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The processes of river erosion and transportation work together to create some spectacular river landforms.
None of these is more iconic than the waterfall.
They form where river processes interact with the geology of landscapes.
From the largest, which create top tourist destinations, and in every climate from the tropics to the Arctic.
Let's look at some of the largest waterfalls on the River Swale in North Yorkshire, here at Kisdon Force.
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You see the processes that form waterfalls like this at work on days when the river is high.
Now, a few years ago, I came to paddle off this waterfall with a group of experienced paddlers.
We kayaked down the river to here and off this waterfall.
On that day, there was so much water in the river that all the rocks either side were covered by the flow.
The paddlers could quite safely go over the waterfall without hitting the rocks beneath.
It's on days like that, when powerful, fast-flow water fills the river channel that the river is really working to erode and transport material, shaping landforms like this waterfall behind me.
But how do waterfalls like this actually form?
If we look at this rock behind the waterfall we can see something really interesting.
There are different layers of rock, here and here.
And if we take some pieces of those different layers out of the river, we can see why this is important.
So, we've got two pieces of rock here.
This is a piece of limestone that we find at the top of the waterfall, and this is a piece of sandstone that we find at the bottom of the waterfall.
Let's see what happens when we hit them with a hammer.
So, the limestone...
absolutely nothing.
I'm hitting that really quite hard and nothing's happening.
Let's see what happens to this one.
The one at the top of the waterfall, and the one at the bottom of the waterfall.
So, this rock is much softer and easily eroded by the water.
Whereas this rock is much harder and more resistant to erosion.
So, we've built a little model here to help us understand how that difference in the erodibility of hard and soft rocks is important in the formation of a waterfall.
This is our model river channel just here.
What we have are some hard rocks up at the top.
This gravel is playing the roll of some soft rock.
And it actually underlies, it goes underneath, this hard rock just here, and it sits downstream of it.
And then these rocks are the edges of our river channel going off downstream in that direction.
Now, as the river starts to flow we'll see what happens to both the hard rock and the soft rock downstream from it.
So, go ahead.
So, the water's flowing over the hard rock.
And you can see that it's not eroding the hard rock but it does erode the soft rock below it.
I'm going to help the river along here, so, my trowel is playing the role of some abrasion, and we're moving material off downstream.
And if we have thousands of thousands of years of erosion, that's what would happen.
And we'd see all this soft material removed but the hard material is much more resistant to erosion and it stays in place.
Over time this vertical drop forms and this is our waterfall.
The water flowing over the top has more energy as gravitational potential energy is converted into kinetic energy.
It keeps on eroding the bed and banks below the waterfall.
And that's what creates our plunge pool.
So, as that water continues to flow, over time it begins to undercut the rock behind the waterfall.
And remember, this soft rock extends underneath the hard rock.
It undercuts under here and creates an overhang.
That hard rock is then undermined and at some point it will become unstable.
Although this is very hard and can resist erosion, at some point this will collapse, and when that does happen, the waterfall will then retreat and move upstream.
When the river is low, like it is today, you can actually see right behind this waterfall.
Those erosional processes, hydraulic action, abrasion, attrition and solution are all working together and working hard to erode the bed of the river, the banks of the river just here, and importantly they also erode the rock behind the waterfall.
And eventually, that rock on top will collapse and come tumbling down into the plunge pool.
When that happens, it becomes tools for abrasion that further erodes the river downstream.
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Each time a waterfall like this collapses, it migrates a few metres upstream.
And we can see evidence that this has happened over thousands of years if we look behind me here.
Over time, again and again and again, when the waterfall collapses it migrates upstream leaving behind a steep-sided gorge.
In geography, the past is the key to the future.
If we study what happened in the past, we can foresee what will happen in the future.
Now, in the case of this waterfall, it will keep migrating upstream.
Many years from now, this waterfall will be much further up the river, and the river here will sit down in a deep gorge, just like we see downstream today.
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