The stones carried by a river are dragged along on the river bed causing abrasion. The river bed is smoothed and worn away producing potholes.
This means that the channel gets deeper.
So why doesn’t every river have deep vertical sides like this?
The steeper the slope the more likely it is that the soil and rocks will fall.
They will land in the river and the river will break up the big pieces and gradually wash everything away.
But slopes don’t have to be vertical for soil to find its way into the river.
Here you can see how the layer of soil and plants is gradually sliding down the hill, leaving bare areas where the ground has slipped away.
The result is that the valley usually takes a V shape. The river is deepening in the middle and the side slope slide determines the angle of the V.
It could be quite narrow like this…
…or wider like this.
Next week we will look at a situation where you will find some vertical slopes.
We saw last time that as the river moves the pebbles around, they crash into each other and bits break off. The pebbles become smaller, smoother and more rounded by this process, which is called attrition.
In some places the river drags its pebbles across solid rock on the river bed. This rubs both the pebbles and the river bed, gradually smoothing the surfaces. The process is called abrasion.
If the pebbles get trapped into a swirl by the current, then they whirl in a circle and the abrasion makes a depression in the river bed.
The circular depression is called a pothole.
Once the pothole shape has started to form, pebbles will tend to collect in it. They will sit there in position, all ready to be whirled round when the river flow is strong enough.
So, the pothole will quickly get deeper, which means it will collect even more pebbles etc.
Abrasion on the river bed will make the river channel deeper and will cut a deeper valley through the landscape.
In the upstream area of a river, where the water isn’t too deep, you can more clearly see what is happening on the river bed.
Here on the River Severn you can see the underlying rock layers tilted at an angle.
In other sections the bed is entirely covered by loose material – pebbles and stones of various sizes.
Most of these stones, especially the biggest ones, will just sit there, but in times of fast flow the river will have enough energy to pick up the smaller stones and jiggle some of the others. The biggest ones might not get moved, but the ones that are moving will swirl about and crash into them and bits will get broken off.
This is called attrition. It results in the pebbles becoming smaller, smoother and more rounded.
Eventually the big stones will have become small enough to be moved downstream by the flow of water.
Last week we saw how flatter land and a drier climate make the east more suitable for arable farming. But what happens if the weather in a certain year is exceptionally wet. The crops won’t ripen so well. They might start to grow mould. The amount harvested will be smaller and the farmer have less money to buy the inputs he needs for the next year and to buy food and other essentials for himself and his family.
Farmers know that some years will be better than others. If the crops are good most of the time, then the farm should be able to keep going.
But to improve the chances of the making enough money each year many farmers do some diversification. This means that they use their land and buildings to make money in other ways – anything that is not the traditional use.
This could be a different type of farming, such as keeping rare breeds or growing an unusual crop.
A field could be turned into a campsite.
Buildings can be adapted for holiday accommodation or a tea shop or a craft barn.
A spare room in the farmhouse could be offered for bed and breakfast accommodation.
I found this example on the internet, while researching next week’s post, but you should easily be able to find your own example in your nearest rural area.
As a river descends a valley, it rarely goes in a straight line.
The water will take the easiest route. If there is a lump of rock in the way it will flow round it.
As a result, a river will tend to swing from side to side.
Up near the source of the Severn, the river is already taking a curvy path. As it cuts itself a valley, that curvy path causes the valley side slopes to interlock with each other like the teeth on a zip.
A piece of higher land sticking out into the lower land (in this case the valley) is called a spur. When they come from one side and then the other, as the river flows around them, then you’ve got interlocking spurs.
As the Severn’s valley gets deeper it goes through trees. The path zigzags around the spurs, with the river, but you can’t see the interlocking pattern. However, the virtually treeless Carding Mill Valley in the Shropshire Hills has a nice view!
So do they spur the river on? If anything they probably slow it down, as the river twists and turns around them and cannot follow a direct path.