2 The Global Hydrological Cycle The hydrological cycle describes the global distribution and movement of water within a system. There is a fixed amount of energy within the system and the movement of water is linked with the transfer of energy at all points in the cycle.The initial energy is provided by the sun (which is outside the system) in the form of latent heat to the stores of water e.g. icecaps and glaciers which store about 2% of the surface water; rivers and lakes and groundwater which store 1% and oceans 97%. This transfer of energy causes evaporation.Transpiration takes place from vegetation and when combined with the evaporation from bodies of water on the land's surface forms evapotranspiration. Geothermal energy e.g. by volcanoes contributes very little to evaporation. There is a greater percentage of evaporation from the oceans (84%) than evapotranspiration from the land (16%).
3 The Global Hydrological Cycle As the moist air moves upwards it cools and condenses forming clouds. Precipitation from these clouds returns water to the earth's surface. Again there is more precipitation over the oceans (77%) than over the land (23%). As 84% is evaporated over the oceans and only 77% is returned by precipitation, 7% of the evaporated moisture is transferred to the land by winds (advection) and is matched by a return to the oceans by run-off from the land.Therefore there is no loss or gain of energy within the system.
4 The Global Hydrological Cycle Emitted EnergySolar EnergyBoundary of systemAdvection(7%)Clouds (0.001%)Precipitation (23%)Precipitation (77%)Evapotranspiration (16%)Run offEvaporation(84%)Ice storage (2%)Run off (7%)InfiltrationSoilWater tableSoilSoilGroundwater storage (1%)Ocean Storage (97%)Rivers & Lakes (0.01%)Geothermal Energy
5 Drainage BasinsA drainage basin is an area of land drained by a river and its tributaries.The boundary of a drainage basin is called a watershed. This is usually found on higher ground.The drainage density of a drainage basin is important -High drainage density on impermeable rocks and clays – greater flooding riskLow drainage density on permeable rocks and sand.Unlike the closed system of the hydrological cycle, the drainage basin system is open. It has inputs, stores, flows and outputs.
6 Drainage Basins Key Input Storage Flow Output Precipitation TranspirationInputInterception by vegetationStorageEvaporationFlow(transfer)OutputSurface StorageSurface runoff (overland flow)Unsaturated rock/soilInfiltrationSoil MoistureRiver carrying water to the ocean/seaSaturated rock/soilPercolationWater tableThroughflowGroundwaterGroundwater flow
7 Stream/Storm Hydrographs A hydrograph is a record of channel flow in a stream or river.It shows variations in discharge with time.
8 Stream/Storm Hydrographs A hydrograph has a number of main characteristics:Precipitation – not always shownBase Flow – flow supplied by groundwater. This is a slow movement therefore base flow rises slower and later.Quickflow/Storm Flow – flow supplied by overland flow and soil throughflow. This gives the peak flows.Rising Limb – includes (a) rainwater that fell into stream(b) water flowing overland and into stream channels as the soil water becomes saturated.(a) & (b) lead to peak discharge.
9 Stream/Storm Hydrographs The shape of the rising limb depends on:The length of the rainstorm – a long period of rainfall causes rise in the water table and increased overland flow.The intensity of rainfall – if heavy – considerable quick flow if the soil is incapable of soaking it in and storing it.- if light – less step rise and flatter shape since rain is easier to store.
10 Stream/Storm Hydrographs The shape of the rising limb depends on:The condition of the soil – If the soil is already saturated more water flows overland into streams and this leads to a higher and earlier peak.Shape of the drainage basin - a long narrow basin with tributaries joining the main channel at regular intervals gives a less steep rise.- a wide basin with tributaries more closely spaced gives a steep rise.
11 Stream/Storm Hydrographs Lag Time – the difference in time between peak rainfall and the peak discharge. Lag time depends on:The condition of the soil – if the soil is fairly dry, the longer the lag time since rainfall can be stored at first before the excess flows to the channels.Angle of slopes – the more gentle the slope the longer the lag time compared to steep slopes.Intensity of rainfall – prolonged, steady rainfall has a longer lag time than intense rainfall.Size of drainage basin – small basins have a small lag time; larger basins with more tributaries have a longer lag because the tributaries act as temporary storage before run-off increases rapidly.
