Not All Who Meander Are Lost - Meandering Streams
[00:00:00]
Chris Bolhuis: Okay. Hey, question before we start. Can you, can you hear this when I do this? can you hear that sound?
Dr. Jesse Reimink: Okay.
Chris Bolhuis: No, seriously.
Dr. Jesse Reimink: doing. So, listener, Chris is right now, he just goes, he's got his chin up looking at me and he goes, can you hear it when I do this? And he [00:00:30] just turns his head softly, side to
side. Are you rubbing your whiskers on your microphone. right now?
Chris Bolhuis: wondering if, can you hear that? I'm serious. That's a serious question. I need to know this because I have the mic right close to my face and I don't want to be like scratchy when we're recording cuz I'm a professional, Jesse, I am a pro.
Dr. Jesse Reimink: I can't hear it
and nobody, nobody
can hear that. But you know what we can hear and you know what listener we are sick of, is that right there? What you just did, you go. Right
Chris Bolhuis: can't,
Dr. Jesse Reimink: about
five times or [00:01:00] eight times an episode, Chris is coughing into the
Chris Bolhuis: you do such a good job of editing all my, coughing
out,
Dr. Jesse Reimink: You know what? That's the first thing I do when I get these files is I go through and I cut out Chris's coughing in the background. It's the
Chris Bolhuis: Jess, that's not, I can't help that. That's not fun.
Dr. Jesse Reimink: is funny.
It's very funny.
Chris Bolhuis: all right, so now I'm self-conscious. So now when I cough, if I have to cough, I'm gonna try to suppress it. It's gonna affect my voice, and then
Dr. Jesse Reimink: And then the quality of
planet geo just goes down so
much
it's
just, [00:01:30] we're gonna be horrible
Chris Bolhuis: All right. Hey, let's get going. Dr. Reimink, what are we doing today?
Dr. Jesse Reimink: Extreme meanders. Really, really cool topic. This I think is one of the topics that every, like first year Geology student who takes the intro class. It's one of these mind-blowing things, you know, like plate tectonics is very cool. Volcanoes are awesome. Earthquakes are awesome. This is awesome in a very, very different way. And it's sort of one of these mind-blowing things. Everybody's seen stream meanders and we're gonna explain 'em [00:02:00] today, which is a really kind of a
mind-blowing concept, I, think.
and, and very, very
Chris Bolhuis: I agree. But we're gonna do something though today that explains meanders and they are not random. And I don't talk about this in my classes. Do you, do you get into the weeds on this?
Dr. Jesse Reimink: no. I mean, usually the way my class is structured,
Chris, I talk about
minerals and I think you do the
Chris Bolhuis: Wait a minute. You have structure in
your
Dr. Jesse Reimink: Yeah, it, it's a bit of whatever I want to talk about any given fall semester. But you know, I usually, so it starts with minerals and we [00:02:30] go into rocks and I obviously take way too long talking about igneous
rocks and plate tectonics and things.
So by the end of
the semester, I run outta time.
for glaciers and streams and all this kind of stuff. I do hit streams cuz streams are super important in Pennsylvania. But sometimes I run
out of time to talk about glaciers, for
Chris Bolhuis: Right. Yeah. Yeah. Oh, I never do that because I, I'm in Michigan and we cannot neglect glaciers in Michigan. That would be an injustice. But what we're gonna do today, if I didn't know this and I listened to this, I would appreciate it much more.
And just like looking at [00:03:00] rivers. Cause we're gonna talk about the geometry of Manders here really,
and I think it's such a cool
topic.
Dr. Jesse Reimink: let's level set. Kind of what we're talking about here is that there's a bunch of different types of rivers or streams, we call them streams, like the, the general classification for flowing water is streams. And so there's a whole bunch of different types of streams. You know, you can picture these in your head with us.
You can think of a mountain stream that's kind of going straight down the mountain valley, you Cascading over boulders and doing all this very interesting stuff. It's usually freaking cold water. you can have streams that are a little bit [00:03:30] quieter and they're in flat terrain, and they kind of wind back and forth.
That's what we're talking about with meanders. And so we're, we're focusing on one specific subset of streams, which is meandering streams. where do these occur, Chris? Like paint the
visual
for like where these occur.
Chris Bolhuis: First of all, you know, you and I, we tend to get a little off topic too, so
we're not just gonna focus on meanders, so we'll bring in some other things we promise. And also we're gonna end with entrenched meanders because those are very, very cool [00:04:00] thing and a dynamic of streams as well.
But manders happen when rivers kind of flatten out? you don't have meandering streams, typically when you have, they, they tend to be much straighter when the gradient or slope is much steeper. So when, as soon as it flattens out, that's when rivers will, will begin their meandering or looping
Dr. Jesse Reimink: Totally. Totally. And so places they flatten out are, floodplains. These are things where rivers hit their floodplain and they, they kind of, the, the [00:04:30] trajectory of the stream or the gradient of the stream from the start, from the headwaters of the stream, way up in the mountains to the mouth where it hits the ocean or hits some lake.
