Lesson video

Hello, my name is Ms. Elliott and I'm going to be your geography teacher for today.

In today's lesson, we are going to be learning about the middle and lower course of a river, which is the fifth lesson of the river's topic.

So our lesson outcome for today is that you can describe the characteristics and land forms associated with the middle and lower course of a river.

We've got five keywords for today's lesson.

The first one is meander, which is a winding bend in a river.

An oxbow lake is a curved lake formed by an old meander.

A levee is an embankment alongside a river.

A floodplain is a flat area of land, either side of a river, which experiences flood in periods of high river flow.

And an estuary is the tidal mouth of a river where the river meets the sea or ocean.

We're first of all gonna learn about what are the middle and lower course of a river.

We're then gonna learn about how meander and oxbow lakes form.

And finally, we're gonna look at what landfalls are found in the lower course of a river.

So let's get going with the first learning cycle.

Now, geographers use specific terminology for talking about rivers.

So a drainage basin is the area of land that a river and its tributaries drain.

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

Okay, so we've already learned about the upper course and previous lessons.

So this lesson, we're gonna focus on the middle and the lower course.

Now the middle course is the section in between the upper and the lower.

And the lower course of the river is the final part of the river's journey.

And this is where the river enters the ocean.

Okay, let's check our understanding.

So can you label parts A, B, and C of the river? Okay, so A is in the upper course, B is in the middle course, and C is in the lower course.

Well done if you got that right.

Okay, so this diagram shows the long profile of a river.

It shows the gradient of the land and the cross section of the channel from the source to the mouth.

Okay, so we can see there in the upper course, the land is the steepest, but alongside that, the river channel is very shallow but also very narrow.

In the middle course of the river, the gradient of the land becomes a little bit more gently flowing, and the river channel widens and deepens.

And in the final part and the lower course of the river, the gradient of the land is much more gently sloping and the river channel is much, much wider.

Okay, so as the river flows downstream, the the gradient becomes flatter and the river channel becomes wider and deeper.

Now this video shows the middle course of a river.

What do you notice about what the river looks like? Might be worth just having a quick chat to the person you're sat next to.

Okay, so the middle course of a river.

The river channel is wider than in the upper course.

You'll find evidence of deposition in the middle course of a river.

There's a fairly high volume of water.

The land is generally quite gently sloping.

There'll be evidence of erosion as well, and there'll often be meanders or bends in the river in the middle course.

Okay, so this is what it is like in the middle course of a river.

The lower course of a river, the river channel is extremely wide.

This is where you'll find the river entering the ocean.

There's a very high volume of water.

Often you'll find deposition within the river and the area on either side of the river channel is generally very flat.

Okay, so that is kind of pieces of evidence of what the lower course of the river looks like.

Now we can also look at evidence from maps to tell which course of the river or which section of the river you are in.

Okay, so if we have a look here, we've got a section of an ordinance survey map.

What evidence can you see that we are in the middle course of a river? Pause the video and have a quick chat with the person you're sat next to.

Okay, so we've got the river channel is wider than in new upper course.

The land is gently sloping and there are meanders or bends within the river.

Well done if you got those.

Now again, looking at this map, how can you tell it's in the lower course of the river? Have a quick chat with your partner.

Okay, so there's evidence of deposition in the form of sander mud.

We can see here that the river enters the ocean and this flat land on either side of the river.

So well done if you got that right.

Okay, we're onto our first task for this lesson.

So what I'd like you to do is fill in the speech bubble describing what the landscape and river channel is like in the middle course of a river, using that photograph.

Second, I'd like you to annotate the OS maps with evidence showing which section of the river the map shows.

Okay, are we in the upper, the middle, and the lower course for each map? And what evidence can you spot to tell us that? Okay, let's check our answers.

So for this, I've put here the river shows there is evidence of deposition on the inside bend of the river.

The land is gently sloping and the river channel is wider than in the upper course.

There is also a slight bend in the river channel.

Might be worth pull video and checking you've got some things similar to what I put here.

Okay, so for this one, the middle course of the river is the map on the left and the lower course of the river was the map on the right.

Okay, so let's have a look at pieces of evidence for this.

So for the middle course of the river, we can see that there's a mid-size river channel.

The land is gently sloping and there are also evidence of meanders and bends.

For the lower course of the river.

We can see this is where the river is entering the ocean.

There is a very wide river channel and is flat lands on either side of the river.

