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Theoretical Background Essay Sample

Theoretical Background Pages
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The source of the river is normally found in an area of high ground. The river usually flows through a steep V-shaped valley. Most of the erosion that occurs in the upper course is vertical and as the river bed is eroded lower the valley becomes steeper. The river is narrow with a shallow channel. As discharge is calculated by measuring the cross sectional area of the channel multiplied by the velocity, the discharge is small. The river is mainly slow flowing as most of its energy is used overcoming the friction of the large angular bedload. The bedload is large and angular as it hasn’t been subjected too much erosion. Any changes to the direction of the river are a result of interlocking spurs as the water wants to follow the path of least resistance. The main feature seen in the upper course is the waterfall. Waterfall

This forms when a layer of harder rock lies over a layer of softer rock. The softer rock erodes quicker and a step forms. Hydraulic action and abrasion deepens the step and a waterfall forms. The erosion makes a plunge pool and the hard rock hangs over the plunge pool. When the hard rock becomes unstable it falls and the waterfall moves backwards. This process repeated can lead to a gorge forming. Middle course

The middle course usually has a gentle gradient and is in a sloping valley. The river is wider and has a deeper channel as more lateral erosion has occurred. The velocity is greater than that of the upper course as there is a greater volume of water in the river (because of tributaries flowing into the river and surface run off) and less energy is used overcoming the lesser friction of the smoother less angular bedload. There is more transportation of material, and more suspended sediment. The bedload is smaller and more rounded as it has been subjected to more erosion (namely attrition or corrosion). The main feature seen is the meander. Meander

These are bends that form when the slope of a river evens out. They are the result in differences in velocity across the river channel. Where the water flows faster it spirals downwards which causes more vertical erosion deepening that side of the channel and creating a river cliff. On the other side of the channel the river flows much slower and does have enough energy to transport material so deposits material starting with the heaviest first. There is also a slip off slope leading to a lop sided cross section. Lower course

The lower course is usually flat and wide with a floodplain. Most deposition occurs here. The channel is very wide and very deep as this is the most efficient way for the large volume of water now in the river to travel(as it offers the least resistance) and the force of hydraulic action is now quite large. The bedload is very small and rounded as by now it has been subjected to a lot of erosion .this is the section of the river where the river typically flows the fastest as there is a large volume of water and minimal friction caused by the bedload. Most of the material being transported is in solution. The main feature is the floodplain and levees. Floodplains and Levees.

These are formed by deposition during time of flood. The rivers load is composed of different sized particles. When a river floods it deposits its heaviest material first. The largest material forms levees after a number of floods. The floodplain usually has very fertile ground due to the alluvium deposited. Processes.

Erosion
Erosion occurs most in the upper and middle sections of a river. It occurs in four ways: Attrition-pieces of rock hit against each other and edges break off leading to smaller and more rounded bedload when the rocks break apart. Hydraulic action-this is the force of the water compressing air in cracks in rocks in the rivers banks and bed.it can undermine the river cliff in a meander causing it to collapse. Abrasion-this is the scraping of the load the river is carrying against the bed or banks of the river. This causes the river to widen and deepen.it is most powerful during times of flood when large rocks are grinding along the river bed. Corrosion-the water dissolves minerals in the load.

Transportation
Transportation occurs most in the middle and lower courses of a river. It also occurs in four ways: Traction-the heaviest material rolling along the bed of the river. Usually only occurs at times of high discharge e.g. floods. Saltation- medium sized pebbles bouncing along the river bed Suspension- very small particles are carried along in the water. Solution- chemicals are dissolved in the water. This may colour the water. Deposition

This happens when the river slows down or does not have enough energy to carry its load and drops its heaviest material first. Found mainly in the lower course. It is most likely when a river reaches a lake or sea.

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