River Landscapes & Physical Processes (Edexcel GCSE Geography A)

Weathering & Mass Movement in the River Landscape

River valley processes

• Fluvial and landscape processes shape the river and surrounding land in a drainage basin

• Landscape processes of weathering and mass movement shape the land surrounding the river channel 

• Fluvial processes shape the river's channel and landforms. They include:

  • Erosion 

  • Transportation 

  • Deposition

• Most (about 95%) of a river's energy is used to overcome friction

• There is more friction in the upper course of the river, where it is shallow and narrow

• The rest of the energy is used in erosion and transportation

• Energy in the river depends on the river discharge and velocity

• The greater the discharge and velocity, the more energy a river has

Weathering

• There are three types of weathering. These happen in-situ:

  • Physical: rock is broken down into smaller pieces. This occurs due to changes in temperature, such as freeze-thaw and exfoliation

  • Chemical: rocks disintegrate and dissolve in slightly acidic rainwater 

  • Biological: rocks are broken apart by the roots of plants 

Freeze-thaw weathering

Mass movement

• There are several types of mass movement 

• Mass movement is the downhill movement of material under the influence of gravity

• Throughflow and runoff, caused by heavy rain, can make cliffs unstable and increase the likelihood of mass movement

• Mass movement includes landslides, slumping and rockfalls

• The type of movement depends on:

  • The angle of the slope (the steeper the slope, the faster the movement)

  • Amount and type of vegetation

  • Water

  • Type and structure of the rock

  • Human activity

  • Climate

• In river valleys, there are two main types of mass movement:

  • Slide

  • Slumping 

Slide

• This is a movement of material [popover id="rkUxqCoGHPiMFx-K" label="'en masse'"] which remains together until hitting the bottom of a slope

• This often occurs along a flat surface

Slumping

• Slumps are usually found on weaker rock types (i.e. clay)

  • The slope become saturated and heavy after prolonged or heavy rainfall

• This type of mass movement is also known as a rotational slip

• Slumping involves a large area of land moving down the slope in one piece

  • Because of the way it slumps, it leaves behind a curved, indented surface

Factors affecting weathering and mass movement

• There are a range of factors which affect weathering and mass movement

• These factors are outlined in the table below

Factors Affecting Weathering and Mass Movement

Erosion, Transportation & Deposition in Rivers

Erosion

• Erosion is the wearing away of surfaces 

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

  • Hydraulic action

  • Abrasion 

  • Attrition

  • Corrosion (solution) 

Types of erosion

• Erosion can be mainly vertical or lateral 

  • Vertical erosion is dominant in the upper courses of rivers. It increases the depth of the river and valley as the river erodes downward

  • 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 

  • Saltation

  • Suspension

  • Solution

Processes of Transportation

Deposition 

• When a river does not have enough energy to carry materials, it drops them. This is deposition

• The causes of reduced energy include the following:

  • Reduced discharge due to a lack of precipitation or abstraction upstream

  • Decreased gradient

  • Slower flow on the inside of a river bend

  • 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

Factors Affecting River Processes