You know, kind of somewhere in there. The stream will start to Become less steep and when it becomes less steep, this is what we're talking about. So more flat
land basically. And if you look at a map of Pennsylvania, that's a really kind of cool thing to do. There's mandarin streams everywhere and they flow through the valley.
So we are in, the Ridge and Valley province and we have these really long ridges that run for miles and miles [00:05:00] and miles parallel to each other with a valley in between. And there'll be a stream meandering down the middle of that, and it's
really cool. You can just look at that.
Chris Bolhuis: is your drainage pattern then, what you just described, is that a trellis drainage
Dr. Jesse Reimink: Yeah, we have?
a trellis. Yep. Exactly. Yep.
Chris Bolhuis: Um, I was getting ready for this episode and I came across a quote that I, I don't think this is verbatim quote, but it's really, really good and it, it hit me, it's something to the effect of.
You can never set foot in the same stream twice,
Dr. Jesse Reimink: love that
Chris Bolhuis: you know? Right. [00:05:30] The water's not the same. The sediment's not the same. Basically nothing is the same.
Dr. Jesse Reimink: You know, Chris, I have this memory and I think it was on a field trip with you when I was a ta, I think in my senior year when I was doing the independent study and I went along as a kind of a TA of your field trip
Where there was cold up in the
Chris Bolhuis: Where you had absolutely no responsibility and
you just,
Dr. Jesse Reimink: responsibilities and we did a hike back along a stream.
is, is this, do, do you do that? Did you do that in those days in your [00:06:00] trip
your spring trip to
the upper peninsula?
we there was
and it was one of these, yes, we went back to a waterfall and we sort of stopped halfway. We weren't at the waterfall yet, or we were on our way back. I'm not sure which one, but we stopped and had like a break and I had zero responsibilities.
And that, quote. I think you and I were discussing that quote cuz suddenly got sunny and warm and we were just sitting next to this stream in the forest. It was just glorious. And, uh,
I I love streams.
They're so great.
Chris Bolhuis: I agree. I think actually what you and I were talking about is I asked you, [00:06:30] have you ever thought about. Where the water that's running in front of us right now, where it has been, I think that's what we
were contemplating, like following the
life cycle of a water molecule
Dr. Jesse Reimink: Totally crazy. Really, really cool to do. Streams are just
thought-provoking
Chris Bolhuis: Mm-hmm. I agree.
Dr. Jesse Reimink: all types of ways. Um, So that's the quote
Chris Bolhuis: Okay, so basically, Rivers in the mountains, they're governed by the slope, okay? Their steepness, and they're confined by the valley, so they tend to be [00:07:00] straighter. As soon as they hit the broader flatter floodplain, they begin to meander. So, In these meanders, let's start to get a little bit into the weeds on this a little bit.
That like these meanders have a function. They actually are managing the energy of the water, right? I mean, if you think about what a meander does at the basic, basic level, they're doing two things, right? They increase the resistance because it's no longer flowing in a straight path, So they [00:07:30] increase the resistance.
And they decrease the gradient.
which is the slope. I'm
sorry? They, They,
decrease
Dr. Jesse Reimink: but let's. the gradient is, is kind of a, this is a term that's hard to, understand, so let's just visualize it really quickly. So imagine you have, there's an elevation when the stream comes outta the mountains and hits this floodplain that's at an elevation.
Let's say it's like 300 feet above sea level, and then the mouth of the stream is at sea level when it pours out in the ocean. So
there's 300 feet.
Over that distance. right.
And if it's a straight stream and it's 10 miles, [00:08:00] it's going 300 feet and 10 miles, that's the gradient. However, if you add meanders in there, it's a lot longer path.
So the path the river takes, if you stretch the meanders out, is a lot longer than 10 miles. And so the gradient is a lot less. So that's what you mean by decreases the gradient here. and
then the first thing you talked about, Chris, the resistance is,
if you've ever walked along a stream that's meandering or that's kind of has bends in it, you'll notice that the deeper parts are on the outside, and that's because the, the river's moving faster on the outside.
You'll often [00:08:30] see this too, like there's faster water on the outside of the bend. It's cutting into there, so there's a lot of energy being expended. Cutting in, there's erosion happening, which is using energy to kind of cut into that bank. And so that's just the
point about the resistance number one that you said.
Chris Bolhuis: so, you know what the river is doing is it's just, It's changing itself to minimize the energy that it's expending.
So it can maintain a state of equilibrium. and, and with a river where you talk about [00:09:00] equilibrium, really then there's no net anything going on. There's no net erosion, there's no net deposition. that's a river that is in a state
of equilibrium.
Dr. Jesse Reimink: That's the
key point here, Chris. Cuz remember when we just went back to that analogy of like 300 feet over 10 miles and, and that that's probably not enough height to, for 10 miles of river.
