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 fo energy avaliable. This is governed by the velocity of the flow and the amount of water flowing within the channel (discharge).
Erosion – theory
River erode because they process energy. Their total energy depends on:
· the weight of the water – the greater the mass of water the more energy it will possess due to the influence of gravity on its movement
· the height of the river above its base level (usually sea level) – this gives it a source of potential energy, and the higher the source of the river the more such energy it
· the steepness of the channel – this controls the speed of the river which determines how much kinetic energy it has
Much kinetic 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 river contains large quantities of water following heavy rain or snowmelt, it does possess the energy to perform great amounts of erosion.
Erosion – the processes
The scraping, scouring and rubbing action of materials carried along by a river (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.
This 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.
This is most active on rocks that contain carbonates, such as limestone and chalk. The minerals in the rock are dissolved by weak acids in the river water and carried away in solution.
This is the reduction in the size of fragments and particles within a river due to the processes describe above. The fragements strike one another as well as the river bed. They therefore become smoother, smaller and more rounded as they move along the river channel.
River energy not used for erosion is not lost through friction can be used to transport a river’s load. A river obtains its load from two main source:
· 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 load in four main ways (see diagram below)
· Traction– large stones and boulders are rolled along the river bed by water moving downstream. This mainly happens during periods of high discharge and consequently high energy levels
· Saltation– small stones bounce or leap-frog along the channel bed. This process is associated with relatively high energy conditions. Small particles may be thrust 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 movement 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 but it is also picked up, mainly through the
turbulence that exists within the river. Suspension normally contributes the largest proportion of sediment to the load of the rover. The suspended load is the main cause of the brown appearance of many rivers and streams
· Solution– dissolved minerals are transported with the mass of moving water
Two other terms are 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
The capacity of the river is a measure of the amount of material it can carry, that is, the total volume of the load.
The competence of a river is the diameter of the largest particles that it can carry for a given velocity. The faster flowing a river is the greater the turbulence and therefore the river is better able to life particles from the river bed.
A river deposits when there is a decrease in its level of energy, it is no longer competent to transprt 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 to a tributary bringing in larger particles, increased erosion along the river’s course, or a landslide
into the river
· The river floods and 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 successivelt smaller particles, although the finest particels may never be deposited. This pattern of depostion is reflected in the sediments found along the course of a river. The channel of upland rivers are often filled with large boulders. Gravels, sands and siults can be carried further and are often depoisted further downstream. Sands and silts are deposited on the flat floodplains either side of the river in its lower course.