12 Stream/Storm Hydrographs Recessional/Falling Limb – shows the speed at which the drainage of the rivers return to normal (base flow) once the storm has passed. The water level falls off as the effects of the storm die away.
13 Stream/Storm Hydrographs One type of question that can arise about hydrographs is a comparison between different hydrographs.When answering these types of questions you must not only describe the features of the hydrographs, but explain why the hydrographs have certain shapes. It is likely that you will be given a diagram or information to help and you should think about what effects on the hydrograph the following have:Size of the drainage basinSteepness of the slopesRock or soil type (Permeable or Impermeable?)Stream densityVegetation
14 The Work of RiversThere are three main processes in the way that a river ‘works’. These are-ErosionTransportationDeposition
15 The Work of Rivers Erosion The amount of erosion a river can achieve depends on its energy or discharge. A river’s energy increases with its volume, velocity and regime (seasonal flow).Erosion in a river is caused by 4 processes:Attrition – attacks the rivers load – pebbles etc are eroded by sticking together, therefore being rounded and reduced in size.Corrasion – the wearing away of the river bed and the banks by the river’s load. This is the main method of erosion.Hydraulic Action – the wearing away of the bed and banks by the weight of water hurled against them.Chemical Solution – dissolving of minerals from the rocks.
16 The Work of Rivers Erosion A river’s valley grows in length by headward erosion i.e. rain wash, soil creep and undercutting where the water table meets the surface extending the valley up the slope.A river’s valley is deepened by vertical erosion – entirely a river process.A river’s valley is widened by lateral erosion – affected by weathering on the valley sides and by the river on the river banks.
17 The Work of Rivers A river transports its load in 4 ways: TransportationA river transports its load in 4 ways:Traction – dragging of pebbles, gravel along its bed.Saltation – bouncing of the load.Suspension – light sediments of silt and clay are held in suspension by the river’s turbulence ( the greater the turbulence, the greater the size of particles which can be held).Solution – chemicals are dissolved in the water.
18 The Work of RiversDepositionSediments are transported by a river until it has insufficient energy to move them further and deposition takes place.A river may lose its energy where:there is a decrease in gradient.there is widening or meandering of its channel.there is an increase in load.
19 River LandscapesRivers create valleys by eroding the land but slope processes also transfer weathered debris down the valley sides to the bottom of the valley, which is then removed by the river.An ‘ideal river’ is divided into three sections – the Upper, Middle and Lower courses.These courses all have characteristic features which form the river landscapes.
21 River LandscapesCharacteristic features of the Upper course of a river and its valleyChannel features – Typically, the upper stretches of a river channel are rocky, with boulders of varying shapes sizes. Under normal conditions the discharge is fairly low whereas under flood conditions, hydraulic action and corrasion processes are at work.Valley features – The valley bottom is narrow and the valley sides, typically, are steep and in cross-section are ‘V’ shaped. There are interlocking spurs This shape is a result of the interaction of processes such as vertical erosion of the river itself, physical weather and mass movements.
22 River LandscapesCharacteristic features of the Upper course of a river and its valleyLong Profile – Generally the gradient is steep and the profile is uneven, particularly where waterfalls and rapids form.
23 WaterfallA waterfall is a steep fall of river water where its course is markedly and suddenly interrupted. This may be the result of:Differences in rock hardnessThe edge of a plateauA fault-line scarpThe overdeepening of a valley through glaciation, producing a hanging valleyThe rejuvenation of the area, giving the river renewed erosional power which in turn gives rise to knickpoints where profiles interact.
24 Waterfall 4 Waterfall retreats upstream 2 Overhang develops and Harder, more resistant rockcollapses when unsupported3Softer, less resistant rock, which is eroded more quickly, which leads to -5Gorge-like sides develop as waterfall retreats upstream1undercuttingRiver continues downstreamAngular, fallen rocksDeep plunge pool. Deepened by hydraulic action and abrasion of fallen, angular rocks, which are swirled at times of high discharge – deepening the pool
25 River LandscapesCharacteristic features of the Middle course of a river and its valleyChannel features – Generally speaking, the channel is now wider and deeper and has smoother banks and floor compared to the upper stretches The bedload consists of smaller more rounded particles.Valley features – Lateral erosion by the river’s meanders broadens the valley flow into a narrow flood plain. Meanders gradually shift their course downstream.Long Profile – The gradient is now much more gentle, however the river’s velocity is only slightly reduced since there is a smoother deeper channel.