But if you have a steep gradient, it's carrying sediment downstream a lot, but.
It can only do that for so long until it [00:09:30] runs out of sediment a straight stream at that gradient will carry a lot of sediment out, but you'll eventually run outta sediment upstream. And so then your stream is kind of out of balance in a way. And so when a meander is happening, There's not a lot of net transport of material downstream.
There is a little bit, but it's very, very, very slow, and that's the thing that has to be balanced here. The physics of like moving particles downstream has to be balanced with the gradient
that it's going through.
Chris Bolhuis: a long time ago, Jesse, We talked about ripples, runs, pools, and glides.
Dr. Jesse Reimink: a long
Chris Bolhuis: this [00:10:00] episode a long
time
ago.
It was a
law. No, I don't. Well, let's throw back to that because this is a like classic example of this dynamic of rivers trying to reach the state of equilibrium.
I mean, rivers, Boiled down to their basic level. They are transporting machines, right? They're transporting water and sediment downstream. when a river encounters a bend on the outside, you described it, it's this kind of cut bank thing. It's erosion. You get a [00:10:30] deeper pool on that outside of the bend. If you looked at a profile A cross-sectional profile of a river along this meander, you would see that the inside of the bend, it's very shallow and gentle sloping, and as you approach the outside of the bend, the river gets much deeper. It's gouged out into this, thing that we call a pool. I mean, it makes sense and it's actively then moving sediment out of that bend. Well then as it as the water comes out of the meander and this stream kind of straightens [00:11:00] itself out, it's gonna deposit this sediment now, which creates a ripple where the water's more kind of more turbulent as it goes off over this deposited sediment. And so it's just a throwback to this old episode a long time ago.
The ripples
Dr. Jesse Reimink: absolutely. And okay, I have, analogy that just popped into my head, which might be terrible. So, but, and you're
the analogy king.
Chris Bolhuis: Uh oh.
I'll
let, I'll let
you know.
Dr. Jesse Reimink: this one, Chris.
Okay. So you just said that that streams are transporting machines, which is a great phrase. I love that phrase. here's an [00:11:30] analogy.
Okay, let's test this out and you can tell me how I did with this. , let's put it into context of something everybody has seen, which is the conveyor belt at the grocery checkout, right? You've got the conveyor belt now, A mountain stream that has a gradient where it goes several thousand feet in a couple miles is a fast moving conveyor.
It is. The conveyor belt sped up really, really fast where you can just chuck a bunch of stuff on there and it'll take it away downstream, no problem. Right. It has that space. What we're talking about demanding streams are streams that are slowing down [00:12:00] that conveyor belt, like really slowing it down.
So you can't put many groceries on there. The groceries kind of pile up and then fall off the sides because it's not moving fast enough. the checkout person's really taking their time about it, or maybe they're new and they're not checking out fast enough In the sediment, the, your groceries have to fall off the edges to maintain this balance.
You just can't pile them up very high. That's the meandering stream. And so this net transport has to slow down. So instead of taking sediment downstream down the conveyor belt really fast, instead of moving groceries down fast, they fall off the sides. And that's [00:12:30] what a meandering stream is doing, is moving sediment laterally, side to side instead
of moving it downstream.
Chris Bolhuis: that works, Jesse. Okay, well done. My young sage, um, let me go at it a different way a second. Rivers move, sediment streams move sediment in three ways. They, they have what's called their dissolved load, which are like these molecules, they're salts that are dissolved in the water.
Right. This is, again, a throwback to way back when, when we talked about
why the oceans are
Dr. Jesse Reimink: like episode number
[00:13:00] five or something like that. We're going
back
Chris Bolhuis: It's way back. I know. I can't believe I actually remember it. Um, anyway, that's the dissolved load it's a pretty small amount in terms of the overall load that a river carries, but the next one is the suspended load.
And these are particles that are, they're carried in the column of water. They're carried in suspension. this includes particles like dust sized particles and silt sized particles, and even sand if it's moving fast enough. So if you can imagine sand moving [00:13:30] in the suspended load of a river, As soon as that water slows down just a little bit, it doesn't have enough energy now to carry that sand, and the sand is deposited.
And that's what happens when a river comes out of the pool and it straightens out. There's a little bit a lower energy, a little slower movement, and some of the suspended material now is gonna be deposited. And then the third way that rivers move sediment is along the bed. Of the river where they roll and they slide and they kind of bounce or salt it along the bottom.
And [00:14:00] the same thing is if the river slows down just a little bit, those things stop moving. so it's the, a combination of the bed load and the suspended load that create the ripples then
coming out of
Dr. Jesse Reimink: Let's move
into that pool dynamic or the bend,
the actually meander part. maybe I'll just cover the the names that we're gonna use and then we can
start talking about the physics of these meanders. Um, and so,
okay, so
if you're looking at a meandering stream, you're walking along a path and the stream is winding
to the path, and then away
from the
path and then [00:14:30] to the path, and
Chris Bolhuis: get in a kayak. Let's get in a
Dr. Jesse Reimink: Well, okay, we could get into Kayak, but you know, in Pennsylvania we don't have that
many. There's some you can kayak, but the streams are small. All these mandering ones.
