River Movement and Erosion

Rivers are an important part of the landscape, eroding, transporting and depositing materials of many different sizes. For the GCSE, you need to demonstrate that you understand how rivers work and the effects that they have on the landscape. This quiz is concerned only with river movement and erosion.

Rivers move because water flows downhill. They start in higher areas and flow to the lowest point that they can reach. For most rivers, this is a sea or an ocean. Some flow into depressions that are either lower than sea level or cut off from the oceans by higher land. An example of this is the world's largest lake - the Caspian Sea. This has no rivers flowing out from it, water seeps out into the underlying rocks or evaporates. If you want to find out more about these, look up 'Endorheic basin'.

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Some rivers do not reach the sea because humans take so much water from them or there is a dry season which significantly reduces the volume of water going into the river.

A river valley changes shape as it is followed from source to mouth. Where the water is fast flowing, different processes of both erosion and transport occur than in the places where the flow is slower. There are three different stages of a river, the upper course, where valleys are narrow, the volume of water is smallest, but the flow is fastest. The middle course is less steep but there is a lot more water that is still flowing quite quickly and finally the lower course where the volume of water is greatest and the flow is slowest. This is where least erosion takes place.

Moving water erodes rocks and surface deposits in several ways. Abrasion is the process in which the river's load smashes into the banks and the bed causing pieces to break off. Solution is the process of dissolving soluble materials from the load, the banks and the bed. Hydraulic action is caused by air being trapped in cracks in a river's banks and bed. Finally, there is attrition which describes the process of the load particles hitting each other and breaking into smaller pieces. In the upper course, there is more vertical erosion and in the lower course near the mouth, there is more horizontal (also called lateral) erosion.

The movement of the river transports the products of erosion by several processes – traction, saltation, suspension and solution. Traction is the process of large pieces of load rolling along a river bed. Saltation is the name given to the process of load bouncing along a river bed. Where smaller pieces of load are being carried in a river's flow, we call it suspension and dissolved material is carried 'in solution'. Occasionally, you will see another process mentioned - flotation, where less dense objects are carried along on the surface of the river.

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  1. Where is the flow of the river fastest?
    The upper course is the steepest part of the river
  2. In which part of a river does solution occur?
    The tricky part of this question is deciding between options three and four. Solution could occur all the way along a river, but it will actually only occur at places where the river is in contact with soluble materials. Option three is therefore only partially accurate but option four is completely accurate
  3. Where in a river valley is lateral erosion dominant?
    Slower moving water has less erosional power and can only erode softer materials like the soil of the river banks
  4. Which of the following correcty describes saltation?
    This occurs to particles that are too large to be kept in suspension
  5. Which of the following features is an erosional feature of the upper course of a river?
    They are caused by vertical erosion which is the dominant form of erosion in the upper course of a river
  6. Hydraulic erosion is caused by:
    Each time air is trapped and compressed into a crack, the crack can widen, eventually the crack gets large enough to cause part of the bank to break off
  7. By the time a river is in its lower course, there are no large boulders being carried along. Which of the following could be a reason for this?
    Make sure that you know the meanings of the four terms in this question. Another reason is that the flow is a lot slower and only fine particles can be moved
  8. In the introduction to this quiz, we mentioned the processes of abrasion and solution. Look at the following pairs of words. Which pair gives the alternative words that are used to describe the two processes in the order of abrasion and solution?
    When processes have alternative names, it pays to remember them both
  9. Which of the following is part of a waterfall?
    This is the deeper area at the base of the waterfall that has been created by the higher speed of the water at the bottom of the waterfall
  10. Waterfalls are often created where there is a resistant band of rock that has softer, less resistant rocks above and below. The rocks below are eroded creating an overhang. From time to time the overhang will collapse and so the waterfall gradually move further and further upstream. This is called:
    This happens only very slowly. Niagra Falls is a good example of waterfall retreat

Author: Kev Woodward

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