Lesson video

Hello geographers, my name's Mrs. Hormigo and I'm looking forward to teaching you today.

I hope you're going to enjoy the lesson and learn lots.

Let's get started.

Today's lesson looks at how a river changes as it moves from its source towards its mouth.

We'll be looking at a long profile and a cross section of a river.

By the end of the lesson, you'll be able to describe changes in the long profile and channel of a river along its course.

There are four key words for today's lesson, gradient, lateral erosion, channel and cross-section.

The gradient refers to the steepness or slope of the land or river bed.

Lateral erosion is the process where a river erodes the sides of its channel, widening the valley.

Channel is the pathway through which water flows in a river or a stream, and cross-section is a section taking sideways across the channel or valley.

There are three parts to today's lesson.

How does a river's long profile change, how does a river's cross-section change, and how does the River Eden change along its course? Let's move to the first learning cycle for today.

Geographers have got specific words and vocabulary that they use for describing and talking about rivers.

Here we can see a drainage basin, and that's the area that a river drains.

So it's the land from which water drains into that river.

Tributaries are smaller rivers that are joining the larger one, and they are part of that same drainage basin.

And you can see the orange dotted line delimits the edge of the drainage basin.

A river can be divided into the upper, middle, and lower course.

The upper course is the area nearest to the source, the middle course is the middle section of the river, and the lower course includes the mouth of the river and the lower area of the river's course.

Here we've got an arrow pointing to the source, which is the start of the river, and this is usually found in higher ground.

The watershed, that's the actual boundary of the drainage basin, a tributary, which is a smaller river that joins the main river, and the mouth, where the river ends and it meets the sea.

Check for you now, can you label these parts of the drainage basin? Pause the video and come back when you've had a go.

Hopefully your arrows were labelled in the same way as these.

So the top left is the source, then you've got the mouth at the bottom on the left, and then the three sections of the river, the upper course, the middle course, and the lower course at A, B, and C.

Well done if you manage to do that.

This image here shows a river's long profile and it shows the river's journey from its source, so its start, down to its mouth where it meets the sea.

We can see here that the gradient of the river changes and that the upper course starts at the highest altitude and the river slowly works its way down to a much lower altitude where it meets the sea.

The steepest section you can see is in the upper course and the flattest section is in the lower course.

We are gonna have a look now at the characteristics of each part of the river.

So in the upper course, the river starts in mountainous or hilly areas, and it's often a very small stream.

So you can see there an image of a stream in the upland areas.

The long profile in the upper course is steep and it's angular, and the river is mainly eroding vertically as it cuts down into the bedrock, which creates this steep profile.

In the middle course, the river moves out of the mountains and onto flatter land.

It's often surrounded by lots of farmland.

And we can see that the long profile is less steep, the profile is starting to flatten out, the gradient is becoming more gentle.

In this part of the river, we have lateral erosion, which is where we are widening the river channel and the river valley.

And there's often a mix of erosion and deposition by the river in this section, which leads to very characteristic land forms being created.

In the lower course, so this course where the river enters the sea, where we find the mouth, the river flows through flat, wide floodplains.

And the long profile is very shallow and flat.

The river here is flowing much more powerfully and it's much quicker and it's able to transport a lot more sediment.

Check for you now, can you complete the five missing labels on the river's long profile? Pause the video and see if you can identify what's missing from this diagram.

Hopefully you manage to add these labels in.

Source and mouth, so the start of the river and the end of the river, and then the three sections of the river, the upper course, the middle course, and the lower course.

Well done if you managed to complete that.

Your first task for today's lesson, can you annotate the diagram of a river's long profile to describe how it changes from source to mouth? So looking at this diagram here, can you add labels with information to describe how the river is changing from its source to its mouth? Pause the video and come back when you are ready.

Your answer may have looked something like this.

You may have identified in the upper course you've got a steep gradient where the river is mainly eroding vertically, in the middle course, the gradient is more gentle and we have lateral erosion, which is widening the river valley and channel, and that there is a mix of erosion and deposition.

And then for the lower course, the gradient is almost flat in the lower course and transportation is the main process.

Well done if your annotations were similar to these.

Let's move now to the second learning cycle for today, how does a river's cross-section change? The cross-section of a river is a section taken sideways across the channel or the valley, so a slice through the river channel.

It changes as it moves through the upper, the middle and the lower course.

But let's think how it changes.

The channel, so the size of the channel, the depth, the width of the channel.

