River Environments PDF

1 PHYSICAL ENVIRONMENTS RIVER ENVIRONMENTS 1.1 THE HYDROLOGICAL CYCLE - A CLOSED SYSTEM 1 FLOWS (TRANSFERS) > RIVER ENVIRONMENTS The transfers of OWS as 5 that take place between stores do so through a variety below. listed the hydrological cycle starts with evaporation due to the hea...

1 PHYSICAL ENVIRONMENTS RIVER ENVIRONMENTS 1.1 THE HYDROLOGICAL CYCLE - A CLOSED SYSTEM 1 FLOWS (TRANSFERS) > RIVER ENVIRONMENTS The transfers of OWS as 5 that take place between stores do so through a variety below. listed the hydrological cycle starts with evaporation due to the heat of the Sun. Water is converted from a liquid into a gas (called water vapour). This takes place from the surface of tha sea and from water surfaces (ponds and lakes) on land. Evaporation is particularty important in the transfer of water from the sea store into the atmosphere. Transpiration plants take up liquid water from the soil and 'breathe' it into atmosphere as water vapour. Evapotransptration the loss of moisture from the ground by direct evaporation from water bodies and the soll, plus transpiration from plants. Condensation the change in the atmosphere when water vapour cools and becomes liquid. The liquid takes the form of water droplets that appear in the atmosphere as clouds. Preciprtation the transfer of water in any form (rain, hail or snow) from the atmosphere to the land or sea surface. Overland flow most precipitation that hits the ground moves due to gravity and eventually enters a stream, nver or lake. This is known as run off. Infiltration and percolaton the transfer of water downwards through the soi and rock into the aquifer or groundwater store. Throughflow ~ this takes place between the ground surface and the top of the groundwater store. As a result of gravity, water moves slowly through the soil until t reaches a stream or nver. @ Groundwater flow this happens in the rocks of the aquifer and is the underground transfer of water to nvers, lakes and the sea. @ Evaporation Fresh water is essential for life on Earth. This water is constantly being racycied as it moves through what is called the hydrological cycle. This is a global circulation of water and itisa qiant closed system (Figure 1.1). This means that there is a fixed amount of water because water neither enters nor leaves the Earth and its atmosphere. - - - - - - - - CHECK YOUR UNDERSTANDING What 1s the difference between a store and a flow? STORES CHECK YOUR UNDERSTANDING Which store da you think holds the largest amount of water? ba ACTIVITY List the possible physical consequences of melting ice sheets and glaciers. > During the hydrological cycle. water is held in a number of stores and then transfers. moves betwaen them by means of a sanies of tlows, also called The stores in the cycle are as follows. 1.2 DRAINAGE BASINS AND THEIR FEATURES (romy The atmosphere Sur' Wee where the water exists either as water vapour or as minute droplets in clouds. @ The land where water is stored on the surface in rivers, lakes and reservoirs, Water is taken in by plants and stored in vegetation for short penads of time. It is also stored below ground in the bedrock. This is known as the groundwater store (see aquifer}. Water mostly exists in these stores in a liquid form. However, it can also exist in a solid form as snow and ice, for example in ice sheets, glaciers and snowfields. @ The sea it is estimated that over 95 per cent of the Earth's water is stored in the sea. This is mostly held in liquid farm, but also as ice, for example the icebergs in high-latitude seas. @ All these transfers come together to form a circle involving the three major stores (the atmosphere, the land and the sea). We might imagine that the cycle starts and finishes in the saa. However, some of tha water that falls as rain on the land may never reach the sea. Instead, it is retumed directly back to the atmosphere from the land by the transpiration of plants and evaporation from both soil and water bodies evacoraton and tra from vegetation - : evapo atin - 'vom water suracas will While the amount of water In the global hydrological cycle cannot change. the proportion held in the different stores can. These variations are caused by changes in the Sun's energy. For example, an increase in the Sun's energy will lead to more evaporation and possibly to the meiting of ice sheets and glaciers. 