River Processes (DP IB Geography)

River Erosion

• 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

Methods of erosion

Factors affecting rates of erosion

River Transportation

• There are five processes of transportation 

  • Suspension

  • Saltation

  • Traction

  • Solution

  • Floatation

• 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

River Deposition

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