The processes taking place, so is there erosion, transportation or deposition, or maybe more than one? The gradient, is it a steep channel or is it a much more gentle gradient? The banks, are they vertical or are they much more gentle a gradient? Is the river eroding them or is it depositing next to them? And the shape of the channel.

We're going to look at each of these as we move through the different stages of the river.

Let's start in the upper course.

So the upper course cross section you can see on this diagram here, it's a very distinct V shape.

The channel is very narrow, the banks are steep and the valley sides have a very steep gradient.

There's a lot of vertical erosion taking place.

We can see here an upper course stream on the OS map, and we can see that the contour lines are very close together, showing that there is steep land surrounding it, and they are forming that distinct V that you can see creates that V-shaped valley.

In the upper course, the cross section shows that the river channel is very narrow.

The banks are often steep and they can be jagged.

Lots of erosion is taking place.

The river channel is fairly shallow and the water flows around the rocks and the boulders because it's not powerful enough to move them or to erode them.

And it's for this reason that the flow of water is slightly slower, there's a lot more friction as the water moves over and around these boulders.

Vertical erosion dominates, so there is the river cuts downwards into the bedrock, which over time deepens the channel.

If we look now at the cross section in the middle course, we can see it's a very different shape to that of the upper course.

It's no longer that distinctive V shaped, it's become wider and the valley sides are less steep.

So the channel and the valley are widening due to a change in the type of erosion taking place.

Instead of eroding downwards, it's now starting to erode sideways in that process known as lateral erosion.

And we can see here on the OS map, this is a river in its middle course, the contour lines are much more widely spaced, showing that the valley surrounding the river is not as steep.

The river channel is widening and deepening and lateral erosion is becoming more significant.

The image there shows a river in its middle course, which is very different to the image we saw of the river in the upper course.

The banks are far less steep.

Erosion is smoothing the riverbed and the banks and there is less friction, so the water is starting to flow more quickly.

There is also some deposition taking place as the river meanders, so as the river starts to bend.

Let's have a check now, which of these statements describe the upper course? Is it A, a V-shaped valley, B, slope becomes gentler and the water flow slows down, or is it C, the channel is narrow and shallow? Pause the video and decide which ones you think are correct.

I hope you ticked A and C, that V-shaped valley because of vertical erosion and the channel being narrow and fairly shallow.

Let's look now at the lower course.

The cross section is again a different shape.

It shows that the channel is much wider and that the valley itself is also much wider and flatter.

And we can see here an OS map of a river in its lower course.

You can see that the blue area showing the river is much wider than on previous OS maps.

And the contour lines are more widely spaced, again, showing that the land surrounding it is flatter still.

And if we look at this image here, the channel is becoming even wider and deeper and the banks are generally low, and next to the river is often flatland.

This flatland is known as floodplains and it can be prone to flooding.

The channel is very smooth, therefore there is very little friction between the river flow and the banks and the river bed.

So the flow speed is much faster.

The river's velocity increases.

Deposition takes place where the river slows down at various points and when the river flows onto the floodplains.

Let's check for you now.

Can you identify which part of the river these images come from? Pause the video and come back when you've decided.

Hopefully you identified these correctly.

Photograph A is in the middle course, photograph B is in the upper course with its distinct V-shaped valley, our photograph C is in the lower course, where the river is the widest and deepest.

Now a task for you.

Can you complete the table summarising how a river's cross-section changes as it moves downstream? We want you to think about the shape of the river, the gradient, the channel, the bank, the river flow, and whether or not erosion or deposition is taking place.

The three different parts of the river are the upper, middle and lower.

Pause the video and have a go at completing your own table.

Your table may have been completed to look something similar to this.

So in the upper course, the shape is a V shape, the gradient is steep, the channel is very narrow and shallow, and the banks are steep.

The river flow here is the slowest and the main process taking place is vertical erosion, as the river cuts down into the bedrock.

in the middle course, there is a much more U-shaped river valley.

The gradient is moderate, so it's much less steep.

The channel becomes wider and deeper and the banks are less steep due to erosion taking place.

The river flow becomes faster because there's less friction due to a more shallow river bed.

And the main processes taking place are lateral erosion, but also some deposition as the river starts to meander.

In the lower course, the shape of the river valley is much flatter and wider.

The gradient is very gentle, it's often very flatland surrounding the river.

The channel is at its widest and deepest here, and the river banks are much lower.

The river flow is fastest because there is very little friction between the river and the riverbed, and the main process is deposition taking place.

Well done if you managed to complete this table comparing the different stages of the river.