1.2 J. evened haw water - nr | Every river has its own drainage basin or catchment area. Eacn drainage basin a system. There is a movement of water within it that is rather like a small-scale hydrological cycle. The drainage basin cycle involves stores and flows (Figure 2 }. However, an mportant difference between the drainage basin and the hydrological cycle is that the drainage basin is an open system (Figure 1.3). A drainage basin has extemal inputs and outputs The amount of water in the WPS basin system vanes over time. In the hydrolog:cal cycle the amount of water remains exactly the same flow j =>, inputs an Stores A Figure 1.2: The drainage basin cycle Oueputs ap Fows 1 PHYS CALENVIRON MENTS RIVER 630018031316115 » The inputs of a drainage basin are: environment ® energy from the Sun @ precipitation formed from moisture picked up outside the basin @ possibly water from tnbutary drainage basins this is not shown in Figure input Suggest two reasons why drainage basins close to one @ the river's discharge @ the water in its basin from another might have different drainage densities. which evaporation and transpiration take place: this water eventually falls as precipitation in another drainage basin. watershed. This is where most precipitation falls. Smaller streams, or tributaries, enter the main river channel at iocations known as confiuances. The mouth or estuary of the river is where it flows out into the sea. Figure 1.3: Open and closed systems Why is the hydrological cycle a closed system and the drainage basin cycle an open system? @ @ CHECK YOUR UNDERSTANDING [s there an inland sea in the country where you live? Those that simply collect and deliver water directly to the sea. Those that are parts of much larger drainage basins. For example, the basin of the River Negro in Brazil is a tributary or part of the huge drainage basin of the River Amazon. The water it collects is conveyed to the Amazon and then eventually to the sea. Some drainage basins do not lead, either directly or indirectly, to the open sea. Rather they lead to 'inland' seas or lakes such as the Caspian Sea located between Europe and Asia, the Aral Sea to the east of it, or Lake Victoria in East Africa. The channel (or drainage) network is the system of surface and underground channels that collects and transports the precipitation falling on the drainage basin. mouth N Kay 20am 4 shea lowand waited Figure .4: Sasic features of the drainage basin Figure 1.5 shows how some of these features can affect overland flow or run off. Rock type and relief are physical factors aver which people have little control. But land use can be easity changed by peopie. Woodiand holds water and slows overland flow. However, once it is cleared for cultivation, run off will speed up. The built-up areas of towns and cities can speed up run off even more. Rainwater hits solid surfaces such as roofs, pavements and roads. It is then quickly channelled into drains which speed its delivery into a stream or river. skaws nite of run off urban area, up rate of run off Be sure you knaw the difference between permeable and impenneabie rocks. limestore grit cla 1.3 RIVER REGIMES AND HYDROGRAPHS We need to know how quickly any rain falling in a drainage basin will reach the drainage network. It is also important to know how much a river's channel can hold. If rainwater reaches the river quickly, the channel may not cope and flooding will occur. The amount of water carried by a river at any one time is known as its discharge. This is measured in cumecs that is, in cubic metres of water per second moving past a particular point along the river's course. - > $0 000 Figure .1: A hydrograph of the River Ganges in Bangladesh. Average monthly discharge is shown in cubic metres per second. Figure 1.4 shows the channel networks of two drainage basins, A and B. Basin A's network is slightly less danse (has fewer channels) than B's, though the main channel of B is shorter. Channel networks can be mapped and their lengths and densities (number of channels per unit area) measured. The network can change over short periods of time. For example, during flooding drainage basins often have many more and longer channeis than they do in periods of low or normal rainfall. Each drainage basin is unique in its combination of features. These features include size, shape, rock type, relief and land use. Thay datermine how quickly or slowly water moves through the basin. - CHECK YOUR UNDERSTANDING Discharge (curnecs) @ Cy] land Drainage basins can be of at least three broad types as follows. CHECK YOUR UNDERSTANDING confiuence ttle cun off perneabis rock, rrars nave disappaared undergrad ACTIVITY The outputs are: As Figure 1.4 shows, we can draw a dividing line between neighbouring drainage basins. It follows the tops of the hills and is called the watershed. The main river has its source in the higher parts of the basin close to the fapid run off - steep sapes; impermeable rock - 1.2; an explanation is givan below. 