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IGCSE Geograph­y: River processe­s and landform­s

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Swiss International School, Zürich - SIS

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7,2014

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IGCSE Geography

River processes and landforms


Introduction

This essay is about how river processes shape landforms at different stages as the river moves from its source to its mouth. A river transports, erodes, or deposits sediment (load).


River processes – erosion, transportation and deposition

1) Erosion involves the wearing away of rock and soil from the river bed and also banks. This way the river can get deeper and/or wider.

There exist four main types of Erosion. Corrasion (abrasion) describes the process when a river picks up material (sand and pebbles) and then rubs it against its bed and banks wearing it away. Second is called attrition. Here boulders collide with one another moving down the river and break into smaller pieces. Over time rocks become more round. In the third process, called hydraulic action, the power of water hits river banks, and then pushes water into cracks. Air becomes compressed; pressure increase and the riverbank may collapse. The last process includes solution. This continual process is a result of the chemical composition of the water.






















Fig. 5.1

2) Transportation

The eroded material becomes than the river’s load which will be transported by the river energy (also called velocity). The faster the river is flowing the more and heavier load can be transported.

There are a three different ways of transportation.

Suspension means that, tiny particles are carried in suspension down the river stream. Solution describes invisible acidic chemicals, which are dissolved in the water. This solution can react with the bed rock. Easy dissolving bed rocks are called soluble or soft rocks. There exist also hard rocks which can not be dissolved. By saltation, pebbles and stones are lifted up by the current and drawn in a skipping motion along the bed. If the load is very large (boulders) and can not be picked up by the rivers flow it will roll or slide along the river bed. This is called traction. Additionally the more volume (water) is in the river, the greater the load.























Fig. 5.2


3) Deposition occurs when a river loses energy. Than it is forced to deposit its load. The bedload involves the heavy material which is carried along the bottom of the river. There are several reasons why deposition happens: Decrease in gradient, decrease in river flow (water drains away after rain), meet of river and sea/ lake or a slower flowing on the inside of bends.

Landforms - A River’s Course

1) In the upper stage the river is close to its source. A source is the place, where a river starts its course. Here the gradient of the river is steep, the river is high above sea level, the valley floor is narrow and the valley sides are steep. Here often occur rapids, because of the rocky, irregular river bed, which make the water rough. The steep gradient gives the river enough gravitational potential energy to carry out vertical erosion. This kind of erosion leads to the formation of gorges, canyons and potholes. Potholes are smooth and round holes in the river bed caused of eddies in the water swirl, which drill stones down into the rock and form those holes. The narrow V-shaped upland, forces the river to adopt and to wind its course around interlocking spurs, because only after storms large boulders can be moved.











Fig.5.9


Small waterfall in the upper course of the Cinca, in Valle de Pineta, near its source in Aragon, Spain.

A different type of the rivers upper stage can be a waterfall or gorge. Waterfalls are formed where a layer of hard rock lies over a layer of soft rock in a river valley. The soft rock erodes more quickly and a plunge pool develops, which undercut together with splashing water and eddy currents the rock. Then the formed overhang collapses, the process repeats and gorges are created.

















Fig. 5.11


Border of Ontario, Canada & New York, United States

Niagara Falls


Through the scenery of waterfalls economic benefits develop such as tourism or use for hydraulic power. However they cause problems with navigation.

2) In the middle stage the gradient begins to level out resulting new river shaping’s. The valley and the valley floor widen, the slopes become gentler and the gradient lessens. That’s why now lateral (sideways) erosions and deposition take over. There exist three different main river channel forms. A few bends mean straight and when the river splits in many channels and sometimes re-join describes a braided shape. In contrast if the river flows in series of bends it is called meandering.













Table 5.1

Meanders have their sweeping shape because of the helical flow inside the water, which sends the energy to the sides (laterally). That leads to the formation of a river cliff (erosion) and slip-off-slope/point bar (deposition).



















Fig. 5.15

Meanders of the Rio Cauto at Guamo Embarcadero, Cuba.


Continued erosion creates a narrow neck, which can be cutted through. Than the river form a new channel and the old meander become an oxbow lake.