So I'm usually by a path, but you can be in a kayak. Chris is in a kayak. I'm by the path. and Jenny are in kayaks and maybe you got a dual
kayak and you know.
Chris is in front and Jenny's
got her legs up
up. Jenny's got her leg
kicking her legs up
front with her beer. And you're doing all the
paddling
in the back. Is that what happens here? Chris?
Bring me over there. Chris, bring me
over
here.
Chris Bolhuis: the other way around. My [00:15:00]
Dr. Jesse Reimink: Okay, so actually this is a good
visual. If I'm walking down the path, straight
path and the the river where that you guys are kayaking in, you're kind
of meandering towards me where the river will be
right next to
the path and then you meander away. You sort of
bend away, you go around another bend and then you come back close to the path, right?
And so if we look at that lateral variation inside of the bends, the water is slow. And you can see this, you can look at this from the path or from your kayak, and you can see that it's slow water.
It's sandy there, and you can look on the
outside of the bend and the waters fast. And the [00:15:30]
the riverbed is deep.
And what's happening here is on the outside of the bend because the water's faster. It's deeper. The water at the top is
moving faster because the water's deeper, it's
cutting. And so on the outside is where the erosion happens. As the water is scraping along that outside of the bend, it's eroding, it's cutting in, it's forming what we call cut banks and US trout fishermen really like cut banks cuz the big trout like to hide underneath of
the, the cutbank there.
They're, they're
safe from eagles and ospreys and all that stuff. And then on the inside of the [00:16:00] bend, it's this shallow sloping thing that we call a point bar, which is this. Shallow sloping. The bank is sandy usually, and it slopes gradually towards the outside of the bend and the river, and that is where deposition is happening.
So sediment is being deposited on the inside again, cuz Chris, you described
this beautifully coming out of the riffs.
There's sediment being dumped off, and when the, the river is flowing straight, it hits the inside of the curve and it slows down. It hits the
outside of the curve and it speeds up. So the outside is [00:16:30] hungry.
The inside is not hungry, is regurgitating some sediment and dropping it off right there. And so this causes that meander to keep cutting further to the outside and keep depositing on the inside. And so obviously if you speed this up, The river keeps meandering or keeps cutting away the outside and cutting
away the outside and depositing the inside.
Depositing the inside in that loop keeps getting
more and more exaggerated and eventually
you on your kayaks
will cut into my path and
the path will [00:17:00] have to change.
Chris Bolhuis: good. I wanna put a pin in something cuz I wanna come back to it, so remind me. All right. I wanna put a pin in the path of Jenny and I in the kayak as we come out of the meander. Okay. So let's, let's come back to that later on. But before we do that, we need to get into some of the weeds.
And this is the cool part about, I think this episode that I think most people that are not like Fluvial Geomorphologists, don't really know. And I think it's a very. Very awesome aspect to River Dynamics and Meanders in [00:17:30] particular. So let's get into the detail. The geometry of meanders is not random.
it's specific and it's, amazing. I love this stuff, but we have to introduce some terminology a second. So let's introduce first the wavelength of Meanders. And
what
we're talking about is,
let's say the crest of one meander. To the crest of another meander further
downstream,
Dr. Jesse Reimink: let me just, double click on this a
minute. This is done with like satellite imagery. This is done top down. You're, [00:18:00] we're
looking at an airplane view or a
satellite image of the river, and you're seeing it
meander up and down, back and forth on your map. And this is
actually how scientists who study these things look at these things.
They take satellite imagery. And measure the distances between these bends. So
think of it like a wave
where the wavelength and the amplitude is, you know, how far the wave goes up and down. This is the same thing, except this, the map
view of this stream.
Chris Bolhuis: That's right. So we're looking at two identical spots on consecutive
meanders. [00:18:30] So like crest de crest, or the same, we're trying to find the same point in two consecutive meanders. We call that the wavelength of the meander because it, it kind of looks like that, like you just said. And, so that's called the wavelength.
And the beauty of this is that regardless of the stream, Regardless of the size of the river, this can be a small thing like in your lab, like a stream table in your lab or a massive river like the Mississippi or the Missouri. It doesn't matter. [00:19:00] The wavelength is gonna be about 11 times the width of the River channel.
Now that's like, I find that to be very exciting. I, I don't know why it just be, I think it's because it just makes perfect sense. Like when you kayak a river, when you're walking along a path and you're looking at these bends, these oppositely looping bends back and forth, they go, they're not random.
And I, just think that you understand that, that there's a reason to this madness that it [00:19:30] just makes it, I can appreciate
then, That's so much more. I, I don't know. I just find it to be very exciting and I don't know why I, I really need to do a better job of this in my classes because I don't talk about this with my students.
