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Sediment in a river comes from a variety of sources. It may be from outside the river (exogenetic) that includes, mass movement, rill and gully erosion and sheet wash. Or from within the rivers channel itself (endogenetic) that could be material from the stream bed and banks, which is influenced by the power of erosion and the resistance of material to erosion.
Critical erosion velocity (the Hjulstrom curve)
Rivers either transport, erode, or deposit sediment (load). The relationship between the size of particles (competence) and water velocity is shown on the graph below:
You should note the following:
- Sand is easily transported at lower velocities.
- More velocity is needed to pick up material than to carry it in suspension.
- In times of highest discharge, velocity increases, as does erosion.
- The division between Transportation and deposition is small. This means that only a small decrease in velocity leads to sedimentation.
- Competence is the maximum size of material a river can transport.
- Capacity is the total load actually transported.
Suspended sediment load:
This is carried with the body of the current. It can consist of suspended bed material, which is fine to medium sands, which have come from the riverbed. The material such as silts and clays is light and can be held in suspension.
Can be either exogenetic or endogenetic, and moves by sliding, saltating, or rolling. It is larger than suspended sediment load.
This is held in solution and can come from erosion, pollution, mineral springs and chemical weathering.
Four main types exist:
When a river picks up material and then rubs it against its bed and banks. Erosion occurs by the process of abrasion, and is most effective during times of flood. It is the main method of both vertical and horizontal erosion.
Boulders collide with one another as they move down the river, and can break into smaller pieces. Over time rocks become more rounded in appearance.
3. Hydraulic action
The force of water that hits river banks, and then pushes water into cracks. Air becomes compressed, pressure increased and the riverbank may, in time collapse.
A continual process, which is as a result of the chemical composition of the water.
Once friction has been overcome, transportation of material in a river begins. There are three main processes of transportation:
Very small particles of clay and silt are carried in suspension. The larger the amount of turbulence and velocity the more particles that can be picked up. Material held in suspension usually accounts for the greatest part of the load of a river.
Water in a river, contains acids. Where the bedrock is easily dissolved running water will remove material in solution. It is a minor process except in areas of limestone.
If particles are very large and not picked up by the current they will move in one of two ways:
Saltation: pebbles sand and gravel are lifted up by the current and bounced along the bed.
Traction: larger boulders rolling or sliding along the riverbed. Only experienced in times of great flood.
This happens when the velocity of a river is reduced. Then the main factors leading to deposition are shown in the table below:
|A||Low rainfall reducing precipitation|
|B||A river entering the sea or a lake, reducing velocity|
|C||Water becoming shallower|
|D||Increase in load|
|E||River overflows its banks, depositing material on the flood plain|