Lesson video

Hello, geographers.

My name's Mrs. Hormigo.

I'm looking forward to teaching you today.

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

Let's get started.

In today's lesson, we're going to look at the landforms that develop along the river's course.

By the end of the lesson, you'll be able to explain the characteristics and formation of river landforms. There are four key words for today's lesson: lateral erosion, hydraulic action, abrasion, and deposition.

Lateral erosion is the process where river erodes the land on its sides, so its banks, rather than just the river bed.

Hydraulic action is the power of water eroding a river's bed and banks.

Abrasion is the wearing away of a river's bed and banks by its load.

And deposition is when material being carried is dropped due to a loss of the river's energy.

There are three parts to today's lesson: landforms created by river erosion, landforms created by river erosion and deposition, and landforms created by river deposition.

Let's get started on our first learning cycle and look at landforms that are created by river erosion.

If we remember, rivers erode the landscape through these four different processes.

Hydraulic action, where water compresses air into the cracks in the river bed and the river banks, forcing them apart over time.

Abrasion, where material carried by the river scrapes and grinds away at the river bed and the river banks.

Attrition, where material carried by the river knocks against each other, breaking it into smaller particles.

And solution, were slightly acidic river water, due to carbon dioxide dissolving in it, reacts with minerals in certain types of rocks.

Calcium carbonate in limestone being one of those.

Due to this erosion, river creates different landforms, which include waterfalls, V-shaped valleys, and gorges.

And we're gonna look at how each of these form in this learning cycle.

Let's have a quick check before we get started.

Which of these are examples of landforms created by erosion? Is it A, hydraulic action, B, V-shaped valleys, C, waterfalls or D, abrasion? Pause the video and make your decisions.

I hope you said V-shaped valleys and waterfalls.

It's important not to get confused between processes and landforms. Hydraulic action and abrasion are processes of erosion, whereas V-shaped valleys and waterfalls are the landforms created by erosion.

Right, let's look at V-shaped valleys.

Now they're found in the upper course.

They are narrow and steep sided, so the cross section is a V-shape.

The river in this section, the river channel, is shallow and its narrow.

And although we've got a steep gradient, friction that's created by the water passing over rocks and boulders that it isn't powerful enough to move or erode, slows the river flow down.

We can also see in this image interlocking spurs.

So those are ridges that extend into the river course that the river winds between.

It doesn't have the power to erode them, so it winds around them.

In the upper course, the river often encounters resistance from harder rock, and this often leads to the formation of waterfalls.

So we can see here the cross section of a river in the upper course.

We have vertical erosion as the main process taking place, which deepens the valley.

The valley sides are steep due both to weathering and erosion.

And rivers may also carry some sediment that contributes to erosion by the process of abrasion.

This is an image of a waterfall, and it's characterised by a sharp drop in the river's course with water cascading over a cliff.

An image there of a waterfall.

As this water falls over the cliff, it erodes the base, the river bed at the base, which creates a deep pool known as a plunge pool.

Waterfalls often form where a layer of harder rocks, so more resistant rock, sits above a softer, less resistant rock.

And this creates a step in the river bit.

But let's break this process down.

So waterfalls are found in the upper course of a river, where a layer of harder rock lies above softer rock, and you've got there a diagram showing you that.

As the water flows over these rocks, it erodes the softer, less resistant rock more quickly than the harder rock, which results in a vertical drop in the riverbed over time.

As the river flows down, the drop is created.

Hydraulic action, so the water compressing air into cracks in the riverbed and the river banks, and abrasion, so the grinding and the wearing away of the banks by material it's carrying, erodes the softer rock below even further, and this creates a plunge pool at the base, so a particularly deep area at the base of the drop.

The band of harder rock is undercut to the extent that it collapses, it's no longer supported and it will eventually collapse.

This rock fall contributes to the erosional process of abrasion, so it adds sediment into the river that the river will then use to erode the river beds of the riverbank.

Once this has happened, we say the waterfall retreats, it moves back up the valley.

It leaves behind a steep-sided gorge, where the river is still running through it.

So you can see there an image of a gorge.

So as the waterfall cuts back into the river channel, it's still surrounded by those steep-sided river valleys, which creates a landform called a gorge.

So another image there of a gorge, it's a narrow, steep-sided valley that is typically found downstream of a waterfall, and that's created as the waterfall retreats upstream.

So as the soft rock undercuts the harder rock, the harder rock collapses and the waterfall retreats.

And this process will keep happening over time to create these gorges.

Now, can you put these diagrams in the correct order to show the formation of a waterfall? Pause the video and have a go at numbering them 1-4.

Hopefully you put them in this order.

The first image is actually the last in the sequence, the second image was the first one, the third image came second, and the fourth image is third in the sequence, showing the plunge pool as the harder rock collapses as it is undercut.

First task for you now.

Can you explain how waterfalls form in the upper course of a river? Now you may want to include a diagram or a sequence of diagrams to help you with this answer.

And Aisha also says, "I should remember to include the correct erosional processes in my answer." So making sure you try to name the process of erosion that is involved in each stage of the formation of a waterfall.

