Lesson video

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

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

Let's get started.

Today's lesson looks at the processes happening in a river, so the different processes of erosion, transportation, and deposition.

By the end of the lesson, you will be able to explain these processes at work in a river.

There are three keywords for today's lesson: sediment, river discharge, and river velocity.

Sediment refers to the particles of rock, soil, and organic material that are carried by a river as it flows, and they vary in size from fine silt to large boulders.

River discharge is the volume of water that flows through a river at a given point over a specific period of time.

And river velocity is the speed at which water flows in a river.

There are three parts to today's lesson.

The first is how do rivers erode the landscape? The second, how is sediment transported by rivers? And the third, where and why do rivers deposit sediment? Let's get started on our first learning cycle.

River erosion is the process by which flowing water wears away the land, shaping the landscape over time.

This here is an image of the Grand Canyon in the USA.

It shows the power of river erosion over a very, very long period of time.

Combined with the uplift that raised the Colorado Plateau and the power of river erosion, this canyon was created.

There are four types of river erosion: hydraulic action, solution, attrition, and abrasion.

And we're going to look at each of these in a little bit more detail.

Let's start with hydraulic action.

This is where moving water traps air into cracks in the river bed and the river banks, causing them to break apart over time.

Abrasion: this is where the sediment that is carried by the river scrapes and grinds against the bed and banks, like sandpaper, wearing them away and making them smoother.

Solution: this is where slightly acidic water dissolves soluble minerals in the rock, particularly limestone, which gradually erodes it away.

So this won't happen in every river channel.

It just depends on the minerals that are present in the rock.

And, finally, attrition: This is where rocks and particles, so sediment that the river is carrying, collide with each other, and in doing that, they'll break into smaller fragments and continue to knock and jostle against each other, which makes them much smoother and rounder over time.

Let's have a check for you now.

Can you label the four processes of erosion pointed out in this diagram? Pause the video and come back when you're ready.

Hopefully you got these correct.

You should have had attrition, pointing to the little pieces of sediment in the river.

Abrasion: the rocks and sediment grinding and being dragged along the base of the river, and hydraulic action, where water is forced into cracks in the river bed and the river banks, making them break down over time.

And solution, where slightly acidic water dissolves soluble minerals such as limestone.

Let's think now about the factors that increase the rate of erosion.

High river velocity: A river that is flowing faster has more energy.

When a river has more energy, it is able to erode more, so a higher river velocity leads to a higher rate of erosion.

Higher river discharge: When a river has a greater volume of water flowing in it, it will have more energy.

Therefore, when a river has a higher river discharge, it will have a higher rate of erosion.

Human activity can also affect the rate of erosion.

Urbanisation alters the natural flow of water.

By creating more impermeable surfaces and more drainage systems, more precipitation and water is flowing into the river.

This increases the river discharge and the river velocity, and so can lead to an increase in the rates of erosion.

And by removing vegetation, you are removing a way that precipitation is intercepted or absorbed, and so more water will end up reaching the river, which increases river discharge and river velocity.

And geology: when a river flows over a rock that is very resistant to erosion, the erosion will slow down.

Where rocks are less resistant to erosion, the rates of erosion increases.

Two or false then? Are all rivers eroded at the same rate? Pause the video and make your decision.

I hope you said false, but can you tell me why? You may have said something like this: "Differences in river velocity and river discharge affect the rate of erosion.

Faster-flowing rivers and those with a greater discharge may erode at a faster rate.

Also, if a rock type is less resistant, it may be eroded more rapidly than a more resistant rock type.

Well done, I hope you've understood that.

First task today for you: Annotate this diagram to explain the four ways a river erodes.

Now remember, it's not just a label, it's an annotation.

So we want to have you explaining each of these four processes of erosion.

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

Right, your answers should look something like this: Attrition: rocks and particles collide with each other, breaking into smaller round of fragments.

Abrasion: sediment carried by the river scrapes and grinds against the bed and banks, wearing them down like sandpaper.

Hydraulic action: moving water compresses air into the cracks in the bed and the banks, which causes them to break apart over time.

And finally, solution: slightly acidic water dissolves soluble minerals in the rock, particularly limestone, and gradually erodes it.

Well done if you managed to annotate your diagram like this.

Let's move now to the second learning cycle.

