River Processes (Cambridge International IGCSE Geography)

Spec point — 2.2.3 · Demonstrate an understanding of the work of a river in eroding, transporting and depositing

River Processes

Erosion

• Erosion is the wearing down of surfaces

• There are four erosion processes that change the shape of the river channel:

  • Hydraulic action

    • The force of the water, which removes material from the bed and banks of the river

  • Abrasion

    • When the materials carried by the river scrape away the banks and bed

  • Attrition

    • When the material being carried by the river hits each other, the pieces become rounder and smaller

  • Corrosion (solution)

    • When rocks are dissolved in slightly acidic water

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 courses of rivers. It increases the width of the river and valley as it erodes sideways

Transportation

• There are four processes of transportation:

  • Traction

    • Occurs when larger rocks and materials are rolled along the riverbed

  • Saltation

    • Smaller material, which can be lifted by the water, bounces along the riverbed

  • Suspension

    • Lighter material carried within the river flow

  • Solution

    • When materials are dissolved in the river water

Exam Hint 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 downstream

• 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 marks]

Final answer: Answer

• Hydraulic action [1] The 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]

• 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 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 steep areas 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 Hint Remember, valley shape and river shape are not the same thing. If you are asked to describe a valley shape, you should focus on the gradient and shape of the land on either side of the river channel.