Let's move now to the third part of this lesson and have a little look at a specific river, the River Eden, and how does the River Eden change along its course? So let's locate the River Eden.

It flows from the Cumbrian Pennines in the northwest of England to its mouth, Solway Firth, an estuary on the Irish Sea, so it flows in a northwesterly direction.

We can see there a map and a satellite image showing the flow of the River Eden.

Its source, which is circled on that OS map, is on Black Fell Moss, which is in the Mallerstang Valley near the village of Outhgill in Cumbria.

It's a mountainous area in the Cumbrian Pennines, so a high land in the Cumbrian Pennines.

And Laura's spotted there that the source of the River Eden is in the grid square 80, 99.

It's really good practise to apply your map skills when you are looking at rivers and OS maps.

It's a chance to combine the two.

Check for you now then, the River Eden flows in what direction from its source in the Cumbrian Pennines to the mouth on the Irish Sea? Is it northeast, northwest or southeast? Use that image and the north arrow to help you make your decision, and come back when you are ready.

Hopefully you worked out or you remembered, it flows in a northwest direction, from its source in the Cumbrian Pennines to its mouth at the Irish Sea.

Here we have the River Eden's long profile.

We can see it's very similar to the long profile that we looked at in learning cycle one.

The source of the river has the steepest gradient, the upper course.

The gradient becomes more gentle through the middle and the lower course.

We have now an image and an OS map extract showing the River Eden's upper course as it flows through the Cumbrian Mountains.

And we can see it has these characteristics that we've discussed.

The OS map shows that the contour lines are very close together, indicating those steep valley sides that we would expect in the upper course, the V-shaped valley.

The river is eroding vertically, so cutting down into the bedrock, creating that V-shaped valley.

The channel itself show it has water flowing around large rocks, and that's because it doesn't have the power to move or erode them.

And the channel is fairly narrow and shallow.

This shows an image and an OS map extract of the middle course, which flows through the Vale of Eden, from Kirkby Stephen through to Carlisle.

And we can see there's a very definite change in the OS map extract.

The contour lines are much more widely spaced, showing that the gradient is much more gentle.

The river is starting to meander as it flows through a much wider valley.

And that wider valley is because of lateral erosion taking place, which widens the valley and the river channel.

And there is some deposition taking place where the river velocity slows, so where the speed of the river slows down.

The lower course of the River Eden is from Carlisle to Solway Firth.

And we can see again, the OS map extract looks different.

The gradient is flat, the contour lines are very widely spaced.

The river's channel is wider and deeper and transportation and deposition are the main processes taking place.

And at this point, the river is transporting a huge amount of very fine sediment.

And we can see there that the river is meandering, so it's bending and turning as it nears its mouth at the Solway Firth and flows out into the sea.

Have a go now at identifying which map extract shows which course of the river.

Pause the video and come back when you are ready.

Hopefully you identified that map extract A is at the lower course because the contour lines are very widely spaced and the river channel is at its widest point here.

Photograph B is the middle course, the contour lines are flattening so they're becoming more widely spaced, the land is becoming flatter, the river channel is wider, and that C, map extract C is the upper course, a very distinct shaped valley, that V shape where the contour lines are very, very close together.

A final task for you today, can you compare the upper, middle and lower courses of the River Eden? Things that you should try to include are how the gradient changes, how the shape of the valley changes, so the idea of the cross section, the slice through the valley, and what processes are taking place, is it erosion or deposition? Pause the video and have a go at this task.

Your answers may have looked something like this.

In the upper course of the River Eden, the gradient is steep.

The river has a narrow, shallow channel and it primarily erodes vertically, creating V-shaped valleys with steep valley sides.

In the middle course, the gradient becomes gentler.

Lateral erosion widens the river channel.

Deposition also occurs in places where velocity slows.

The river valley becomes more of a U-shape as a result of this lateral erosion.

And in the lower course, the gradient is very shallow, nearly flat, and both the valley and the river channel are much wider and the channel is deeper.

Well done if your answers were similar to this.

Let's now have a look at the summary of today's lesson.

So a river's long profile shows how the gradient changes along the course of the river, so from the source to the mouth.

The upper course of a river has a steep gradient, which decreases through the middle and the lower course.

The cross section of the river channel and the river valley also change along the course of the river.

The course of the River Eden shows these changes from its source in the Cumbria Mountains towards its mouth at the Irish Sea.

Well done today.

I hope you now feel a lot more confident in describing how a river's landscape changes along its course.

Look forward to seeing you again soon.