26 Meander A meander is a sinuous bend in a river. How meanders begin to form is uncertain. They appear to begin during times of flood in sections of the river where there are riffles and pools.Riffles are shallow areas of fast flowing water. Pools are deeper areas of slow moving water. The riffles and pools are regularly spaced. As they enlarge, the channels become more sinuous.Water tends to flow in a helicoidal movement (corkscrew motion) and this means that material is moved form the outside of one meander bend and deposited on the inside of the next bend. This increases the sinuosity of the river.
27 Meander Meanders first develop in relation to riffles and pools - DepositionErosionRifflePool
28 Meander Meander becomes more sinuous - Deposition on convex bank Velocity is lowest, friction greatest Point-bar deposits; slip-off slopeLargest material deposited on upstream sideMeander Wavelength = 7-10 X channel widthErosion on concave bank by hydraulic action and corrasion Velocity is greatest, friction minimal. River cliff developsAB
29 Helicoidal motion of water MeanderCross section A - BABSlip-off slopeRiver cliffShallow, slow flowingHelicoidal motion of waterDeep, fast flowing
30 Meander Deep fast flowing Shallow, slow flowing Deposition on convex bank – slip-off slope developsErosion on concave bank – river cliff develops
31 River LandscapesCharacteristic features of the Lower course of a river and its valleyChannel features – The channel is now at its broadest and deepest. Bedload is carried entirely in suspension and in solution. Deposition now dominates, particularly during floods when the river’s energy is expended in spreading its load over the lower course.Long Profile – As the river reaches base level, the gradient is very gentle in this section. The volume of water is now at its greatest and velocity differs little from that registered in the upper reaches.
32 River LandscapesCharacteristic features of the Lower course of a river and its valleyValley features – Typically the broad, low-lying valley floor has the following features:1) Floodplain and natural levées2) Braiding3) Large Meanders4) Ox-bow lakes5) Estuaries and Deltas6) Terraces
33 Floodplain and Natural Levees Over many years, the river has covered the valley floor with enormous quantities of alluvium (sedimentary deposits). It was deposited by migrating meanders and floodwater. Across the resulting level floodplain, variations in relief are very slight so that any rapid increase in discharge almost inevitably results in flooding.When a river floods , the floodwater’s speed reduces most quickly at the sides of the channel. Consequently coarser alluvial sediments are deposited at the channel edge, gradually building up into natural ridges or levees. Occasionally levees act as natural embankments and often they have been strengthened by man as a part of flood prevention measures.
34 Floodplain and Natural Levees Bluff lineWidth of floodplainCoarser material deposited firstLevéeChannelFiner material carried furtherLayers of silt deposited by earlier floodsRiverBedload causes bed of river to rise
35 BraidingFor short periods of the year, some rivers carry a very high load in relation to their velocity. When a river’s velocity falls rapidly, competence and capacity are reduced, and the channel may become choked with material, causing the river to braid i.e. the channel splits up into several smaller channels which flow around fresh ‘islands’ of deposited material before rejoining and further dividing.
36 Ox-bow LakesAn ox-bow lake is a horse-shoe shaped lake separated from an adjacent river.Ox-bow lakes
37 Ox-bow Lakes1)2)Erosion on concave banks – gradually narrowing the neck of landRiver eventually breaks through neck of land – usually at times of floodDeposition beginning to seal cut-offCut-off formed
38 Ox-bow Lakes3)* Note that these three diagrams alone with not gain you the required amount of marks in a question on ox-bow lakes. You must also include the relevant information about meanders.River flows in new, straight channelOx-bow lakeDeposition completely seals cut-off
39 Estuaries and DeltasWhen a river enters a loch or the sea, its flow characteristics are markedly change. Velocity is reduced and load is deposited.Where the river’s mouth broadens to form an estuary, tidal currents are able to scour out most of the sediments, transferring them to the sea’s transportation system. The sediment that is not removed from the estuary forms extensive sand and mud banks, often colonised by salt-tolerant vegetation and exposed at low tide.Deltas are essentially the seaward extension of the floodplain and form when tides are weak. They also grow where streams enter freshwater lochs.