It's, and again, it's the same reason I'm running out of time. My exam for my students from Grand Valley is coming up next
week,
Tuesday. I'm out of
time,
Dr. Jesse Reimink: And I think that, you know, this is, you know, we live in an ordered system. Really, this is like an ordered, environment. And the reason that it's like 11 times, or, you
know, 10 to
14 times the [00:20:00] width, there's obviously variation. There's like
Chris Bolhuis: look, can I finish that real quick?
Dr. Jesse Reimink: for it.
Chris Bolhuis: So it's usually 11 times the width of the river. So the wavelength is 11 times greater than the width of the river, no matter what the width of the river happens to be. And it is always 10 to 14 times the width of the river.
Dr. Jesse Reimink: and this is, I think, you know, it's very complicated in detail and it's very hard for people to accurately model the details here. So if people say, okay, I'm gonna, Create a stream. I've got a stream out [00:20:30] there in the world. I'm gonna look at it, I'm gonna create a physical model of this stream and sort of forward model it.
So look at satellite imagery from 1980 of this Mandarin stream, and then say, I'm gonna model it forward to 2023. With some physics, they don't do a great job of matching up perfectly, but they get some of the main principles, right? So it's really hard. My point is, is it's really hard to model the, the detailed physics of this, but the.
The broad scale physics makes sense. Like the width of the stream, that's amount of water and amount of water moving downstream is [00:21:00] energy.
And so the wider the stream, more energy that's expended and the more energy is available to
cut these meanders. And so
you get wider. Broader meanders that have a greater wavelength when you have a wider stream, cuz there's just more energy, more water flowing, more energy to do this erosion and deposition in
a broader wavelength pattern. So that's why there's this relationship between stream width and
wavelength of the meander.
Chris Bolhuis: let me try to simplify what you just said a little bit if I could. And, [00:21:30] and that's where I want to come back to me and my kayak is if I get Jenny and I into the swift part of the river and we're just drifting with very little direction, maybe a little bit of direction, but I'm just basically gonna drift, I'm
gonna drift
then to the
Dr. Jesse Reimink: be by your choice. This would be at Jenny's direction. I mean, Jenny,
Jenny is controlling the situation here. Let's not
say, Chris,
I did this. It's Jenny directed you to do this, so,
Continue.
Chris Bolhuis: true. Okay, so we're just drifting in the swift part of the river. So it's gonna naturally then take [00:22:00] us to the outside of this mander. Right? And I'm gonna go around the meander and my kayak is basically gonna stay in that swift
part of the
Dr. Jesse Reimink: No, you're gonna try the the important thing, you're you're gonna have to work hard to stay
outta like the brambles and the branches, cuz you're gonna be like, if you just let it go, that cock would nose into the bank and you know, there'd be a branch falling on Jenny and you know,
it'd be really
Chris Bolhuis: Well, I know, and she does get irritated when that happens. Yeah. But as I come out of that meander, the waters, as it slams into that bank, [00:22:30] kind of gets swept. Then across the channel. It's, it's almost like a, a slingshot effect, if you will. And so my kayak, if I keep it in that swift part of the river, is now gonna drift me slowly across the other side of the river.
And, and so the, the wider that river is, the longer it's gonna take me to slingshot or sweep across to the other side of the river. And that's where when the river then slams into that other side, that's the beginning of the [00:23:00] other meander.
Dr. Jesse Reimink: Let me just double click on something you said. There I'm kind of imagining this with a left-handed bank. So you're coming and you're kind of the, the fast parts on the left side of the river, you're trying, fighting to keep off that bank.
And then when you come out and the river straightens out into a riff section, you're gonna. Have a bit of a
right word, momentum, like you're coming
out of this this curve that's curving from left to right and you're gonna still be curving from left to right and you're gonna cross that ripple section, not
straight down the river, but
going from left to right and you're following the [00:23:30] water, like you said, by the time you hit.
The the next curve, you're on the right side and you're impacting the right side of the stream.
Hence, that's where the erosion happens and it starts to
curve back towards the left. Like that's what we're
talking about, this slingshot thing you're describing. You get sling shotted
out of one curve into the next one, and that
exaggerates, that creates exaggerated
loops and exaggerated loops and the eventual thing, which we didn't actually talk about is that the eventual outcome of this is that. These loops get bigger and bigger and bigger until they become [00:24:00] circles
the stream cuts itself off and will cut off a meander. And then that cutoff meander is no longer connected to the river.
So it becomes
this big horseshoe bend,
and then eventually it'll cut across the horseshoe part, and so it cuts off that meander and that
becomes an oxbow lake.
And then, The river will straighten out for a section gain speed, and then
start this meandering process over again. So these
meanders happen progressively over and over and over and over.
And if you ever wanna see something really cool, look at a
[00:24:30] lidar map of a meandering stream.
