River Processes

Rivers

Rivers shape the landscape over which they flow in three ways:

• Erosion: Wearing away the landscape

• Transportation: Moving the river's load (eroded material) downstream

• Deposition: Dropping the material How much a river erodes the landscape depends on a number of factors:

• Gradient: Steeper the slope, faster the river flows, more erosion

• Discharge: More water results in a faster river, a faster river results in more erosion and transportation

• Shape of the river channel: If the wetted perimeter is large then the river will be in more contact with the beds and banks. This friction will decrease the rate of flow of the river

• Roughness of the river channel: If the beds and banks are rough friction will increase and the rivers velocity will decrease

Processes of Fluvial Erosion

• Rivers can erode vertically down into the riverbed deepening the channel, laterally into the riverbank widening it or backwards, against the flow of the river through headward erosion
• All of this is carried out by the following processes.

Hydraulic Action:

• Force of moving water
• Active on beds and banks of rivers by dislodging material as it passes
• Higher velocity results in more erosion
• In some places the river will undercut the riverbank and bank caving will occur which can cause the river bank to collapse
• Cavitation can also occur as a result of hydraulic action where air bubbles get trapped in cracks in sediment and burst

Abrasion/Corrasion:

• When the load of a river hits off the bed and banks
• This smoothens the river channel which further increases the rivers erosive power
• Most active when the river is in flood and it is able to carry a larger load as the volume and velocity of water is greater

Solution/Corrosion

• The chemical break down of rocks
• Calcium Carbonate in Limestone is especially vulnerable to this type of erosion
• Dissolved minerals are carried away in solution

Attrition:

• Rounding of stones and pebbles carried by the river as they hit off each other and the bed and banks of the river
• This reduces the load to fine particles of alluvium

Processes of Transportation

Solution

• When chemicals in rocks are dissolved by the river, they are carried away in solution
• This type of transportation is not visible

Suspension

• Fine materials such as sand or silt are light enough to be carried along by the water
• They are 'suspended' in the water
• Rivers can take on a brown appearance, especially during times of flood – this is when suspension is greatest, and the river is carrying a large load of sand/silt

Saltation

• Also known as bouncing
• Larger particles such as pebbles are too heavy to be carried by the river for long periods of time
• They are picked up and carried for short periods of time, being dropped repeatedly
• They appear to almost 'bounce' along the river bed

Traction

• The largest stones and boulders that make up part of the rivers load are rolled and dragged along the riverbed by the force of moving water
• Most active during times of flood, as the river has more power due to increased velocity and discharge