(upbeat music)
The erosive power of river carves out valleys, removing rock that has been uplifted by plate tectonics, creating river gorges and mountain landscapes like this.
All the rock eroded by rivers has to go somewhere, and it is carried downstream as sediment, through river transport processes.
Those processes are important.
They're the reason that this valley and river gorge exist, and is not full of rock and soil.
They carry material and nutrients downstream, creating and sustaining fertile farmland and river ecosystems.
And ultimately, every river flows into the sea, where the material and nutrients from the river feeds marine life.
So from the landscape we live in to the food we eat, river transport plays an essential role.
Let's go and have a look at how the river actually transports sediment.
At some point, all of this material has been removed from the bed and banks of the river, and transported to here.
If you have a look at this material, it's all different sizes, and it's the size of material that determines how it actually moves down a river.
We're going to use this tank and some sediment that we've borrowed from the river, to show you how the size of material changes the way in which material is transported down the river.
Now as you can see, we've got a range of sizes of sediment here.
In this pot here we've got some salt, then we've got some silt that we've taken from the riverbed, they get a little bit bigger into some gravel, we've then got some smaller pebbles, and then some bigger cobbles at this end.
Now if we look closely at the water, you can see it's already a little bit of a murky brown colour, and that's because the River Swale flows over an area of peat.
And that material in the peat is actually already being carried by the river, which gives it that yellow tinge.
The smallest particles carried by the river are those which are dissolved into the water, just like this salt.
When this material dissolves into the water, it becomes part of the river's dissolved load.
We say this material is carried in Solution.
But when the material is slightly larger, like this silt, and when we make the water very turbulent, you can see that the colour of the water actually changes.
That's because some of that material is being suspended, and held in the water, and it gets carried along in a process that we call Suspension.
When the river looks muddy or cloudy, this tells us that it's carrying a lot of sediment in suspension.
(water gurgling)
When the material gets slightly larger, just like this gravel, we can see that sometimes these particles are getting picked up off the base of the river, and they actually get bounced along.
We call that process Saltation.
(water running)
For larger material, like these pebbles, or even these cobbles, we need a very high discharge and high flow velocity to get this material moving.
And when it does, it doesn't actually lift off the bed of the river, it simply rolls along the riverbed in a process we call Traction.
Because of its weight, this material often only moves after heavy rain, during very high flow, when the force of the water is enough to move it along.
Can you--
Go on.
Right, so it takes two of us to pick that rock up, or even just to roll it over.
So you can imagine how powerful the flow of the water must be when it's been raining heavily, and this river is in a really high flow state.
and this river is in a really high flow state.
So there's four processes of river transport.
Solution Suspension Saltation and Traction.
And they take place in every river in the world, transporting material from the sources of rivers up in mountain ranges, all the way down to the sea, shaping the Earth's landscapes and ecosystems.