Edexcel IGCSE Geography 复习笔记 1.2.1 Fluvial Processes

River valley processes

• Both fluvial and landscape processes shape the river and surrounding land in the drainage basin
• The landscape processes of weathering and mass movement shape land surrounding the river channel
• Fluvial processes shape the river channels and the landforms linked to them. They include:
  • Erosion 
  • Transportation
  • Deposition
• Most (about 95%) of a river's energy is used in overcoming 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

Weathering and mass movement

• 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
• There are a number of types of mass movement where large scale movement of material occurs
• In river valleys there are two main types of mass movement:
  • Slumping - where the slope is eroded by the river. This undercuts the slope causing large scale movement of material down the slope
  • Soil creep - the influence of gravity causes weathered material to slowly move down the slope towards the river

Factors affecting weathering and mass movement

• 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)

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

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:
  • 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