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Rivers

Rivers

imageThe water cycle

Understanding how the water cycle operates is key to understanding how rivers work. The water cycle is also known as the hydrological cycle. It is called a cycle because water continuously moves around the system. Rivers are part of this cycle. The illustration below shows how water changes state through the cycle. It can be a liquid, a vapour or a solid. How does the water cycle work? • Energy from the Sun heats the surface of the Earth. • Water is evaporated from oceans, rivers, lakes, etc. • The warm, moist air rises because it is less dense. • Condensation occurs when water vapour is turned back into water droplets as it cools down. Clouds are formed. • Precipitation occurs as water droplets get bigger and heavier they begin to fall as rain, snow and sleet, etc. When the precipitation reaches the surface, some falls directly into the sea but other water falls on land: • Some water is intercepted by vegetation. Some water may then slowly reach the ground. Some will evaporate from the surface of leaves or be taken up by the plant roots, and some of this water will eventually return to the air as vapour through the process of transpiration. This slows down or prevents some water flowing back to the river. • Some water flows across the surface of the ground – surface run-off. This happens when the surface doesn’t allow water to penetrate. Surface run-off is more likely to occur if the ground is saturated with water or when the rock is impermeable. This water moves quickly to the river. • Some water infiltrates into the soil. This through flow moves more slowly back to the river than surface run-off. • Some water percolates deeper into the ground and is slowly transferred back to the river or sea. Stores and transfers The major stores of water are the ocean, ice caps, land and the atmosphere. The movement of water between these stores is called transfers.

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Drainage basin

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A drainage basin – the area of land drained by a river and its tributaries (the catchment area or river basin) Key words: ​​- Watershed – area of highland that divides two drainage basins ​​- River – a stream of water flowing in a channel (high to low ground) ​​- Source – the beginning of a river. ​​- Tributary – a stream or river joining the main river. ​​- Mouth – the end of the river – usually the sea or a lake. The watershed This is the boundary between two rivers systems, marking the divide between drainage basins.

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River Processes There are 3 processes: Erosion, transportation and deposition Erosion

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Rivers erode the land in four ways: Hydraulic action – the force of the river on the beds and the banks. Corrasion / abrasion – stones and pebbles carried by the river wear away the channel like sandpaper) Corrosion – river water dissolves the chalk (limestone) in the banks and bed. Attrition – stones and pebbles carried by the river smash into each other and break up. They become rounder.

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Transportation The material carried by the river is called it load (just like a lorry). The load is transported in 4 different ways: Traction – rocks are rolled along the river bed. Saltation – stones and pebbles bounce along the bed. Suspension – fine particles of silt hang in the water. Solution – minerals dissolve in the river water.

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Deposition When a river slows down it loses energy and deposits its load. The heaviest material, rocks are deposited first and then the lightest, silt.

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River Valley (long profile) The long profile is a cross-section of a river from the source to the mouth. These profiles are divided into 3 stages upper middle and lower course stages

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River Valley (Cross-sections) The shape of the river valley changes along the course (from source to mouth). Erosion is the key process in the upper course whereas deposition is the key process in the lower course.

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Features found in each of the stages (courses) The upper course

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As the river cuts down it swings from side to side between alternate interlocking spurs. These are a series of ridges of land protruding alternately from either side of a valley, with the river winding between them.

imageRapids

image Like waterfalls rapids are formed as they flow over rocks of differing hardness. Softer rocks wear away more quickly, leaving a series of small steps formed by harder rocks. They vary in size and the amount of turbulence.

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Potholes These are circular depressions found on river beds. Potholes are formed by corrasion. Pebbles carried by the river are swirled around on the riverbed. This action erodes the rock on the riverbed forming potholes. Over time, they may widen and join with other potholes to form larger potholes, and the whole riverbed is deepened.

