River Processes

Vertical Erosion

• Makes a river deeper
• Occurs in upper stages

Lateral Erosion

• Makes a river wider
• Happens in middle and lower courses

Hydraulic Action: Occurs where the pressure of the water breaks rock particles away from the bed and banks.

Abrasion: Eroded pieces of rock in the water scrape and rub against the bed and banks, removing material. Most erosion of river beds and banks happen by abrasion.

Attrition:Eroded rocks collide into each other and break into smaller fragments. Attrition doesn't erode the bed and banks - it only makes the pieces of rock smaller and more rounded.

Solution:Occurs where rocks dissolve into the water and are carried away. Carbone dioxide dissolves in the water to form a weak acid that reacts with limestone and chalk, breaking them down.

Solution: dissolved substances carried along in the water e.g. limestone that's dissolved in slightly acidic water

Suspension: very fine material such as clay and silt particles carried along in the water

Saltation:the force of the water causes larger particles such as pebbles or gravel to bounce along the river bed

Traction:very large particles such as boulders are pushed along the river bed by the force of the water

Material transported by traction or saltation is called the river's bedload.

Deposition occurs when the river loses energy. When it slows down, it loses energy and drops some of its load.

Ways in which speed and energy can be reduced:

• Reduced rainfall causing lower discharge
• Increased evaporation or abstraction causing lower discharge
• Friction e.g. shallow areas of the river where the water is close to the banks the river's speed reduces, reducing its energy
• When the river is forced to slow down e.g. before a narrow section in the channel
• Energy is lost when the river meets the sea - the sea absorbs the energy.

The capacity is the total load (measured in volume, weight or mass) that a river can transport at a given point.

The load of a river can be divided into different categories according to particle size:

• Clay
• Silt
• Sand
• Gravel
• Pebbles
• Cobbles
• Boulders

The competence describes the maximum particle size that a river is capable of transporting at a given point

The Hjulstrom Curve shows the relationshiop between river velocity and competence. The competenece of a river is affected by the amount of energy it has - greater velocity = greater energy

The Hjulstrom Curve shows how erosion, deposition and transportation vary with river velocity. 

• The critical erosion velocity curve shows the minimum velocity needed for the river to erode and transport particles of different sizes
• The mean settling curve shows the velocities at which particles of different sizes are deposited i.e. the compentence of the river at different velocities.