Changing River Shape (AQA GCSE Geography)

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 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

Diagram of a typical 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

• Erosion is the wearing away of surfaces 
• There are four erosion processes which change the shape of the river channel:
  
  • Hydraulic action
  • Abrasion 
  • Attrition
  • Corrosion (solution) 

Processes 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

• Deposition is when a river does not have enough energy to carry its material and it drops it
• The causes 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 as the gradient is more gentle and the river flow is affected by tides
• The heaviest material is deposited first, this is known as the bedload
• Larger rocks are only moved short distances before being deposited 
  • This happens at times of high discharge and in the upper section of the river
• 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