How is sediment transported by rivers? River transportation refers to the movement of sediment by a river as it flows downstream.

Now, rivers carry various types of sediment, from very, very fine particles, like clay, to much larger rocks and boulders.

And this transportation is influenced by the river velocity to the speed of the river flowing.

River discharge, the amount of water flowing past a given point in a certain amount of time.

And the sediment size, so the size of the particles.

There are four ways a river transports sediment: traction, saltation, suspension, and solution.

Now, you may notice, that the size of the sediment changes for each of these different processes of transportation, and that's an important observation.

Right, check for you before we look at them in a little bit more detail.

Can you label the four ways a river transports sediment? What were those four key terms? Pause the video and see if you can remember.

You might have been able to remember suspension, solution, saltation, and traction.

Well done if so.

So, let's have a little look now at traction.

This is when large heavy particles, like boulders and pebbles, are rolled or dragged along the river bed by the force of the water.

They require the river to have the most energy.

Saltation: this is where slightly smaller particles, such as sand grains, are lifted off the riverbed and then fall back down after a short distance.

It's often described as a bouncing movement.

And this happens because the particles are slightly easier for the river to move.

So the river needs less energy to use this type of transportation.

Then we have suspension: Very fine particles, like silt and clay, are carried in the water.

So they're suspended by the river's flow, and that's what often makes the river look very dirty, a muddy colour because of all the particles suspended within the river's flow.

And then, finally, solution: Rainwater often contains dissolved gases that make it slightly acidic, carbon dioxide being one of them.

This means that minerals, such as calcium carbonate, which is found in limestone, can dissolve within the water so a chemical reaction takes place.

And so you cannot see materials carried in solution, because they've dissolved into the water.

Right, have you been listening carefully? Can you match these four processes of transportation with the correct description? Pause the video and read through them carefully, matching them up to the correct term.

I hope you thought through this and you managed to answer it correctly.

Traction are the large heavy particles, like boulders and pebbles, that are rolled or dragged along the riverbed by the force of the water.

Saltation is when smaller particles, such as sand grains, are lifted off the riverbed and then fall back down after a short distance.

Suspension is when fine particles, like silt and clay, are carried in the water, suspended by the river's flow, which often gives the river a muddy appearance.

And finally, solution: dissolved minerals and chemicals, such as calcium carbonate, are transported in the river's water.

Well done if you matched those correctly.

Lots of terms to think about.

River velocity and sediment size influence how a river can transport.

A river needs energy to pick up sediment to erode material and to transport it.

Higher river velocity means that the river has more energy for both erosion and transportation.

We can see here, silt particles, that are carried in suspension, take very little energy to transport.

So rivers can transport silt for very long distances, even if the river velocity is low.

Boulders are larger and take up the most energy to pick up and transport.

Remember, that they are transported by the process of traction.

So a river velocity needs to be very high, and even then, they are often not transported very far.

Let's have a quick check now.

What factors influence sediment transport? Air temperature, river velocity, or sediment size? Pause the video and come back when you're ready.

I hope you remembered it was river velocity, so the speed of the river, and sediment size.

So the higher the river velocity, the more energy a river has, and the smaller the sediment, the easier it is for the river to transport.

Here's a task for you then.

Can you answer the following short answer questions? Number one, describe traction, and explain when it is most likely to occur.

Number two, how does the river velocity influence the type of transportation that occurs? And for number three, explain how solution works in river transportation, and give an example of a type of rock that is transported this way.

Pause the video and come back when you've had a go at answering these questions.

For number one, your answer might have included this: " Traction is the process in which large, heavy particles, such as boulders and pebbles, are dragged or rolled along the riverbed by the force of the river's flow.

And it occurs when the river velocity is high enough to move these larger materials." And for number two, how does river velocity influence the type of transportation that occurs? You may have said that "In fast-flowing rivers, so at high velocity, the water has more energy, so it can transport larger particles, like pebbles and boulders, through those processes of traction and saltation." And then "In slower-flowing rivers where there is less energy, so lower river velocity, the river is likely to only be able to transport smaller particles, primarily by suspension." And finally number three: Explain how solution works in river transportation and give an example of a type of rock that is transported this way.

So, solution is the process by which soluble minerals in rocks dissolve in the river water, allowing them to be transported in solution.