So these really high resolution laser maps, topographic maps, you can see generations of meanders in these floodplains, and they're really beautiful people make artwork out of
'em, and then they're really, really cool.
Chris Bolhuis: that's, that's a really good point. meanders ox expos, they are. I don't know. They get
me.
They, they just, they grab me at my emotions when you see 'em. Like I, I think of one of my favorite places to see this is, is in the Appalachian Mountains looking at the Cumberland River and it's [00:25:00] just broad, sweeping, meandering, dirty looking river cuz it's got a lot of suspended load in it just ox exposed all over the place.
And it's just one of the best examples on the planet to see this kind of looping, curving. But yet, it's not random. And you know, again, to, come back to this point and, and really what I want all of our listeners to do is look at meandering rivers a little bit differently and see if, that makes sense.
You can look at the width of the river, multiply that by [00:25:30] about 11, and that's where the next meander is gonna be. And it just keeps doing this again and again and again with regularity. It's just
such an
Dr. Jesse Reimink: So let's just
tidy up this physics part here, Chris, or the
the sort of regularity here. So we described
wavelength, which is distance between meanders in the same direction. So these right hand bends the distance between two right-hand bends is the wavelength.
There's the radius of curvature. So like,
as this horseshoe develops, the size of that horseshoe is also a function [00:26:00] of the physics of the river. And it's usually about one fifth the wavelength. So smaller rivers have tighter loops, larger rivers have much broader loops. This is proportional to the width of the river and to the, uh, wavelength of these meanders.
And I think this brings up like, why. These happen? Like why do MANDERS happen or how
do they start in the first place?
I want you to answer this, Chris, but I just wanna say one thing that the people who study these things in the lab, I saw one of these, I went and gave a talk at Lafayette College up in Northeastern
Pennsylvania, and they have [00:26:30] a, a flume, which is.
A huge room with basically a big tank in it that sits on pistons. And so this tank is, It's like the size of a, a semi-truck trailer, probably of this clear glass tank that
has tons of sand in it.
It has water, a big water tank upstream. there's like viewing sides to it, and you can change the pitch of this thing.
So you can like turn, you can
tilt it, you can put one side up and one side down. And there's a ton of sand in this thing. And so you can put whatever [00:27:00] sediment you want in there. You can start a stream by just.
Jumping water into one side of it and
watch how the stream changes.
And so people see these meanders occur. Start with a straight path, turn it on. And then within hours to days, you have a meandering stream.
And so we can see this in the lab.
This people do experiments like this in the lab. in
these big flumes. They're really cool.
Chris Bolhuis: I have one. It's not the size of a semi. It is, instead it is the size, uh, it's about six feet long and three feet wide, [00:27:30] it, it circulates its own water back and forth. So it's kind of cool that way. It's got a pump and it, you know, so I have on one end of it where water, I can control the outflow of the water.
I can have it a trickle, I can flood it, I can, you know, I can control that. And then it goes down into a lake. And it creates a delta and so on. It's, I'd like to just turn it on and let it run, and then just, students will just migrate into your class then and just stand in front of it and they're mesmerized by this, this dynamic going on in front of 'em.
It's really a, really a cool thing. [00:28:00] But the, you got, man, I have an idea. I think I want to build a sandbox outside of my room, like a big, big sandbox outside of my classroom. And then I, I have a water source
nearby.
I can just, you know,
I can create
my own. Yeah. Yeah,
man. Yeah. I'm, I'm gonna make this happen.
This is, I need another TA Jessie.
Dr. Jesse Reimink: Maybe
Chris Bolhuis: Actually, I need, I need a TA that's actually gonna do
something
though.
Um, you are such
a piece of work. All right.
Dr. Jesse Reimink: so Chris, you know, I just described how we
can do experiments of [00:28:30] this. why do meanders happen? Like what starts at the rivers? Let's assume it's straight to start with.
Why does it start to meander.
Chris Bolhuis: there are so many things that can cause manners to begin, and once they begin, then they take on this regularity that we talked about just a, a few minutes ago. It could be a log falling across the river. It can be a bridge with pilings in the middle of the river. It can be a muskrat. Digging a tunnel underneath the river into the bank on one side of the river, and that tunnel weakens that [00:29:00] part of the river.
erosion begins and it creates a pool. And once you get a pool, then you have faster flowing water and the process is on. And so there are so many things that can cause meandering to
begin.
Anything that diverts water to one side of
the channel
is, is, uh, is
Dr. Jesse Reimink: And you know this from your, your flume in your classroom and people who do these experiments, you take completely
homogenous sand, you completely
homogenize it, have a [00:29:30] perfectly straight stream starting. This will naturally happen. Meanders will naturally happen because there's very subtle variations.
Any little tiny pebble difference. Can shift the water intensity to one side of the stream. And then because the slingshot effect, it just starts happening. So this is like unavoidable almost, and very, very slight perturbations, like you described, a muskrat drilling into the
side or digging into the side.
