River Processes
- Erosion is the wearing down of surfaces
- There are four erosion processes which change the shape of the river channel:
- Hydraulic action
- Abrasion
- Attrition
- Corrosion (solution)
Types of Erosion
- Erosion can be mainly vertical or lateral:
- Vertical erosion is dominant in the upper course of rivers. It increases the depth of the river and valley, as the river erodes downwards
- Lateral erosion is dominant in the middle and lower course of rivers. It increases the width of the river and valley as it erodes sideways
- There are four processes of transportation:
- Traction
- Saltation
- Suspension
- Solution
Transportation Processes
Exam Tip
It can sometimes help to remember a word and the process it refers to if you know what the word means.
Traction - the action of pulling something over a surface
Saltation - leaping or jumping
Deposition
- When a river does not have enough energy to carry materials it drops them. This is deposition
- The causes of reduced energy include:
- Reduced discharge due to a lack of precipitation or abstraction upstream
- Decreased gradient
- Slower flow on the inside of a river bend or where the river is shallower
- When the river enters a sea/ocean or lake
- The heaviest material is deposited first, this is known as the bedload
- The lighter materials, gravel, sand and silt are known as alluvium and they are carried further downstream
- The dissolved materials are carried out to sea
- As a result of erosion, transportation and deposition the character of a river changes as it moves down stream
- These changes are summarised in the Bradshaw model
The Bradshaw Model
Worked example
Study Fig 1, which is a photography of a river in an upland area.
Explain how the river in Fig 1 is likely to carry out erosion.
[5]
- Answer:
- Hydraulic action [1] Power of water wears away bed and banks releasing air compressed in
cracks [1]
Abrasion [1] Material carried by river grinds bed and banks/sandpaper action [1]
Corrosion / solution [1] Rocks dissolved by chemical reactions [1]
Attrition [1] Load reduced in size as particles hit each other when being carried by
water [1]
Vertical erosion [1]
- Hydraulic action [1] Power of water wears away bed and banks releasing air compressed in
- Remember you will receive 1 mark for the type of erosion and the second mark for explaining how the erosion process works.
River characteristics
- All rivers have a long and cross profiles
- Each river's long and cross profiles are unique but they do have some characteristics in common
- These profiles show changes in river characteristics from the source to the mouth
Long profile
- The long profile of a river shows the changes in the river gradient from the source to the mouth
- Most long profiles have a concave shape with similar characteristics:
- The source is usually in an upland area
- The upper course of the river includes areas which are steep with uneven surfaces
- In the middle course the gradient decreases
- In the lower section the gradient decreases further until it becomes almost flat
Long Profile
Cross profiles
- The cross profiles of a river are cross-sections from one bank to another
- Cross profiles of the upper, middle and lower courses show the changes in the river channel
- Upper course characteristics include:
- Shallow
- Steep valley sides
- Narrow
- Low velocity
- Large bedload
- Rough channel bed
- High levels of friction
- Vertical erosion
- Middle course characteristics:
- Deeper than upper course channel
- Gentle valley sides
- Wider than upper course channel
- Greater velocity than upper course channel
- Material in river decreases in size
- Smoother channel bed
- Lower levels of friction than upper course channel
- Lateral erosion
- Lower course characteristics:
- Deeper than middle course channel
- Flat floodplains
- Wider than middle course channel
- Greater velocity than the middle course channel (apart from as the river enters the mouth)
- Material carried mainly sediment and alluvium
- Smooth channel bed
- Lowest friction
- Deposition is dominant
Exam Tip
Remember valley shape and river shape are not the same thing. If you are asked to describe valley shape you should focus on the gradient and shape of the land either side of the river channel.