4 Figure .5: Factors affecting run off REASLMIFE - environment RIVER ENVIRONMENTS 1 - 40 000 30 000 20 000 140 000 0 Jan Feb Mar Apr May June Ju y Aug Sept Oct Nov Dec RIVER DISCHARGES > River discharges vary throughout the year, from month to month, from day to day. These variations make up what is termed the river regime. In most rivers, the regime closely reflects local climatic conditions, particularly the rainfall. Figure 1.6 shows the average monthly discharge of the River Ganges as it passes through Bangladesh. This diagram is a hydrograph. Clearly, mean (average) discharge is high between June and October. This period of high discharge coincides with the monsoon season, during which total rainfall can exceed 2750 mm. » 150 Figure 1.7: Hydrograph of the River Thames at Reading, 2001-02 PHYS CALENV RON MENTS RIVER ENVIRONMENTS 1 RIVER ENVIRONMENTS The storm hydrograph shows discharge of the river as being made up of two flows: 7 1404 @ the base tlow @ the storm flow 120 120 1 4 - - the 'normal' discharge of the nver the additional discharge of the river as a result of the rainstorm. MTERPRITTIQ INDIERIC a ACTIVITY FACTORS AFFECTING RIVER REGIMES 88 cs 114 1 @ The amount and the intensity of the rain. Heavy rain will not sink into the ground. instead, it will become overtand flow or run off and quickty reach the river. @ Temperatures affect the form of precipitation. For example, if ternperatures are below freezing, precipitation will be in the form of snow. This can take 20 70 54 Compara the hydrograph of the Ganges with that of the Thames as shown by Figures 1.6 and 1.7. Try to explain the differences. 404 30 20 10 1 Oct 200 Nov Dec Jan Feb Mar Apr May Jun 2002 Jul Aug Sep EXECUTIVE FUNC TICN ACTIVITY Draw an annotated diagram showing impacts of the six factors on river regimes. 1.4 FLUVIAL PROCESSES WEATHERING AND MASS MOVEMENT Figure .8: A storm hydrograph B - Weathering involves elaments of the weather, particularly rainfall and temperatures (Table 1.1). level of water storm flow Rivers play a major part in shaping landforms. Three processes are at work here crosion. transport and deposition. These river processes artner two other processes weathering and mass movement. Let us first look at weathering and mass movement and then examine more closely the work that rivers do. - river levels to rise. This delay between peak rainfall and peak discharge is called the lag time. The shorter the lag time, the quicker the water reaches the river channel. A short lag time causes the nver discharge to rise steeply. Tha steeper the rise in discharge, the greater the chances of flooding. It is possible to mark on the storm hydrograph the level of discharge above which the river will flood. Once the storm and its peak discharge have passed, the amount of water in the river starts to decrease. > - - The majority of the world's drainage basins are home to many people. The big attractions of such areas are their fertile soils and the ability to grow food. Today much money is invested in drainage basins, not just in farmland, but also in homes, businesses and transport. It is therefore important to know how nvers will behave following heavy rainfall. This data allows people to work out the risk of flooding, and over what area. This is where a storm hydrograph is useful. A storm hydrograph records the changing discharge of a river after a rainstorm. The bars in the left-hand comer of Figure 1.8 show the input of rain. After rain hits the ground, it takes time for rainwater to reach the river and cause weeks to melt. If the ground ramains frozen, melting snow on the surface can reach the river quickly. Steep slopes will cause rapid surface run off, so water will reach the river more quickly. Flat and gently sloping land may lead to water sinking into the soil. This will delay it reaching the river. Rock type impermeable rocks will not allow rainwater to sink into it, so will speed up run off. Permeable rock allows infiltration and percolation of water into the bedrock. This in turn slows delivery of water to the river. Vegetation and land use trees and other plants intercept and delay the rain reaching the ground. Bare soil and rock speed up run off and reduce the time fag. So. too, will urban areas under tarmac and concrete. Human intervention dams and reservoirs are an obvious form of intervention in river regimes. By hoiding back discharge, dams reduce the risk of flaoding downstream. The flow of water out of reservoirs can also be controlled by opening and closing sluice gates. The increasing abstraction of water from rivers for a range of growing human needs (see Part 1.7) Impacts on the regimes of many rivers, not just on their storm hydrographs. - Figure 1.7 shows the regime of the River Thames (England) during one year. Unlike the Ganges. the highest discharges generally occur in winter, in February and March. Another feature of the hydrograph is its 'jaggedness', Discharge varies from day to day. The peaks indicate the impact of passing showers and short penods of heavy rainfall. STORM HVDROGRAPHS Six factors affect river regimes in general and storm hydrographs in particular. PHYSICAL WEATHERING discharge above which the river will flood raintall peak discnarge ot nve base flow high river discharge 4 Figure 1.9: A landslide in Sri Lanka CHEMICA iL WEATHERING lag time BIOLOGIC. WEATHERING rainfall REASONING CHECK YOUR UNDERSTANDING Explain why lag time > so important. low river discharge channel water level channel water level atA atB Table ,1: Different types of weathering This breaks rocks down into smaller and smaller pieces It is done by changes in temperatures and by rainfall freezing and thawing in rock cracks. This causes rocks to decay and disintegrate. It is largely done by slightly acidic rain seeping into porous rocks. The roots of plants, especially trees, growing into cracks in the rocks gradually split the rock apart. All this destructive activity takes place where rocks are found above the surface of the surrounding land. Once rocks are really broken down, the weathered material starts to move down the slope under the influence of gravity. This is mass movement. It takes several forms. In river valleys, there are two main types of mass movement as follows. DNMENTS# CHECK YOUR UNDERSTANDING What made the landslide in Figure 1.9 into a senous hazard and natural disaster? 