This occurs because river water often contains dissolved gases, like carbon dioxide, that make it slightly acidic, which allows it to dissolve certain minerals.

An example of a type of rock transported by solution is limestone, which is made primarily of calcium carbonates, and you remember you can't see these as they're dissolved in the water.

Let's move now to learning cycle three.

Why and where do rivers deposit sediment? River deposition: this is the process by which rivers drop sediment they have been carrying.

And we can see here an image of lots of material that has been deposited in the river channel.

But why do rivers deposit material? They deposit material because they lose energy.

How do they lose energy? There may be a decrease in river velocity, so the speed of a river's flow, or a decrease in river discharge, so the amount of water, the volume of water, flowing through a river at a given point.

When river velocity is high, rivers have lots of energy for transportation.

Slowing down a river means it loses energy and deposition occurs.

Lucas says: "Is that why dams silt up?" So behind a dam, a river slows down, and it loses energy, and so it will deposit material that it is carrying River velocity and river discharge are strongly linked.

If the velocity of the river decreases, the discharge will also decrease.

And Izzy says: "So deposition might increase in a drought?" She's correct, isn't she? During a drought, there will be less water in the river.

With less water in the river, the river's velocity will decrease, the discharge will decrease, and so deposition will increase.

Well done, Izzy.

Well done, Lucas.

Let's have a check now.

When would a river be most likely to deposit sediment? Pause the video and make your decision.

I hope you said, when a river velocity decreases.

The slower a river flows, the less energy it's got, and so as it has less energy, it will be more likely to deposit its load, its sediment.

Well done.

Where do rivers deposit sediment? They deposit sediment on floodplains.

These are the areas next to the river in the lower course, so when a river overflows its banks or floods, river velocity decreases as the water spreads out.

Sediment is deposited over a wide area, and that creates a floodplain.

They also deposit sediment on the inside of a river bend.

As the river flows around a bend, river velocity decreases on the inside of the bend.

Sediment is deposited on the inside of the bend, so where there is less energy because of less velocity, sediment is deposited.

At the river mouth, so as a river enters the sea, there will also be some sediment deposited.

The river flows into the sea, river velocity slows and energy is lost, sediment is deposited, and this forms estuaries.

True or false for you then.

Deposition only happens at the mouth of the river because this is where the river loses all velocity.

Pause the video and come back when you've decided.

I hope you remembered that that was false, but can you tell me why? You may have said something like this to your partner: "Deposition happens at the mouth of the river, but it also occurs when the river loses the energy it needs to transport all of its sediment load, for example, on the inside of river bends, or when a river floods, or during a period of dry weather and lower discharge." Well done, if you managed to think of a range of examples as this answer has.

Final task for you today: Describe two locations where river deposition is most likely to occur and explain why deposition happens there.

Let's have a look at this image.

Sofia says, "This diagram is helpful because it shows river bends, and that reminds me of one location where river deposition happens," and we can see it's labelled on the inside of the river bend.

But can you remember why? You might want to use this as one of your examples? Pause the video and come back when you're ready.

Your answer may have looked something like this: So for your first location, on the inside of a river bend.

"Deposition occurs here because river velocity slows down on the inside of the bend, and this means the river loses the energy it needs to transport all of its sediment load, and so some sediment settles out of the water." The second place you may have chosen, is on a floodplain, which is the flat area next to a river in the lower course.

"A river in flood has high discharge and it can transport lots of sediment, but when it bursts its banks, the water spreads out across the wide area of the floodplain, and all the energy from the high river discharge is lost, and so, sediment is deposited." Well done if you had chosen those two and written something similar to that.

Let's have a look now at the summary for today's lesson: River erosion is the process by which flowing water wears away the land, shaping the landscape over time.

Let's remember, there are four processes of erosion: hydraulic action, abrasion, attrition, and solution.

Rivers transport sediment material downstream via four processes: traction, saltation, suspension, and solution.

And if we remember, river velocity, river discharge, and sediment size, affects the type of transportation taking place.

Rivers deposit sediment due to a decrease in river velocity and river discharge.

And this happens, not only at the mouth of the river, but also inside a river bend and on floodplains in the lower course.

Well done, I hope you now understand the different processes of erosion, transportation, and where and why deposition occurs in a river.

I look forward to seeing you all again soon.