40 Estuaries and Deltas Deltas have been grouped into three basic forms Arcuate: having a rounded, convex outer margin.Cuspate: where the material brought down by a river is spread out evenly on either side of its channel.Bird’s foot / Digitate: where the river has many distributaries bounded by sediment and which extend out to the sea like claws of a bird’s foot.
41 Estuaries and Deltas Bird’s Foot / Digitate Arcuate Cuspate Estuarine SeaRiverBird’s Foot / DigitateArcuateDistributariesDistributariesRiverSeaSeaSeaSeaCuspateEstuarineRiverSeaEstuary eventually fills up with materialRiver
42 TerracesA river terrace is a remnant of a former floodplain, which after rejuvenation has been left at a higher level.Rejuvenation is where a fall in sea level relative to the land or rise in land relative to the sea enables a river to revive its erosional activity.When a river is rejuvenated, adjustment to the new base level starts at the sea and gradually works its way up the river course and the point of change to the existing profile is known as the knickpoint.
43 Terraces Graded profile of a river source mouthSmooth, concave profile, decreasing in angle and gradient towards the mouthEffect of rejuvenation on the long profileOriginal graded profileOriginal sea levelKnickpoints11st fall in sea level22nd fall in sea level and present day sea level11st regraded profileMost recent regraded profile2
44 TerracesIf a river is rejuvenated, the downcutting (vertical erosion) of the river produces terraces.These are paired i.e. the height of the terraces on one side of the river will correspond with the heights of the terraces on the other side1122Knickpoint33
45 O.S. Rivers WorkIn the hydrosphere question in the exam it is possible that you will be given a question relating to the Ordnance Survey map. The most typical type of question is“Using appropriate grid references describe the physical characteristics of the river ………… and its valley from GR… to GR….”Notice that the question says “physical characteristics”. This means that you do not include anything altered by man e.g. artificial straightening of the river.
46 O.S. Rivers Work River Valley Direction of flow Degree of straightness The following should be taken into account when describing the river and its valley.RiverValleyDirection of flowDegree of straightnessWidthHeight and Steepness of valley sidesTributariesWidth and gradient of valley floorMeandering / StraightFeatures e.g. glacial, upland courseFeatures? E.g. waterfalls ox-bow lakes etc.
47 O.S. Rivers Work Recognising features on an O.S. map Change in the direction of flowConfluence of the two riversWidth of floodplain (approx. 1km)Ox-bow lakeRiver is meanderingGentle valley sides
48 O.S. Rivers Work Recognising features on an O.S. map Small width of the rivers and no floodplain – river takes up the whole width of the valleyGenerally, the rivers are flowing from North to southRivers in their Upper courseSteep valley sidesConfluence of riversV-shaped ValleyRiver flowing in a generally straight – little signs of meandering (likely to be flowing quickly)Confluence of riversWaterfallsDecrease in gradient
49 O.S. Rivers WorkOn the following slides, an example of an answer to a ‘describing the river question’ will be shown. The map is divided into a number of sections, however, if you would prefer to follow the whole map, you should be able to borrow the map from the Geography department. The map is the Thirlmere map.The question is -“Using appropriate grid references, describe the physical characteristics of the River Derwent and its valley from (Stockley Bridge) to (its entrance to Derwent Water).”
50 O.S. Rivers WorkThe river flows in a general South to North Direction. At , just North of Stockley Bridge, three streams meet to form the main River Derwent. This increases the volume of water in the river and the river increases in width. The river is flowing relatively straight in a narrow valley, between steep slopes and falls from 170m to 140m. At the river enters the floor of a U-shaped valley.
51 O.S. Rivers WorkHere, the river flows along the West side of the valley in almost a straight line hugging the base of the valley side. A number of tributaries flow down and join the river on the eastern bank in G.S At the river begins to meander and continues to do so until its confluence with the Stonethwaite Beck at During this stage, the valley varies in width from 0.2km to 1km wide in G.S.’s 2514 and 2515 where both those rivers occupy the valley.
52 O.S. Rivers WorkUnusually, the river valley then narrows through Borrowdale. Here the river appears to be flowing through a gorge with steep sides. The river has no floodplain at this point. At , the river emerges from the gorge and again meanders across a wide floodplain (1km). The river enters Derwent Water at Here the river is forming a delta (arcuate), extending the land into the lake. This land is marshy.