The main processes of fluvial erosion occur throughout the course of the river. These are outlined below.
The river itself, however, will try to erode in different directions, depending on how far down the course you are. Very basically, rivers are trying to erode down to their base level. In most cases this is sea level, but it can also be the level of a lake that the river might be flowing into.
At the top of the river, near its source, the river has a huge amount of material to get through to reach base level, so it primarily cuts downwards, creating a steep-sided v-shaped valley.
In the mid-course of the river it continues to cut downwards but is also starting to cut sideways or laterally.
Once it has reached the lower course, and is nearing the sea, the river has almost reached its base level, so most of its erosive energy is concentrated on cutting laterally, creating features such as meanders.
The main processes of fluvial erosion are:
Abrasion: The erosion of the river bottom and the riverbank by material being carried by the river itself.
Attrition: The rocks and pebbles being carried by the river crash against each other, wearing them down to become smaller, rounded pebbles.
Corrasion: see Attrition.
Corrosion: The chemical erosion of the rocks of the riverbank by the slightly acidic water. This occurs in streams running through rocks such as chalk and limestone.
Hydraulic Action: The water forces air to be trapped and pressured into cracks in the rocks on the bank of the river. This constant pressure eventually causes the rocks to crack and break apart.
Once it has been eroded, material in the river is transported down the river. Whilst this is happening, erosion processes such as attrition and abrasion continue to occur. There are four main processes of fluvial transportation,depending on the size of the material being moved:
Traction: The largest rocks in the river are slowly rolled along the bottom of the river by the force of the water.This primarily occurs in the upper reaches of the river.
Saltation: Smaller rocks are bounced along the river bed. This occurs in the upper and middle sections of the river in general.
Suspension: The water carries smaller particles of material. This process occurs throughout the course of the river, but increases the closer you are to the mouth of the river.
Solution: Material is dissolved within the water and carried along by it. Primarily this occurs in the middle and lower reaches of the river.
Fluvial deposition occurs where the river losses energy and therefore cannot continue to carry the material it is transporting. This could happen in an estuary when the river meets the sea and slows down, depositing its load, which may eventually lead to the formation of salt marshes or a delta. Material is also deposited further up the course of the river. For instance the slower moving water on the inside of a bend of a river will have less energy and therefore drop its load, helping to create a meander.
A major depositional feature of a river is the flood plain, in its lower reaches. This is made up of deposited sand and silt, which is known as alluvium. This is often very fertile and is the reason why many areas near rivers have large amounts of agricultural activity.
|Upper Course||Mid Course||Lower Course|
|Erosion & Deposition||Primarily vertical erosion, through attrition, abrasion and hydraulic action. Large boulders deposited and eroded in situ.||Continues to cut vertically, but it also begins to cut laterally as it gets closer to base level. Deposition occurs in the slower moving insides of meanders.||Primarily cuts laterally as it has almost reached base level. The erosive energy of the river is almost totally concentrated on cutting sideways. Much deposition occurs.|
|Transportation||Traction and saltation.||Saltation, suspension and solution.||Mainly suspension and solution.|