You know, that's a pretty small perturbation, but it
can cause massive meanders to, to sort of initiate. So it's kind of a natural outcome of [00:30:00] rivers in
this, uh,
type of setting.
Chris Bolhuis: All right. So Jesse, I think next you wanna, did you want to talk about this, this kind of paper that you know, meanders and river dynamics are tied to vegetation? A little
bit. Like,
Dr. Jesse Reimink: well, Chris, you know me.
Chris Bolhuis: on this?
Dr. Jesse Reimink: you know me. I, I, I kind of, I always like to highlight that the early earth was different than the modern earth in some way or shape or form. And, a colleague of mine has been focusing a, a fair bit of research on this, looking at. How plants
affect meandering streams,
[00:30:30] really.
and there's a couple different aspects to this, but if we think of like earth history, plants have been, plants have been on land and have colonized the land for the last 350, 400 50 million years. Before that, there weren't many land plants.
And so a lot of the rivers did not have plants.
Anchoring the banks of the rivers and things like that. So, so river systems on the early Earth before 450 million years
ago might have
been very, very different. Um, and
so what, what.
Chris Bolhuis: and erosion must have
looked way
Dr. Jesse Reimink: Totally different, right?
Just completely [00:31:00] different stuff. And so, people who study vegetated rivers and Unvegetated rivers, like think about a meander in a desert versus a meander in a lush jungle environment.
Those are very different. And actually the plants do a lot. The plants impact How much energy it takes to erode the
stream bank really is the key thing. More plants
equals more energy required to make meanders. And so I think the, the important highlights here, Chris, are that the channel width of Unvegetated rivers, so rivers [00:31:30] in arid environments or in, you know, without vegetation on the banks, those usually have a smaller channel width and they migrate a lot faster.
the migration rate in meters per year is like, An order of magnitude faster than vegetated rivers. So the stream that we were
describing
through the forest of Pennsylvania is moving slower than if there were not trees there,
like the, the lateral migration here, just because
it's gotta fight through all the roots and the biology, I think, you know, that's
the, the sort of take home physics.
Chris Bolhuis: [00:32:00] Okay. Interesting. I have an idea. We have been throwing around this idea of doing a topic for an episode on how to read a scientific paper. And this is you and it's basically me talking to you about this. What do you say? We put together a little series, then following this episode up with we can use that and talk more about River Dynamics and also talk about how to read a scientific paper because I think that's something that's an
important
topic.
Dr. Jesse Reimink: that. That's a really
good one. [00:32:30] That's a good idea, Chris. This would be a great one.
It's sort of a
review of stream meanders and, and vegetated unvegetated, and it's a, it's a learned skill, this reading a scientific paper, because it's
not easy to start
with. So Yeah, I like that. That's a great idea. Good idea.
Let's, let's do that in a, in a coming episode here for sure.
Chris Bolhuis: Okay, good. Well, hey, let's wrap today's episode up with one final thing, that I, I don't know, I don't want to gloss over it too much, but I think we've covered meanders well enough. Uh, well, What I want to talk about are entrenched meanders, because these are some of the most beautiful [00:33:00] geologic features that exist.
famous examples of entrenched meanders are Horseshoe Canyon in Canyonlands National Park. Both you and I have seen this, the goosenecks. Gooseneck State Park in Utah. These are just a couple of like examples of very, very famous
entrenched
meanders.
Dr. Jesse Reimink: you don't know what those look like, the visual in your head here is a big bend in a river, a horseshoe bend in a river that's cut through rock, where there's cliffs on either side of that river. So very [00:33:30] different from the floodplain
that you and I were just talking about, Chris,
that you and Jenny were casually kayaking through.
This would not be a casual kayak in this river system. You know, this is
class four rapids on these types of rivers, but
that's the visual we're painting here. That's what an entrenched meander. It's entrenched in
cliffs, basically rock.
Chris Bolhuis: And they have very little migration. They're not gonna move much because they are
confined now by the valley
walls.
Dr. Jesse Reimink: that's a key point, Chris. So, okay. That's a really key point cuz I think these entrenched meanders are very commonly [00:34:00] misunderstood . Because people look at it and think,
oh, it cut through the rock. Like
the meanders are cutting through the rock. But you just said that there's very little lateral migration.
And if you look at these rivers, you can see that there's no like undercut bank. There's no like, uh, well maybe there's a
little bit of undercut bank, but there's a little bit of deposition on the inside of the bends, but not a lot. Like it's, it's not the same dynamics and the stream is not
cutting outward as much.
And that's a key part of how these things form . The question I [00:34:30] think that people get confused about is when
did the meanders happen
and what's the answer, Chris?
Chris Bolhuis: the meanders happened first. you have this river that was doing what we've described for the last half hour is, they're establishing this equilibrium. It was broad, it was flat, and it began to, you know, minimize energy and deposit and erode at the same time and established a state of equilibrium.
