River Processes Revision Notes and Questions.

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River processes (Page 6-7)

Erosion can affect the length, Depth and Width of a river

Vertical erosion deepens river channels. It happens in the upper stages of a river.

Lateral erosion makes the river wide. This happens in the middle and lower stages of a river.

Headward erosion makes the river longer. It happens near a rivers source as throughflow and surface runoff causes erosion at the point the water enters the river channel.

  1. Hydraulic Action – the pressure of the water breaks rock particles away from the bed and the banks. Its strongest in rapids and waterfalls and during floods.
  2. Abrasion (Corrasion) – eroded pieces of rock in the water scrape and rub against banks, removing material. Most erosion occurs this way.
  3. Attrition – eroded rock smash into each other and break into smaller fragments. Their edges get rounded off as they rub together. Attrition doesn’t erode the bed and banks, just makes rock in river smaller and rounded.
  4. Cavitation – air bubbles in turbulent stretches of water implode causing shockwaves that break pieces off the banks and bed
  5. Corrosion (solution) - the dissolving of rock by chemical proces. Carbon dioxide dissolves in water to form a weak acid, which reacts with rock like limestone and chalk, breaking them down.


Deposition: process of dropping ERODED material

Occours when the river loses energy. When it slows down, it loses energy and drops some of its load.
The speed and energy of a river can be reduced by:
- Reduced rainfall causes lower discharge, which means the river slows down and has less energy.
- increased evaporation or abstraction (removal of water by humans )also causes lower discharge
- Friction, e.g. in shallow areas of a river and close to the banks, reduces the speed of the river, reducing its energy.
- When a river is forced to slow down e.g., before a narrow section of the channel it loses energy
- A lot of energy is lost when the river meets the sea (the sea absorbs the energy)

The capacity of a river is the total amount of material it can carry

  • Capacity is the total load (measured in volume, weight or mass) that a river can transport at a given point.
  • The load can be divided into different categories according to particle size. Sizes range from silt to big boulders
  • The competence describes the maximum particle size that a river is capable of transporting at a given point.

The Hjulstrom curve shows the link between River velocity and competence

Shows the relationship between river velocity and competence. It shows how the processed of erosion, deposition and transportation vary with river velocity.

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  • The critical erosion velocity curve on the graph shows the minimum velocity needed for the river to pick up (erode) and transport particles of different sizes (in suspension or as bedload). It takes a higher velocity to erode material than it does to just transport material
  • The mean settling velocity curve shows the velocities at which particles of different sizes are deposited.  i.e it shows the competence of the river at different velocities.

Exam Questions

  1. Describe the 5 ways in which river erosion happens.

Hydraulic action is a form of erosion. The sheer force of ...

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