.

It's about probability, not timing.

A 100 -year flood has a 1 % chance of happening in any given year.

So you could have two in five years, or none for 150 years.

Precisely.

And the estimates depend heavily on the length of reliable data we have.

Plus, changing climate patterns and land use can throw those historical probabilities off.

Building a mall and a watershed changes everything.

So how do we manage floods?

What are the strategies?

Historically, the main approach was structural controls.

Building things.

Artificial levees are the most common earthen mounds built along banks to increase channel capacity.

If they can fail.

They can.

And sometimes spectacularly, like during the 1993 Mississippi floods,

sometimes engineers even create floodways, areas designed to be intentionally flooded to relieve pressure elsewhere.

What else?

Channelization.

Basically modifying the stream channel itself, striking it, widening it, dredging it, clearing vegetation.

The idea is to move water through faster.

Downsides.

Oh yeah.

Straightening increases the gradient, which often causes faster erosion downstream, requiring bank protection.

It can also harm habitats.

It's often a constant battle against the river's natural tendencies.

And dams.

Flood -controlled dams are built to store floodwater in reservoirs and release slowly.

They can be very effective and provide other benefits like hydropower or recreation.

But they flood valuable land permanently, disrupt ecosystems,

trap sediment the river needs downstream, and eventually the reservoirs themselves fill with silt.

So structures aren't a perfect solution, what's the alternative?

The non -structural approach.

Instead of trying to control the river, we adapt to it.

It focuses on sound floodplain management.

Meaning?

Meaning identifying high -risk areas and restricting development there.

Using zoning regulations, building codes, flood -proofing requirements,

basically keeping people and vulnerable infrastructure out of the river's way when it inevitably needs more room.

Working with the river.

Exactly.

Recognizing that floods are natural processes and managing our exposure to the risk.

Applying our knowledge of how these systems work is key.

Well this has been quite the journey.

From a single raindrop influencing infiltration or runoff.

To the immense collective power of a river system, carving canyons, building deltas.

And shaping not just the land, but human history and how we live.

It's this constant sculpting, this dynamic process.

That interplay of erosion, transport, deposition, the adjustment to base level, the drama of floods, it really underscores how alive and changing our planet is.

Understanding it is, well, fundamental.

So next time you're out, maybe you see a treak, a river bend, even just how water flows after rain.

Remember these forces.

Think about your local landscape, the shape of the hills, the valley floor.

How might running water have played a role?

Perhaps unseen for years, but powerful nonetheless.

Something to think about.

Thank you for joining us on this deep dive into the powerful world of running water.

We hope you feel a little more well -informed.

And maybe a lot more curious about the ground beneath your feet.