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Waterfalls Waterfall Formation – waterfalls form where the river bed has a layer of harder rock overlying the softer rocks. 1. As waterfalls the softer rock is eroded much more quickly than the harder rock; therefore undercutting the harder rock. 2. The hard, overhanging rock eventually collapses. 3. The collapsed rocks fall into the plunge pool causing more erosion of the soft rocks and a deeper plunge pool. 4. This goes on continuously causing the waterfall to retreat upstream. 5. The waterfall leaves a steep sided gorge as it retreats.

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Middle and lower course As the river comes out of the upper course so the gradient decreases. This has a major impact on the river, river valley and the type of landforms found in both the middle and lower courses. Here the river channel has become much wider and deeper as the channel has been eroded and the river has been fed by many tributaries upstream. Consequently, despite the more gentle gradient the velocity of flow may be as fast as in the uplands. As well as changes in the river channel, its surrounding valley has also become wider and flatter in cross-section with a more extensive floodplain. One of the most distinctive features of the river in the middle course is its increased sinuosity (not travelling in a straight line). Unlike the relatively straight channel of the upper course, in the middle course there are many meanders (bends) in the river.

Features

Meanders

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They form due to the greater volume of water carried by the river in lowland areas which results in lateral (sideways) erosion being more dominant than vertical erosion, causing the channel to cut into its banks forming meanders. Water flows fastest on the outer bend of the river where the channel is deeper and there is less friction. This is due to water being flung towards the outer bend as it flows around the meander, this causes greater erosion which deepens the channel, in turn the reduction in friction and increase in energy results in greater erosion. This lateral erosion results in undercutting of the river bank and the formation of a steep sided river cliff. In contrast, on the inner bend water is slow flowing, due to it being a low energy zone, deposition occurs resulting in a shallower channel. This increased friction further reduces the velocity (thus further reducing energy), encouraging further deposition. Over time a small beach of material builds up on the inner bend; this is called a slip-off slope.

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Ox-bow lake  : Over time meanders gradually change shape and migrate across the floodplain. As they do so meander bends becomes pronounced due to further lateral erosion and eventually an ox-bow lake may form. 1 As the outer banks of a meander continue to be eroded through processes such as hydraulic action the neck of the meander becomes narrow and narrower. 2 Eventually due to the narrowing of the neck, the two outer bends meet and the river cuts through the neck of the meander. The water now takes its shortest route rather than flowing around the bend. 3 Deposition gradually seals off the old meander bend forming a new straighter river channel. 4 Due to deposition the old meander bend is left isolated from the main channel as an ox-bow lake. 5 Over time this feature may fill up with sediment and may gradually dry up (except for periods of heavy rain). When the water dries up, the feature left behind is known as a meander scar.

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Lower course Floodplains

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As a river continues its journey towards the sea, the valley cross section continues to become wider and flatter with an extensive floodplain either side of the channel. The river erodes laterally and deposition also becomes important. By the time it reaches the lower course the river is wider and deeper and may contain a large amount of suspended sediment. When the river floods over the surrounding land it loses energy and deposition of its suspended load occurs. Regular flooding results in the building up of layers of nutrient rich alluvium which forms a flat and fertile floodplain.

Levees

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Flooding is also responsible for the deposition of alluvium across the flood plain, left behind once the floodwaters subside. Every time the river overflows, more silt and sand builds up on the flood plain. Material deposited during flooding can form natural embankments or levees on the banks either side of the river channel.

Estuaries and deltas Estuary – Where the river meets the sea it is the river’s mouth. Most rivers flow into the sea via an estuary. Water flows out to sea and back in twice a day via tides. Deposition is dominant, with fine material being deposited over a large area. Sometimes rivers form deltas where they enter the sea. This happens when the river slows down and it drops large amounts of sediment. If more sediment is deposited than the tide can remove, a delta will build. This may cause the river to split into smaller channels called distributaries. Types of delta – arcuate and birds-foot.

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2 Comments
  1. Isabel Steedman permalink

    It has every thing we have studied. It is so useful

  2. Ilaria Puckett permalink

    It is full of information. I learned a lot from it. There are no adverts, which is very good.

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