An oxbow lake is forming. The Nowitna River, Alaska.











Fig. 5.15

3) In the lower stage the river loses its velocity and the vertical erosion stops. Levées form as the river floods. When the river overflows the bank, it loses energy to transport bedload. Consequently, the river deposits its load and the bank and bed, which become raised, forming gentle sloping banks towards the floodplain. The floodplain is the flat, marshy and poorly drained land next to a river and is build up through depositions of point bars and load.


















Fig. 5.17 & 5.18


A Delta is a low and flat area where a river enters a lake/sea and because of loosing its velocity deposits there its load. The river channel divides into distributaries (small rivers joining big ones) which re-join to form braided drainage pattern.














Fig. 5.20

Aerial view of the Lower Prana Delta, north of Buenos Aires

Long and cross profile of a river valley

The long profile shows how a river’s gradient changes as it flow from its source to its mouth and the river cross profile shows a cross-section of a river’s channel and valley at different stages of the river’s course. The energy carried out by a river changes from one stage to another because of the river’s height, gradient and speed. The river starts flowing high in the mountains but the gradient gradually decrease over the middle stage to the lower stage. In the upper course the river flows over series of waterfalls rapids and sometimes lakes. The cross profile of the valley floor is narrow or non-existent. Erosion cuts down the river bed and carves out steep-sided V-shaped valleys. As the river flow towards the mouth, the slopes become less steep. Same as the flood plain begins to develop and widen. Additionally the cross profile is often asymmetrical, because of river cliffs and slip-off slopes.









Fig 5.3

Hydrological cycle

After a precipitation tree leaves catch raindrops, what is called interception. Surface run-off describes water that flow overground in case soil is hard or very wet. Throughflow includes water flowing through the soil meanwhile infiltration happens if the soil is soft and rain soaks into the ground. Permeable rock allows water to infiltrate, impermeable rock not. Water continues flowing down until finally reaches the water table and from the groundwater and flow as groundwater flow. Groundwater needs hundred of years until it reaches the river/sea, whereas surface run-off and throughflow water need only weeks.

Fig. 5.22

There are five main factors affecting discharge. Antecedent rainfall means the soil is already saturated and further rainfall will run-off towards river. Relief includes the gradient of the slope. Steeps slopes infiltrate more badly than gently slopes. Weather conditions can reduce discharge with evaporation or prevent infiltration with frozen ground. Permeable rocks and soils absorb water easily, in contrast impermeable do not and porous rocks have hollow space between rock particles so they can observe water even more quickly. It also depends on how the land is used. In cities it is regulated by drains where the water gathers, because it cannot infiltrate and in rural areas the water reaches thorough hillsides the river even faster. Finally deforestation also affects the discharge because of less interception the surface run-off increases.

Hydrograph


A hydrograph predicts the change of a river after a storm by showing rainfall and discharge. After rain water flows gradually into the river and entering the water the discharge increases. This is shown by the rising limb-line. Lag time is the gap between the peak of rainfall and peak of discharge. Hydrograph graphs can approximately predict how high a discharge after a rainfall could be. That means calculations can show how strong areas might be affected. This way hydrographs can be used to drainage basin management.

Causes of flooding

Flooding tends to be caused by heavy rain. The faster the rainwater reaches the river channel, the higher is the likelihood to flood. For example heavy rainfall led to catastrophic flooding in Australia or monsoon rains in Pakistan. Other reasons are steep-sided channels where around the river channel steep are slopes that cause fast surface run-off. Also causes for flooding are lack of vegetation or woodland or drainage basin in an urban area.

To prevent flooding there are several solutions possible.

Planting vegetation would provide interception and transpiration. Straightening and dredging the channel adopt the river artificially to the environment. Same as artificial levees and bridge designs. By washing land and reservoirs, actions around the river can be solved.


Conclusion


Most rain that falls on the land travels to the sea through rivers. Large numbers op people live near rivers and, which provide great benefits for them. But rivers also can cause problems and hazards for people.


Bibliography



Complete Geography for Camridge IGCSE

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