Fluvial Processes (Edexcel IGCSE Geography)

Weathering & mass movement

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 the land surrounding the river channel 

• Fluvial processes shape the river channels and the landforms linked to them. They include:

  • Erosion 

  • Transportation 

  • Deposition

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 weathering

• Rocks are broken apart by the roots of plants 

Mass movement

• There are several types of mass movement where large-scale movement of material occurs

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

  • Slumping occurs when the slope is eroded by the river.

    • This undercuts the slope causing large-scale movement of material down the slope

    • Slumping often occurs when a softer less resistant material overlies a harder more resistant material

  • Soil creep happens when the influence of gravity causes weathered material to slowly move down the slope towards the river

Factors affecting weathering and mass movement

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

  • climate - in hot, wet climates, chemical and biological weathering are dominant

  • weather - heavy rainfall increases mass movement

  • slope - any slope over 5^o experiences mass movement

    • The steeper the gradient of the slope, the more mass movement there will be

  • geology - different rocks have different levels of resistance to weathering

  • altitude - at higher altitudes, freeze-thaw weathering occurs frequently due to the low temperatures

  • aspect - physical weathering is more common on a colder, north-facing slope due to a higher likelihood of freeze-thaw

  • vegetation - roots bind the soil together which limits mass movement

The process of erosion

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

  • There is more friction in the upper course of the river

    • It is shallow and narrow so more water is in contact with the bed and banks

    • The rocks in the river are larger so more energy is used to overcome the friction to move over and around them

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

• Energy in the river depends on the river's:

  • discharge

  • velocity

• The greater the discharge and velocity the more energy a river has for erosion and transportation

Erosion

• Erosion is the wearing away and movement of material

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

  • Hydraulic action is when the force of the water removes material from the banks and bed of the river

  • Abrasion (corrosion) occurs when materials carried by the river scrape away at the banks and bed 

  • Attrition is when the material being carried by the river hits each other, as a result, the pieces become rounder and smoother

  • Corrosion (solution) occurs when some rocks are dissolved by the slightly acidic water

• 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

Processes of transportation & deposition

Transportation processes

• There are four processes of transportation:

  • Traction occurs when larger rocks and materials are rolled along the riverbed 

  • Saltation is when smaller material is lifted by the water and bounces along the riverbed

  • Suspension occurs when lighter material is carried within the river flow

  • Solution is when materials are dissolved in the water

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

Factors affecting processes

• There are several factors which affect the fluvial and landscape processes including:

  • climate

  • slope

  • geology

  • altitude

  • aspect

Climate

• Heavy rainfall and/or low temperatures lead to higher discharge which increases erosion and transportation

• Below average rainfall and/or high temperatures lead to lower discharge and decreased erosion and transportation

Slope

• Rivers on a steep slope will be fast flowing and there will be increased erosion

• Gentle slopes will result in more deposition

Geology

• Softer, less resistant rocks erode more rapidly than harder, more resistant rocks

Altitude

• Melting snow and ice increase discharge and therefore there is more erosion and transportation

Aspect

• South-facing slopes have higher rates of evaporation and transpiration which decreases discharge