Edexcel IGCSE Geography Chapter 1 - Pearson Schools

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50 00040 00030 00020 00010 0000Average monthly discharge in cubic metres per secondJan Feb Mar Apr May June July Aug Sept Oct NovDecRiver discharges vary throughout the year, frommonth to month, from day to day. These variationsmake up what is termed the river regime. In mostrivers, the regime closely reflects local climaticconditions, particularly the rainfall regime. Figure 1.6shows the average monthly discharge of the RiverGanges as it passes through Bangladesh. This diagramis a hydrograph. Clearly, mean (average) discharge ishigh between June and October. This period of highdischarge from the river coincides with the monsoonseason, during which total rainfall can exceed 275 cm.<strong>Chapter</strong> 1: River environmentsFigure 1.6: A hydrograph of the River Ganges in BangladeshDischarge (cumecs)1501401301201101009080706050403020100Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep2001 2002Figure 1.7: Hydrograph showing the regime of the Thames at Reading, 2001–2002rainfallrainfall peakAchannel water levelat AFigure 1.8: A storm hydrographdischarge of riverlag timerainstormBbase flowchannel water levelat Blevel of waterdischarge abovewhich the riverwill floodhigh riverdischargelow riverdischargeFigure 1.7 shows the regime of the River Thames(England) during one year. Unlike the Ganges, thehighest discharges generally occur in the winter, inFebruary and March. Another feature of thehydrograph is its ‘jaggedness’. Discharge clearly variesfrom day to day. The peaks reflect the impact of passingshowers and short periods of heavy rainfall.The majority of the world’s drainage basins are hometo many people. The big attraction of such areas topeople was their fertile soils and the ability to growfood. Today much money is invested, not just infarmland, but also in homes, businesses and transportin these drainage basins. Thus it is important that weknow how rivers will behave following a period ofheavy rainfall. How great will be the risk of flooding –and over what area? This is where a storm hydrographcomes in useful.A storm hydrograph records the changing dischargeof a river after a passing rainstorm. The bars in theleft-hand corner of Figure 1.8 show the input of rain.It takes time for the rain to reach the river and causeriver levels to rise once it has hit the ground. This delaybetween peak rainfall and peak discharge is called thelag time. The shorter the lag time, the quicker thewater reaches the river channel. A short lag time willcause the river discharge to rise steeply. The steeper therise in discharge, the greater the chances of flooding. Itis possible to mark on the storm hydrograph the levelof discharge above which the river will flood. Once thestorm and its peak discharge have passed, the amountof water in the river starts to decrease.The storm hydrograph shows that discharge of theriver as being made up of two flows:●●the base flow − the ‘normal’ discharge of the riverthe storm flow – the additional discharge of theriver directly related to the passing of the rainstorm.6
There are a number of factors affecting the critical time lag of a storm. These include:●●●the amount and the intensity of the rain – heavy rain will not sink into the ground;instead it will become overland flow or run off and quickly reach the rivertemperatures are significant because they affect the form of precipitation. Forexample, if temperatures are below freezing, precipitation will be in the form ofsnow. This can take weeks to melt. If the ground remains frozen, melting snowon the surface can reach the river quicklyland use − trees and plants will intercept and delay the rain reaching theground. Bare soil and rock will speed up runoff and reduce the time lag. So toowill urban areas covered by tarmac and concrete●●●steep slopes will cause rapid surface runoff, so water will reach the river morequickly. Flat and gently sloping land may lead to water sinking into the soil. Thiswill delay it reaching the riverrock type – impermeable rocks will not allow rainwater to sink into it, so willspeed up runoff. Permeable rock will allow infiltration and percolation of waterinto the bedrock. This in turn will slow the delivery of the water to the riverdams and reservoirs regulate and even out discharges. They hold back dischargeand so reduce the risk of flooding downstream (Figure 1.9).Discharge (cubic metres per second)15001000500impact of damand waterabstraction<strong>Chapter</strong> 1: River environmentsstorm hydrograph0Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov DecMonthsFigure 1.9: The impact of a dam and water abstraction on a river’s storm hydrographIn your own words, describe thedifferences between the two hydrographsin Figure 1.9. Try to explain thosedifferences.1.4 River processesWeathering and mass movementRivers play a major part in shaping landforms. They do this work by means of threeprocesses – erosion, transport and deposition. These river processes act in a sort ofpartnership with two other processes – weathering and mass movement. Let ustake a look at these two processes before looking more closely at the work thatrivers actually do.Weathering involves elements of the weather, particularly rainfall andtemperatures (Table 1.1).7
Page 1 and 2: Section A:The natural environment a
Page 3 and 4: ●●●●overland flow - most of
Page 5: shows the channel networks of two d
Page 9 and 10: ErosionThere are several different
Page 11 and 12: KeyDifference inheight betweenX and
Page 13 and 14: As lateral erosion continues, the b
Page 15 and 16: The height of the drainage basin an
Page 17 and 18: ●●●The rising standard of liv
Page 19 and 20: uses less water (now 14%) (Figure 1
Page 21 and 22: 1.7 Water qualityIn Part 1.6, we lo
Page 23 and 24: Agriculture●●●Liquid from far
Page 25 and 26: There is a fourth and relatively ne
Page 27 and 28: FACTORPhysicalWeatherRockSoilRelief
Page 29 and 30: In the longer term, there is the co
Page 31 and 32: the monsoon climate with its heavy

Edexcel IGCSE Geography Chapter 1 - Pearson Schools

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