Rivers are dynamic systems and they have shaped the UK in many ways over thousands of years. There are many processes operating in different parts of rivers. These processes are known as Fluvial processes which relate to erosion, transport and deposition by a river.
Erosion is the wearing away of the land by moving forces - in this case the river.
Hydraulic action - where the sheer force of the water erodes the stones, bed and banks of the river
Abrasion- where stones in transport are thrown into the bed and the banks eroding them
Corrosion - where weak acids within the water react with the rocks and bed and bank of the river
Attrition - where stones in transport are thrown into one another.
In the upper parts of rivers the water tend to erode vertically downwards into the river bed, whereas as we progress downstream this changes to lateral erosion, or a side to side erosion of the river valley.
Transport is the movement of material, in this case by river water. Processes include;
• Solution - where material is dissolved within the water
• Suspension - where small particles are held up or suspended in the water
• Saltation - the bouncing motion of larger particles along the river bed
• Traction - the rolling motion of sediment along the bed - normally much larger sediment.
Obviously the larger sediments will only be moved during periods of high river flow.
Deposition - this is the laying down of sediment in the river channel or on floodplains. This occurs when river velocities slow within the channel, or when velocities slow over floodplains or when the river enters a sea or lake. Velocities slow because the river might widen and become shallower, increasing the friction between the water and the river bed, or variations in channel shape leave some parts shallower and therefore slower than others. When rivers enter the sea they tend to spread out and counter currents from the tidal motion of the sea slow river water and encourage sediment to be dropped.