The work of a river involves three main processes: erosion, transportation and deposition. At any one time the dominant process operating within the river depends on the amount of energy available. This is governed by the velocity of the flow and the amount of water flowing within the channel (discharge).
Rivers erode because they posses energy. Their total energy depends on:
- the weight of the water - the greater the mass of the water the more energy it will posses due to the influence of gravity on its movement.
- the height of the river above its base level - this gives it a source of potential energy, and the higher the source of the river the more such energy it has.
- the steepness of the channel - this controls the speed of the river which determines how much kinetic energy it has.
Erosional Energy Loss
Much of the for mentioned energy is lost through friction, either internally through turbulence within the flow of the river, or externally through contact with the bed and banks of the river channel. Energy loss through friction can be great in all parts of a river, but it is perhaps easier to understand in the context of an upland river channel. Here the channel often has a rocky bed with many large boulders. The rough shape of the channel means that the wetted perimeter - the overall length of the bed and banks that the river is in contact with, is large. More energy is thus lost through friction, and the river's velocity, and therefore its energy level, is reduced. Hence, in normal conditions, the river is unable to perform much erosion. However, when the rain contains large quantities of water following heavy rain or snowmelt, it does possess the energy to perform great amounts of erosion.
The 4 Main Processes of River Erosion
- Abrasion (corrosion) - is the scraping, scouring and rubbing action of material carried along by a river (the load). Rivers carry rock fragments in the flow of the water or drag them along the bed, and in doing so wear away the banks and bed of the river channel. Abrasion is most effective in short turbulent periods when the river is at bankfull or in flood. During times when river levels are low, the load consists of small particles, such as sand grains, and these tend to smooth the surface of the river channel.
- Hydraulic action - is caused by the sheer power of moving water. It is the movement of loose unconsolidated material due to the frictional drag of the moving water on sediment lying on the channel bed. As velocity increases, turbulent flow lifts a larger number of grains, particularly sand sized particles, from the floor of the channel. Hydraulic action is particularly effective removing loose material in the banks of meanders, which can lead to under-cutting and collapse. It can be locally strong within rapids or below waterfalls where it may cause the rocks to fragment along joints and bedding planes or other lines of weakness.
The 4 Main Processes of River Erosion Continued...
- Corrosion - the most active on rocks that contain carbonates, such as limestone and chalk. The minerals in rocks are dissolved by weak acids in the river water and carried away in solution.
- Attrition - is the reduction in the size of fragments and particles within a river due to the processes described above and before. The fragments strike one another as well as the river bed. They therefore become smoother, smaller and more rounded as they move along the river channel. Consequently larger, more angular fragments tend to be found upstream while smaller, more rounded fragments are found downstream.
In the upper reaches of a river, where the land lies high above sea level, river erosion is predominantly vertical. Vertical erosion dominates because the river is attempting to cut down to its base level, which is usually sea level. In times of spate, when the river level and velocity are high, the river cuts down into its valley mainly by abrasion and hydraulic action. Such rivers often produce steep-sided valleys.
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Lateral erosion occurs more frequently in the middle and lower stretches of the river, where the valley floor lies closer to sea level. Here the river possesses a great deal of energy, particularly when close to bankfull. However, energy is used laterally to widen the valley as the river meanders. The strongest current is found on the outside of the bend and hydraulic action causes the bank to be undermined and to collapse.
River energy not used for erosion or not lost through friction can be used to transport the river's load. A river obtains its load from two main sources:
- material that has been washed, or has fallen, into the river from the valley sides.
- material that has been eroded by the river itself from the bed or banks.
A river transports its loads in 4 main ways:
- Traction - large stones & boulders are rolled along the river bed by water moving downstream. This process operates only at times of high discharge (and high energy levels). During the Boscastle floods of August 2004, when the river Valency burst its banks, large boulders transported from further upstream contributed significantly to the damage of the town.
- Solution - dissolved minerals are transported within the mass of the moving water.
Transportation - 2
- Saltation - small stones bounce or leap-frog along the channel bed. This process is associated with relatively high energy conditions. Small particles may be trust up from the bed of the river only to fall back to the bottom again further downstream. As these particles land they in turn dislodge other particles upwards, causing more such bouncing movements to take place.
- Suspension - very small particles of sand and silt are carried along by the flow of the river. Such material is not only carried by is also picked up, mainly through the turbulence that exists within water. Suspension normally contributes the largest proportion of sediment to the load of the river. The suspended load is the main cause of the brown appearance of namny river and streams.
Two other terms often used in the context of river transport - capacity and competence. Both of these are influenced by the velocity, and therefore the discharge of the river.
Transportation - 3
The capacity of a river is the measure of the amount of material it can carry, that is, the total volume of the load. Research has found that a river's capacity increases according to the third power of its velocity. For example, if a river's velocity doubles, that its capacity increases eight times. The competence of a river is the diameter of the largest particle that can carry for a given velocity. Again, research has shown that a river's comptence increases according to the sixth power of its velocity. For example, if a rivers velocity doubles, then its comptence increases 64times. This is because fast-flowing rivers have greater turbulence and are therefore better able to lift particles from the river bed.
A river deposits when, owning to a decrease in its level of energy, it is no longer competent to transport its load. Deposition usually occurs when:
- there is a reduction in the gradient of the river, for example when it enters a lake.
- the discharge is reduced, such as during and after a dry spell of weather.
- there is shallow water, for example on the inside of a meander.
- there is an increase in the calibre (size) of the load. This may be due toa tributary bringing in larger particles, increased erosion along the river's course, or a landslide into the river.
- the river over flows or overtops its banks, resulting in a reduced velocity on the floodplain outside the main channel.
In general, the largest fragments are the first to be deposited, followed by successively smaller particles, although the finest particles may never be deposited. This pattern of deposition is reflected in the sediments found along the course of a river. The channels of upland rivers are often filled with large boulders. Gravel, sands and silts can be carried further and are often deposited further downstream. Sands & silts are deposited on the flat floodplains either side of the river in its lower course.