Pause the video and come back when you're ready.

Your answer may have looked something like this if you chose to use a paragraph as your style, "Waterfalls form in the upper course of river where a layer of hard rock lies above softer rock.

The river erodes the softer rock more quickly through processes like hydraulic action and abrasion.

This creates a step in the riverbed.

The harder rock resists erosion and remains intact, causing the river to drop vertically, forming a waterfall.

As the water flows over the drop, it continues to erode the softer rock by hydraulic action, creating a plunge pool.

The harder rock is undercut and eventually collapses, causing the waterfall to retreat upstream, forming a gorge." Now you'll see in that answer, it's very clear as to the sequence of events and the types of erosion, the processes of erosion have been included at the relevant points.

So well done if your answer does this as well.

Let's move now to the second learning cycle, landforms created by river erosion and deposition.

So some river landforms are formed by both erosion and deposition, and they're often found in the middle course.

So if we think back to our long profile, they are found in that section where the gradient is decreasing, it's becoming much more gentle, and erosion is primarily focused on lateral erosion, the channel is getting wider and deeper.

A meander is a bend or a curve in the path of the river.

This map extract shows lots and lots of meanders across the central part of the image.

And these, the meanders are typically found in the middle and the lower courses of a river.

You can see there that the meanders are circled or some of the meanders are circled on the map.

Meanders form due to the process of lateral erosion, and we know that is one of the main processes taking place in the middle course of a river.

The river velocity, so the speed of the river, is faster on the outside of the bend and it's slower on the inside of the bend.

Erosion occurs where the flow is fast because the river has more energy, and deposition occurs where the river slows and it has less energy.

So we can see there, that brown area, shows where the river is eroding on the outside of the bend, and the white area shows where deposition of sediment is taking place on the inside of the bend.

So the erosion is taking place where the flow is fastest and the deposition is taking place where the flow is slowest.

Can you now have a go at filling out these labels on the diagram? Pause the video and come back when you're ready.

I hope you managed to finish labelling it correctly.

On the outside bend, river velocity is faster and so the banks are eroded.

On the inside bend, river velocity is slower and deposition takes place.

Well done if you're understanding this.

So here we have a cross section through a meander, so like we've taken a slice through the river channel.

Erosion on the outside bend of the river forms a cliff, so a steep river bank known as a river cliff.

Deposition of sediment on the inside of the bend forms a slip-off slope.

So as material is deposited, the gradient of the bank becomes very gentle, and it's known as a slip-off slope.

Check again for you now.

Can you put these words into the correct box around the diagram? So you need to think back to where is erosion and deposition happening? Where is fast river velocity? Where is slower river velocity? And where do we find a slip-off slope and a river cliff? Can you connect these different words.

Pause the video and come back when you're ready.

I hope you managed to get on the inside of the river bend slower river velocity, so therefore the river doesn't have as much energy so deposition takes place.

And as deposition takes place, we get the landform known as a slip-off slope.

So material builds up and the gradient of the bank is very gentle.

And on the outside of the river bend, the river velocity is fast, therefore the river has lots of energy and erosion takes place.

Because erosion is taking place, we have a steep river bank known as a river cliff.

Well done if you managed to connect all of those words correctly.

An oxbow lake is a crescent-shaped lake that forms when a meander becomes more pronounced and is eventually cut through.

You can see on the diagram, the neck of the meander is labelled.

This is the land between the two bends.

And over time, due to erosion on the outside of the bend and deposition on the inside of the bend, the neck becomes narrower During a period of high flow, like a flood, the river may cut through the neck and create a new straighter channel.

The river will then continue to flow in this straighter channel and not around the loop, the old meander.

The fastest flow now is in the centre of the channel, and so deposition will occur along the edges.

This means that the old meander is abandoned by the river and water is trapped in the former loop due to this deposition taking place at the neck.

This old abandoned meander is what we call an oxbow lake and eventually it will dry up.

Which of the following features is formed on the outside of a river bend because of the faster river velocity here? Is it A, a slip-off slope, B, a river cliff, or C, an oxbow lake? Pause the video and have a go.

Hopefully you said a river cliff, well done.

If you remember, the flow is fastest on the outside of the river bend so erosion takes place, creating a steep river bank known as a river cliff.

True or false for you now then, river erosion is the only process involved in the formation of a meander? I hope you remembered that that was false, but can you explain why? Have a go by chatting to your partner or talking as a class.

Your discussions may have been similar to this.

Erosion takes place on the outside bend of the river due to the faster flow, but deposition occurs on the inside bend as the flow is slower, and together these processes create a meandering river.

Well done.

Right, Task B for you now.

There's an answer on the screen now, but can you correct and improve this answer to explain how an oxbow lake forms? Read it through carefully, thinking about processes and sequencing and see if you can make some improvements.

Hopefully you corrected this answer to say, "An oxbow lake forms when a meander in a river becomes larger over time.

Erosion on the outer or outside bend due to faster river velocity and deposition on the inner bend due to slower river velocity causes the meander to grow.

