Geography Essay
Rivers are complicated systems with many human and physical variables. Fluvial landforms tend to be formed by either erosional or depositional processes. These vary depending on which course of the river is examined. Deposition occurs when material is laid down due to a number of both physical and human factors. The landforms that are created are usually found in the middle or lower course of a river system.
The Mississippi river which runs through North America, and continues 4,070km from source to mouth. Here the river deposits much of its load forming a river delta. This type of feature is located in the lower course of the river, where it enters the sea. Deltas are usually composed of fine sediment which is deposited when a river loses energy. This occurs when it flows into an area of slow moving water such as a lake or the sea. The finest materials such as clay are carried furthest by the process of suspension and form the bottomset beds, which are then covered by a layer of coarser materials usually silts and clays. These are deposited to form a slope and create to foreset beds. Lastly the upper layers nearest to the land, are made of even coarser materials such as sand and silt. There are typically 3 formations of river deltas:
Arcuate – a rounded outer margin. e.g Nile
Cuspate – material spread out evenly. e.g Tiber
Bird’s foot – many distributaries inbetween sediment islands. e.g Mississippi
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One factor responsible for the formation of river deltas is flocculation. This is when fresh water from the river meets salt water from the sea. Here the combination of different water produces an electric charge which causes particles such as clay to change to a solid or semi-solid state and settle on the sea bed. This can be found in the Mississippi river delta.
Secondly, natural levees can be found in the lower course of the river. One example can be found in the Mississippi river. These are formed when a river overflows its banks, due to the friction from the floodplain causing the river to deposit material. The coarsest material such as sand is deposited closer to the river channel, forming a small natural embankment alongside the river channel. Then finer materials are deposited furthest away from the channel, typically material such as clay which is carried by suspension is deposited further into the floodplain. The levee system along the Mississippi River has some of the longest individual levees in the world. One of these levees stretches south along the river from Pine Bluff, Arkansas, for an entire 611 kilometres (380 miles).
Over time this process is repeated and the river will build up natural levees, however they are often artificially strengthened and heightened in an attempt to contain the river, especially in a time of a flood. Usually the river banks are laid with concrete so that the river will not overflow these artificial levees and civilization is protected from a flood. Although this particular type of management is long-term and costs a lot of time and money, therefore it is more common in MEDC’s.
Thirdly, braided channels can be found in most rivers a straight course of a river. One example of a braided channel is located in the river Conwy, although another more renown example can be seen in the river Rakaia, New Zealand. These are formed due to the river carrying a very high load in relation to their velocity. This means the the rivers energy will fall and so it cannot transport the heavy load of material and so it is deposited. It is this that causes the formation of braided channels and sediment is deposited often forming islands or eyots in the middle of the river channel. In the river Conwy river braiding can be seen approximately 30km from the river mouth and some islands have been inhabited by vegetation, making them a permanent feature. Often these eyots are temporary as material can easily be picked up and transported elsewhere during a time of a flood, where the rivers energy and load capacity is higher. Sometimes rivers can change from meandering channels to braided channels but also from braided channels to meandering channels, so this landform is constantly changing.
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Lastly, meanders are also a landform formed by deposition, as well as erosion. One example of a meander can also be found in the river Conwy. The rivers thalweg travels on the outside of the river and so erosion takes place on the outside of the bend, creating a river cliff. This means that the rivers slowest flow travels on the inside of as bend where its energy is minimal, causing deposition to take place, as is does not have the energy to carry the sediment any longer. This process causes the formation of a point bar (slip-off slope) Furthermore the pool and riffle theory support this, as riffles are formed in the centre of the river channel inbetween bend in the river, where friction is highest and deposition is more likely to occur. In addition, pools are found on the outside bend where the river has higher velocity and erosional capacity. Consequently the rivers thalweg flows around the riffles and into the pools where its velocity is increased, creating the sinuous nature of a meander. Eventually these processes (hydraulic action, abrasion, corrosion) change the shape of the river into the s-shape river channel. This s-shape bend in the river is known as a meander, which relies on erosional and depositional processes to form and can lead to the formation of ox-bow lakes in times of high river discharge.
Overall there are many factors responsible for the formation of depositional landforms, and that these can be both physical and human. One example of this is found when examining levees and that they are often strengthened or made artificially to prevent casualties in cities during times of high river discharge. Moreover, dams are another human feature that influence the development of depositional features. Dams store water creating a change in a river base level and so as the water has a controlled speed it losing velocity and the capacity to carry sediment, often leading to the formation of reservoirs. The change in base level will mean that the river will regrade its longitudinal stream profile to achieve that curve structure, which will lead to more erosional an depositional features along the way. Finally, climate caused by human activity (pollution, global warming etc)will affect deposition as it will cause a rise in the sea level and a change in many rivers base levels. This will lead to erosion and deposition taking place at a faster rate due to the base level being significantly closer to the source of a river. As well as many glaciers melting causing much higher discharge in some rivers and possibly the bursting and formation of river levees.
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