And the manders developed and [00:35:00] then, then Geology happened.
uplift probably happened where the land began to elevate and the river now is gonna accelerate and it's going to, gonna try to keep up with the rate of uplift. And it's gonna down cut as the uplift is happening happening.
So this is very different than what I think a lot of people think when they look at this. They think that it down cut through the rock like you described. Instead, we had the manders that were established, [00:35:30] uplift began and the river just kept
up.
Dr. Jesse Reimink: can I kind of summarize that a minute, Chris, and, and, and correct me if
I get this wrong or if I missed something key here, but I wanna
just summarize it. The meander pattern is inherited. By a previous generation of river, by the
ancient river system that was actually meandering
and cutting out side to side,
right?
That the meander shape is inherited. And the way I like to describe this in class is that a river, when it's at high elevation in the mountains, when it has a lot of distance to go [00:36:00] high, gradient, it wants to cut down. So if it has to go down 3000 feet in a mile, it is cutting down first.
And when it doesn't have anywhere to cut down, it'll cut out side to side
i e, it'll start to meander
side to side. So when it has a shallow gradient, it's cutting out to side to side. It's meandering. And so we've kind of reversed this. We've taken a meandering stream. That had a low gradient and all of a sudden we made it want to cut down and it just cut straight down.
It kept the same stream profile and just [00:36:30] went straight down very fast, which means
it inherited that shape,
uh, that seniority, whereas it would've
normally been a straight stream if you kind of started
from scratch.
Chris Bolhuis: No, I like the way you put that. In fact, I'm gonna steal that. That's the way I'm gonna use it now on is The shape of the manders is inherited
because they came first.
So I
like that. That's a
good way of
putting it.
Dr. Jesse Reimink: I think this highlights an important concept here and an important thing about meandering streams is that there's a couple ways to force this, to want to make the river like cut down. The first [00:37:00] is to uplift the land, you force the stream to be at higher elevations and it wants to cut down to the same. Like sea level gradient. The other thing to do is change what we call
base level, like where the mouth of the stream is.
And so we can, if we put a dam somewhere that changes base level, if we remove a dam, if we drain a lake,
if we change the base level
where the stream is ending, it changes that gradient and how the
stream is cutting in and out.
And
meanders are really important mostly because, in [00:37:30] society. We have built up a lot of industry and we've built our society around a meandering stream's current profile, and which means we don't wanna let it. Naturally meander like it should be doing. Like the Mississippi River is a classic example.
There's a
bunch of little rivers in Pennsylvania that are
great examples of this. We build a little town around a meandering stream and all of a sudden we put concrete embankments all the way around that stream cuz we don't want it to change its profile anymore. So we're changing the behavior of that stream because we've [00:38:00] sort of built up our civilization around.
A profile
that is changing. Right. It's kind of like, uh, we talked about,
barrier islands and how they're changing all
the time.
Meandering streams are, are very
similar to that. They meander pretty
quickly and we're kinda locking 'em in.
Chris Bolhuis: That's a great idea for another episode in a series of, what is the effect on the river when we levy a river?
we try to control the Mississippi by Levy in it. What are the, pros to that? What's good about it and what's bad about it?
Cause [00:38:30] whenever you try to affect a river, you're gonna affect the Geology. I mean, what's, what's going on?
You know, it's a, I think that's a
Dr. Jesse Reimink: You know, I, we're, we're kind of, I think we should, we should tidy these all together and make a, you know, something about ephemeral.
Geological features, you know, like barrier islands and meandering
streams. These things that change a lot and
we're sort of
forcing them to not change or we're not wanting them.
to change
because we've built stuff around them.
You know, like
that'd
be a, a, a good episode. Um,
okay. Well, Chris, did we cover [00:39:00] entrenched meanders? I think we got the main point across. That's a whole big, I mean, that and water gaps and all these interesting morphological features. Those are, long
episodes in and of themselves, but
Chris Bolhuis: Yeah, I think it's a good idea. Good suggestion. Just push pause and Google Image and Entrenched Meander or Google, horseshoe Canyon, and you'll get it. You'll see exactly what we're talking
about if you have that visual.
Dr. Jesse Reimink: Hey Chris, you know, we haven't done a stream chapter.
We've got it on our docket of things to do for Camp
Geo, [00:39:30] but if you want all of the other basics of geoscience, we've got most of our content up on Camp Geo now. We've got eight or nine chapters if you wanna learn all the basics of geoscience.
With images the way we think you really need to, to understand it with Chris and I, you can go to geo.camp courses.com, the first link in your show notes. Check that out. Leave us a rating and review on your podcast platform. Send us an email, planet geo cast gmail.com. That's super useful. We're building, , office hours episodes, always in the background here.
We got a lot of good suggestions [00:40:00] and questions and we're putting those together in office hours, episodes as we speak here. go to our website, planet geo cast.com. There you can subscribe. Support us. We always love that and appreciate those of you who have supported us so far.
Streams
are beautiful.
Chris Bolhuis: They are cheers.
