(upbeat music)
M1: Compared with the whitewater rapids and waterfalls
that we see in the upper course of rivers,
here in the river's middle course,
this wide meandering channel snakes its way
back and forth across the flat land in the valley bottom.
Here the river is actually carrying much more water
and far more sediment than we saw
in the river's upper course,
because it has collected from all those tributaries
that have joined the river upstream.
F1: Now today, the river is flowing inside of its channel
but occasionally after we've had really heavy rainfall
this river will burst its banks and the river will
flood the entire bottom of the valley,
causing the river to be over a kilometre wide.
We call this part of the flat land in the valley bottom,
the floodplain, because it floods really frequently.
M1: This is actually part of the natural
behaviour of the river.
Most rivers like this burst their banks
and spill onto their floodplains every two to three years.
F1: So if we look closely at the river,
we can see quite a lot of clues that tell us
why the landscape looks like this and how it was formed.
(gentle music)
M2: So we're down here by the river and
one thing you'll notice if you look at the material
that makes up the bed and banks of the river is
it's really, really fine.
That's completely different to the size of
the boulders and the cobbles that we've seen
up in the upper course of the river.
But remember, this is just that material
that has been broken down by those attritional processes,
bumping into each other, making them smaller and smaller
until we get this fine, sandy material
down in the middle course.
Once that material's been broken down
by those attritional processes,
it's transported down here by the river, via
solution, suspension, saltation, and traction
until eventually when the river bursts its banks
it's deposited right on the floodplain.
One other thing that you'll notice that is different
from the upper course to where we are now
is that we can't see the bedrock.
And that's because again
deposition has taken place and the bedrock has been
covered in layer upon layer upon layer of sediment
that has been deposited by the flooding river.
When rivers burst their banks and spill out
on to the floodplain, the friction caused by
the vegetation slows the water down,
and we know that slower water causes deposition to occur.
The courser, larger sediment is deposited first
right next to the river, whereas the finer sediment
is carried slightly further away.
This causes what we call a levee or a natural embankment
that follows the course of the river.
(uptempo music)
M1: By flooding every two to three years
over thousands of years,
the river has created this floodplain.
Now, as humans we're very good at adapting
to both the risks and the opportunities
posed by the physical environment.
The fine alluvial sediment deposited here by the river
provides fertile land for cows and sheep to graze.
If the river floods, the farmers can
move their animals up onto higher ground,
safely away from the floodwaters.
If we look at the valley here,
we also notice that there are no houses, no buildings,
down in the valley bottom.
This is because the river is known to flood frequently.
The houses have been built up on the hillsides
safely out of reach of the floodwaters.
This is even reflected in the history of this landscape
with the sites of ancient castles placed
high up on hilltops.
(gentle music)
Thank you to Bedales School
and Birkdale School for helping to support
this Time for Geography video.
To find out how your school can help support
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on one of our videos, visit the patrons page of our website.