2.2.3 River Processes

River Processes

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

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

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