1 RIVBI ENVIRONMENTS 1 this occurs when the bottom of a valley side slope is cut away by the river flowing at its base. It makes the siope unstable and weathered material slumps down towards the river. Slumping is also helped when the weathered material on the slope is saturated by heavy rain. Tha water does two things. It makes the weathered material heavier and acts as a tubricant. Figure 1.9 is an example of sudden slumping leading to a major landslide. Scil creep weathered material moves slowly down slope under the influence of gravity. It collects at the bottom of the valley side and ts eroded by the river. ® Slumping - - DEPOSITION > CHECK YOUR UNOERSTANDING What sort of material is transported and deposited by nvers? There are several different ways in which rivers erode their channels and valleys (Table 1.2). HYDRAULIC ACTION 4 Table 1.2: The processes af river erosion Check that you know the difference DOWNSTREAM UPSTREAM discharge Attrition is another river process. It involves particles of material being carried by a nver, and becoming rounder and smaller as they collide with each other. This process does not cause erosion of nver channels and valleys. occupied channel width channel depth between weathering and erasion. TRANSPORT > Erosion, transport and deposition are affacted by a number of factors. A wetter climate means more discharge and therefore more erosion and transport. Softer rocks are more easily eroded and transported. Gentle slopes encourage deposition. CHARACTERISTICS Minerals in the rocks forming the sides of the river channel are dissolved by the water flowing past them. CHECK YOUR UNDERSTANDING Deposition is the laying down of material transported by the river. This occurs when there is a decrease in the energy, speed and discharge of the river. Deposition is most likely to happen when a river enters a lake or the sea. It also happens wherever there is a decrease in the gradient of the river's channel. 1.5 DOWNSTREAM CHANGES IN RIVER Water hits the river bed and banks with such force that material is dislodged and carried away. This is most likely to happen dunng periods when the river's discharge is high. The material being carried by a river is rubbed against the sides and floor of the channel. This 'sandpaper' action widens and deepens the channel. (SOLUTION) RIVER ENVIRONMENTS average velocity This Is the movement of material (known as the load) by the river. The load is material that has been washed or fallen inta the river. tt also contains materials eroded by the river from the sides of the channel. The load can be transported in a number of different ways (Figure 1.10). load quantity The long profile of a river runs from its source to the point where it enters the sea, a lake or joins another and larger river. The character of the long profite changes downstream. Overall, it has a smooth concave shape. It is steep and in places irregular where the river is flowing well above sea level in upland country. The irregularities occur where outcrops of hard rock run across the valley. Natural lakes and reservoirs can disrupt the smoothness of the long profile. However, the profile becomes much gentler and srnoother as the river runs through lowland country and reaches its destination. Of the many changes that take place along the long profile, changes to the river channel are particularly striking. The channel starts narrow and shallow (it is V-shaped) with rough edges. it gradually becomes wider, deeper and smoother. Figure 1.11 shows also that discharge and average velocity increase downstream, as doas the amount of load being transported. At the same time, the size of sediment in the load becomes progressively finer and more rounded. toad partcie size Figure 1.10: Ways in which rivers transport thelr oad Ca ! sohaion channel bed roughness slope angie igndiant} # t ~ along at cared along by ACTIVITY traction Explain why the size of load material affects the way it is transported by a river. ® load particle size ® load quantity ® channel depth. : small bouiders bounced Explain the downstream changes, as shown in Figure 1.11, to each of the following: channe! bed roughness material deeoived In the water rotted on the bed Figure 1.11: Long-profite changes in channel characteristics ACTIVITY : H