River Processes (SL IB Geography)

• River erosion is the wearing away of land as water flows past the bed and banks
• Erosion by rivers also provides material, which is carried down the river
• A river channel is eroded by:
  • Vertical erosion – the cutting down of the river into the bed and deepening the channel
  • Lateral erosion – the cutting in of the river to the bank and widening the channel

Four Methods of River Erosion

Erosional term	Definition
Hydraulic action	The force of the air and water into cracks in the river banks and beds
Attrition	The wearing away of the load as rocks knock against each other, making them smaller and rounder
Abrasion or corrasion	The scraping away of the bed and banks by material transported by the river
Solution or corrosion	Chemicals in the river dissolve minerals in the rocks within the bed and bank, carrying them away in solution

Methods of erosion

Methods of erosion

Factors affecting rates of erosion

Factors affecting the rate of erosion

• There are five processes of transportation 
  • Suspension
  • Saltation
  • Traction
  • Solution
  • Floatation

Types of Transportation

• The load of a river changes with discharge and velocity
• Stream capacity is the maximum amount of debris a stream can carry.
• Competence is the diameter of the largest particle carried
• Critical erosion velocity is the minimum velocity to move grains of a specific size
• The relationship between velocity and discharge is illustrated by a Hjulström curve
• There are three important features of the Hjulström curve:
  • High velocities are required to lift particles
  • Higher velocities are needed for entrainment compared to transport
  • When velocity falls below a certain level (settling velocity) particles are deposited

The Hjulström curve

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The Hjulström curve illustrates the relationship between velocity and efficiency. It shows the velocities at which sediment will normally be eroded, transported or deposited

Deposition

• Deposition can take place for a number of reasons:
• A decrease in the volume of water
• A shallow gradient that decreases velocity 
• An increase in friction between water and the channel

Braided channels 

• Braided channels exhibit dynamic river environments
• Common in glacial rivers and arid regions
• Experience rapid changes in discharge, flow velocity and sediment processes
• Arid areas face flash floods, which cause erosion and sediment transport
• Glacial regions undergo surges and diurnal flow variations
• During peak flow, the erosion and transport processes dominate
• River braids merge and deposits are eroded
• Slower flow periods emphasise deposition
• The river divides into smaller braids.

Seasonality

• River processes vary seasonally
• Monsoonal rivers erode and transport more sediment during the wet season
• Some rivers experience high spring flow due to snowmelt, such as those in Iceland