So well done if you got that right.

Okay, we're moving on to our second learning cycle, which is looking at how to meander and oxbow lakes form.

Meanders are bends along a river and rivers meander because it's the most efficient way of moving water.

They're typically found in the middle and lower courses of the river, but sometimes they can also appear in the upper course.

Now from meander to form, you need both processes of erosion and deposition to take place.

Erosion occurs on the outside of the river bend, and deposition occurs on the inside of the river bend.

So let's have a look at this in a bit more detail.

Now, the river velocity.

So the speed of the river water is different in different sections of the channel.

Now it's faster on the outside bend of a meander, okay? Now, because it's faster on the outside of the bend, that leads to more river erosion.

On the inside bend of the river, the water is flowing much slower, and therefore deposition takes place.

Let's check our understanding.

So can you fill in the blanks here about where the river speed is faster and slower and where erosion and deposition are taking place.

So pause the video and see if you can fill in the gaps.

Okay, let's check this then.

So on the outside bend river speed is faster, and so the banks are eroded.

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

Well done if you've got that right.

Okay, here we've got a cross section of a meander.

So this is if you cut a meander or a river channel in half to see what's going on.

Now, as we can see here, that's on the outside bend of the river erosion is taking place and that's forming a little river cliff, which is just quite a steep section of the bank of the river.

Now, on the inside bend of the river deposition is taking place, forming a river beach.

Now in geography we call this a slip off slope.

Okay, let's check that.

So can you put those words that I've got on the slide? Erosion deposition, fast river speed, slow river speed, and slip off slope and river cliff into the correct box.

Okay, so on the inside bend of the river, you've got the slow river speed deposition and a slip off slope.

And then on the outside bend of the river, you've got fast river speed, erosion and river cliff.

Well done.

(mouse clicking) Okay, so we've got a lovely picture here of a meander.

Now I'm just gonna highlight the river in blue so you can see it a little bit more clearly.

What do you think could happen next? Have a quick chat with the person that you sat next to.

Did you suggest that the river might potentially cut through the meander neck? Well, you're absolutely spot on if you did, okay.

So that's exactly what can happen, is that over time, if that meander neck becomes really, really narrow, at one point the river might cut straight through and it's usually in a period of high river flow after a flood or something like that, that it'll do that.

Now, over time, the river will decide to take this faster route rather than going all around the bend, which can mean that that bit of water gets cut off.

Now if that bit of water does get cut off, it can form something called an oxbow Lake.

So oxbow lakes form when a meander becomes more pronounced and the neck of a meander of the land between the two bends narrows.

Okay, so if we have a look here, we can see a meander on the left.

The neck is still quite small there, but the meander on the right, the neck has got really, really small.

Okay, during periods of high flow like floods, the river may cut through the narrow neck, creating a new straighter channel.

The old meander is abandoned by the river and water is trapped in the formal loop due to deposition of the sediment at the neck.

Okay, so the river decides not to go around this meander anymore so that water becomes cut off and deposition can occur on the sides of the river.

So that turns into a lake rather than a river, okay? And it's known as an oxbow lake.

Okay, let's check our understanding.

I've got statements there about the formation of a oxbow lake.

Can you put them in the correct order? Okay, let's check.

So the first one, a meander forms and becomes more pronounced in the neck of the line between the two bends narrows.

Second one, during periods of high flow light floods, the river may cut through the narrow neck, creating a new straighter channel.

Third, the old meander is abandoned by the river and water is trapped in the formal loop due to deposition of sediment at the neck.

And finally, this forms an oxbow lake.

Well done if you got that right.

Now, over time, the position of the river will change as meanders continue a road on the outside bend and deposit on the inside bend.

Now what that can lead to lots of these oxbow lakes forming.

Okay, now this satellite image here, if you can see all of those horseshoe shapes, those are old oxbow lakes, okay? That have created scars.

So the these meander scars show the past position of the Rio Negra in Argentina.

So quite a cool picture that isn't it? Okay, we're onto task B for this lesson.

So what I'd like you to do is annotate the diagram of a meander using the labels provided and explain how the slip off slope and river cliffs forms. So you can use those labels, add them to the diagram, and then write a quick explanation.

Let's check our understanding.

So here we've got the river speed is slower, on the left a slip off slope forms here due to deposition, as the speed of the river is slower.

On the right, the river cliff forms due to erosion as the river speed here is faster.

