(gentle instrumental music)
M1: 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.
(gentle instrumental music)
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.
M2: 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.
M1: 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.
(gentle instrumental music)
M2: 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.
M1: 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.
(gentle instrumental music)