The River ClwydChapter 3

Does the river change over time?

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Does The River Clwyd change over time?

To help answer this question we are going to look at a short section of the river between Denbigh and St. Asaph, near

Llanerch Hall. We are going to look at an old map from 1880 and a new map from 2004 and see if the river has changed over this

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This is how the river looks today.

Take a good look at the course of the river.

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Click on the red circle and it will follow the course of the river.

This is how the river used to look. Can you see any changes?

Click on the red circle and it will follow the course of the river.

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To help you see the changes the river on the old map has been coloured red. When you click on this writing the map of today's river will go over the old map. Can you see how the river has

changed?

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© Hawlfraint y Goron. Cedwir pob hawl. Cyngor Sir Ddinbych. 100023408. 2006.

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In 2 places the river has taken a straighter course.

The meanders are gone.

© Crown copyright. All rights reserved. Denbighshire County Council. 100023408. 2006.© Hawlfraint y Goron. Cedwir pob hawl. Cyngor Sir Ddinbych. 100023408. 2006.

These 2 old sections of river are known as Oxbow Lakes.

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© Crown copyright. All rights reserved. Denbighshire County Council. 100023408. 2006.© Hawlfraint y Goron. Cedwir pob hawl. Cyngor Sir Ddinbych. 100023408. 2006.

What has caused this to happen?

To understand we need to know about three different processes.

1. Erosion

2. Transportation

3. Deposition

The three processes are really quite complicated but we will take a basic look at them.

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ErosionErosion is the wearing away of the bottom or

sides of the river. This can be caused by:

1. the force of the water (think of pointing a hosepipe at some soil – what happens to the soil?)

2. rocks and stones rolling along the bottom. These wear away the bottom. The force of the water can sometimes move even large rocks.

3. items floating in the river which can knock against the banks and wear it away.

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Discuss the picture. What can you see?

Erosion

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The water has worn away the bank. You can see that the grass used to be growing on the bank but is now hanging over the water. In order to do this the water must have got higher and therefore it must have happened after a lot of rain. The river would be deeper and flow faster and therefore have more force.

TransportationThe river is able to transport or carry objects from an area that has been eroded further

upstream.

1. Objects can float on the rivers surface e.g. leaves or wood.

2. Soil or sand can be carried within the river. After very heavy rain the river can look dirty and this is because of the load it is carrying.

3. The force of the water can also roll stones and rocks along the bottom.

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Deposition

This is the river dropping the load that it has transported. This will happen when the force of the water is not strong enough to carry the load any more. Heavy objects will be deposited first and the lightest last. Floating objects will be the last to be deposited.

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Can you see anything in the picture to suggest deposition?

Deposition

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Wood

Sand

Plastic

Small pebbles

Use the terms Erosion (Eroded), Transportation (Transported) and Deposition (Deposited) to discuss the pictures on this slide.

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Before we go any further you will notice that the outside of this bend in the river is eroded and the inside of the bend has pebbles etc. deposited on it.

Outside

Inside

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Back to Oxbow Lakes!

Now you know the 3 principles:-

Erosion, Transportation and Deposition,

You are ready to see why the river changes and Oxbow lakes are formed.

Click on the website below.

www.school-portal.co.uk/GroupDownloadFile.asp?file=21606

The Erosion of banks and the formation of oxbow lakes causes problems e.g.

• a farmer losing land on one side of a river and the farmer on the other side gaining land. Which one is happier?

• the bank near a building could wear away and the building fall in the river

and therefore in some places people take action to prevent erosion.

What do you think they could do?

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How to stop erosion.

To stop erosion you can support the bank with materials that are more difficult to erode. In the top example they have built a stone wall. In the example on the side they have put large stones on the bank. Trees and their roots also hold the bank together.

New river course(in red)

Old river course(in yellow)

Maps and aerial photo

of river.

Here are a map and some photos of the River Clwyd coming into Ruthin. Can you explain what has happened in the last few years? Can you explain what they have done in one place to stop it happening again?

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© Crown copyright. All rights reserved. Denbighshire County Council. 100023408. 2006.© Hawlfraint y Goron. Cedwir pob hawl. Cyngor Sir Ddinbych. 100023408. 2006.