Okay, we're onto our third learning cycle for this lesson, which is looking at what landforms are found in the lower course of a river.

Now river deposition occurs when the river loses energy and can no longer carry the same amount of material.

This happens in areas where the river speed decreases or the amount of water in the river decreases.

This often happens close to the mouth of river and then in the lower course.

So remember the lower course is the final part of its journey.

Now, river deposition helps to create these landforms. So floodplains can form, deltas, natural levees and estuaries.

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

So if we have a look at the OS map here, we can see because the contour lines are very far apart, that the area of land on either side of the river is flat.

Now that means if there is a flood, yeah, flooding period, because there's a high amount of rainfall.

If the river channel bursts its banks and the water overflows, then that water can spread a very far away, okay? Because of the flat land.

So either side of the river becomes completely flooded.

Now as the water spreads, it will bring sediment with it.

So it deposits sediment on either part of that flat land.

So during times of high float, the river may overflow its banks.

Now we can see in the picture here that is exactly what has happened.

As the water spreads out over the surrounding land, it loses energy and deposits the material that it's carrying.

Now, floodplains are very fertile due to the nutrient rich silt deposited by the river, but they are at risk of flooding, of course.

So a lot of floodplains are used as farmland.

So in some ways that's a benefit for farmers because it's really high quality soil because of all that sediment that's been deposited.

However, there is a risk involved that the farmer's fields may get flooded.

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

When a river floods and spreads out into the surrounding land, it slows down and loses energy, depositing sediment.

So we can see here that the largest sediment, it's deposit is closest to the river forming raised banks known as levees.

Now, sometimes you may see a river which has got these banks on either side of it, which have actually been built by humans.

So levees can be built as a flood defence as well.

So sometimes it's actually hard to know whether these levees are naturally formed or whether they have been built by humans.

So smaller sediment is deposited in layers further away from the river.

So if you were to take a cross section and look at the sediment further away from the river, the biggest pieces of sediment are deposited right next to the river channel.

Where the river had more energy, and then as it moves further out, the sediment gets smaller and smaller.

Let's check our understanding.

So which of these is not a land form often found in the lower course of a river? The answer is waterfall that's found in the upper course because it's created by erosion.

Now, an estuary is where the river meets the sea, okay? So large deposits of sediment form and mudflats and salt marshes conform here as well.

So if we have a look at the satellite image that we've got on the slide here, we can see that where the river has entered the ocean, that a lot of the water looks quite murky.

Now the reason for that is because the river has brought down lots of sediment, and as that river water slows down as it reached the ocean, sediment has been dropped off and deposited.

A delta may form at the mouth of a river.

So it is a land form created by the deposition of sediment, the river often breaks into separate streams across the delta.

So we've got a nice picture of one on the slide here.

So how this happens, well, the river flows into the sea and its velocity decreases.

We know that when rivers enter ocean, the speed of the river goes down.

Sediment transported by the river is deposited, can't hold the sediment, so it drops it.

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

Okay, let's check our understanding.

Can you name each of these land forms created by deposition? Okay, so we've got a delta and estuary in a floodplain.

Well done if got those right? Okay, we're onto our final tasks for this lesson.

So first of all, I'd like you to identify the following landform on the map.

So a floodplain and estuary and a meander.

Next, I'd like you to explain the formation of a floodplain.

Okay, let's check our understanding.

So you've got an estuary there where the river is entering the ocean.

A floodplain anywhere that you've labelled the flat side of the river channel on either side, that's absolutely fine.

And a meander is a bend in the river.

So let's read what I've put for this answer here.

A floodplain is a flat area of land next to a river shaped by erosion and built up over time by the deposition of layers of sediment during floods, they form during times of high flow when the river may overflow its banks.

As the water spreads out over the surrounding land, it loses energy and deposits the material it's carrying.

So it might be worth pausing the video there and just double checking that you've got all the correct information within your own answer.

Okay, we've got a summary for today's lesson.

Now, as the river flows downstream, the gradient becomes flatter and the river becomes wider and deeper.

Landform, such as meanders form in the middle course of a river.

Over time, oxbow lakes conform when meander loops become cut off following a flood.

And landform such as levees, floodplains, and estuaries, and deltas can be found in the lower course.

There was a lot of information in that lesson.

So well done for keeping going right to the end.

You've done a fantastic job.

Now that's it from me, so I'll see you next time.

Bye.