Eventually, during a period of high flow or a flood, the river cuts through the narrow neck of the meander, creating a new, straighter channel.

The old meander is then cut off from the main flow of the river due to deposition, leaving behind a crescent-shaped oxbow lake.

Over time this lake dries up and is often filled with sediment." Well done if you managed to spot the corrections that were needed and make some improvements.

Let's move now to our final learning cycle, landforms created by river deposition.

River deposition occurs when the river loses energy and it can no longer carry the same load of sediment.

And this happens in the lower course primarily.

This is where the river's velocity decreases or the river's discharge decreases, and it often happens close to the mouth of the river, which is in the lower course.

So river deposition can help to create these different landforms: floodplains, deltas, levees, and estuaries.

And let's have a look now at each of these.

So a floodplain is a flat area of land next to a river that's shaped by erosion and built up over time by the deposition of sediment during floods.

We can see here that this is in the lower course, so contour lines are very widely spaced.

Circle shows an area of the floodplain of this image.

So during times of high flow, the river can overflow its banks.

This image here shows a river that has clearly burst its banks onto the surrounding land.

You can see there that farmland and trees are now within the river's channel.

As the water spreads out, it loses energy, and as it loses energy, it deposits its sediment.

And this happens over time, repeated flooding deposits sediment, and gradually it builds up a wide, flat floodplain.

There are some benefits to floodplains.

They're very fertile due to the nutrient-rich silt which is deposited by the river, so they can make very productive farmland.

Natural levees are raised banks of sediment that form alongside rivers, particularly during flooding events.

So when a river floods and spreads out onto the surrounding land, it slows down.

And as it slows down and loses energy, material is deposited.

As this happens, the river will deposit its largest sediment first.

And we can see on this diagram that this large sediment, which is deposited close to the river channel forms raised banks, and these raised banks are known as levees.

The smallest sediment can be carried further because it doesn't need as much energy to be transported by the river, and so it's deposited a little bit further away from the river channel.

These natural levees actually raise the sides of the river banks.

And so each time this happens, the channel, river channel will get a little bit deeper.

True or false, deposition creates distinct river landforms? I hope you said true, but why? Can you remember any of these landforms that are created by deposition? You may have remembered this.

So when a river looses energy and deposits sediment, distinct landforms are created, especially in the lower course.

And these include floodplains and natural levees.

Well done.

Let's look now at the next two.

So an estuary is where the river meets the sea.

We can see here an aerial view of an estuary on the southeast coast of the UK.

Large deposits of sediment form mudflats and salt marshes.

So we can see there that the river flows out into the sea.

It's fresh water flowing out into salt water, which the sea's tide or flow coming into towards the river flow.

When the river meets the sea, the river velocity decreases, and therefore deposition occurs.

And that circle on the image now shows where this deposition is occurring 'cause the two different waters are meeting and river velocity is decreasing.

Deposited sediment builds up in layers.

And we can see this salt marsh in the Thames Estuary.

This is a very unique and special ecosystem and takes time, but vegetation will start to grow on these mud flats and form salt marshes, which provides a very special habitat.

The last example of a landform is a delta, which is found the mouth of a river.

And again, it's created as material is deposited by the river.

The river will often break into separate streams across the delta.

So as the river loses energy towards its mouth, material is deposited and you get the river breaking up into separate streams as it navigates its way across this material.

So the river flows into the sea, its velocity decreases, sediment transported by the river is deposited.

And when sediment is deposited more quickly than it can be eroded, silt builds up, forming a delta.

We have here an example of a delta in Alaska, and you can see on it the braided streams. Right, task for you now.

Can you name each of these landforms created by deposition? Pause the video and have a go.

Should have been able to identify the first one, so photograph A as a delta, photograph B as an estuary, and photograph C as a floodplain.

Well done.

Final task for you now.

Can you describe the formation of natural levees? You may choose to use an annotated diagram in your answer.

You might find it easier to write your explanations around a diagram rather than just in a paragraph.

Pause the video and come back when you've had a go.

So your answer may include, "Levees form when a river overflows its banks during periods of flooding.

And as the water spreads out, it slows down and loses energy.

Sediment is deposited in order of particle size.

The larger and heavier sediment, such as sand and gravel, which take the most energy to transport, are deposited first, nearest to the river.

And this raises the banks of the river.

The smallest sediments, fine silts, are deposited further from the river.

And over time, repeated flooded continues this process to create natural embankments along the river called levees or natural levees." Well done if your answer is similar to this.

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

River erosion creates landforms in the upper course of a river.

These include V-shaped valleys, interlocking spurs, waterfalls, and gorges.

River erosion and deposition create meanders and oxbow lakes, often found in the middle course of a river.

And river deposition happens as the river slows and loses energy.

Depositional landforms such as floodplains and natural levees, estuaries, and deltas are characteristic of a lower course of a river.

Well done.

I hope you now feel confident in explaining the different landforms that are created by erosion and deposition.

I look forward to seeing you all again soon.