River Processes (DP IB Geography)
Revision Note
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
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
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
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