Channels and characteristics AQA


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Channels and characteristics AQA

  1. 1. The long profile of a river illustrates thechanges in altitude of the course of theriver from its source, along the entirelength of its channel, to the river mouth.In general, the long profile is smoothlyconcave, with the gradient being steeperin the upper course and becomingprogressively gentler towards the mouth.
  2. 2. Irregularities in the gradient frequentlyoccur and may be represented by rapids,waterfalls or lakes.There may also be marked breaks orchanges in slope, known as knick points,which generally are the product ofrejuvenation.Rejuvenation occurs either when sea levelfalls or when the land surface rises.
  3. 3. Either situation allows the river torevive its erosion activity in a verticaldirection.The river adjust to the new base level,at first in its lowest reaches, and thenprogressively inland.The processes of erosion anddeposition and transportation along thelong profile of a typical river aresummarised in Figure 1.7
  4. 4. The valley cross profile is the view ofthe valley from one side to another. For example, the valley cross profile ofa river in an upland area typically has aV-shape, with steep sides and a narrowbottom.
  5. 5. Variations in the crossprofile can be described andexplained as follows (alsolook at Figure 1.8): In the upper course - anarrow steep-sided valleywhere rivers occupies theentire valley floor. This is aresult of dominant verticalerosion by the river. In the middle course - awider valley with distinctvalley bluffs, and a flatfloodplain. This is as aresult of lateralerosion, which widens thevalley floor. In the lower course - avery wide, flat floodplain inwhich the valley sides aredifficult to locate. Herethere is lack of erosion, andreduced competence of theriver, which results in large-scale deposition.
  6. 6.  Over a long period of time a river may display an even and progressive decrease in gradient down the valley, creating the typical smooth concave shape which has adjusted to the discharge and the load of the river. This may also be referred to as the profile of dynamic equilibrium, where a balance has been achieved between the processes of erosion and deposition.
  7. 7. Recently it has been accepted that the channel may still be graded if it exhibits some irregularities in its long profile.Some geographers define the graded river as that which has been attained when the river uses up all the energy in the movement of water and sediment so that no free energy is left to undertake further erosion.
  8. 8. In this situation the gradient at each point along the river is sufficient to discharge the water and load but there is little excess energy available for further erosion.If the volume and load of the river change over the long term, then the river’s channel and its long profile will also adjust to the new conditions.
  9. 9. Theoretically, river systems should reach an equilibrium when the inputs and outputs are balanced, but changes in the system bring adjustments to the profile as the river attempts to counter the change. In this way it regulates the system.
  10. 10. In relation to rivers, potential or (stored) energy is fixed by the altitude of the source of the stream in relation to base level.Kinetic energy, or energy due to movement, is generated by the flow of the river which converts potential energy into moving energy.
  11. 11. The amount of kinetic energy is determined by the volume of flowing water (discharge), the slope or channel gradient down which it is flowing and its average velocity.An increase in velocity and/or discharge results in an increase in kinetic energy.All channel processes are dependent on the amount of energy available. This is a delicate balance.
  12. 12. If there is excess aftertransportation of load the river willerode, but if energy is insufficient tomove the load, deposition willoccur.The river channel adjusts inshape and size to accommodatechanges in the volume of water andsediment.
  13. 13. • The channel cross profile (or section) is the view of the river bed and banks from one side to the other side at any one point on its course.• As a river flows from its source to its mouth, a number of typical changes take place in the channel. In the upper course, the channel is narrow and uneven, because of deposited boulders.
  14. 14. • As the river enters its middle course and starts to meander, the channel becomes asymmetrical on the river bends but mainly smooth and symmetrical on the straight stretches.• In the lower course, the river widens and deepens further, but banks of deposition and eyots (islands of deposition) can disrupt the shape of the channel cross section, leading to a braided channel. Sometimes embankments called levees can be seen on either side of the channel. Levees can also be man-made.
  15. 15. • The shape of the channel influences the velocity of the river.• In the upper course, where the channel is narrow and uneven due to the presence of large boulders, there is a large wetted perimeter.• The wetted perimeter is the total length of the river bed and banks in cross section that are in contact with the water in the channel.
  16. 16. • River levels only rise after heavy rain or snowmelt and in the upper course the river is relatively shallow.• When there is a large wetted perimeter in relation to the amount of water in the river, there is more friction.• Friction results in energy loss, and consequently, the velocity of the river is slowed.
  17. 17. • As channels become larger and smoother, in the middle and lower course of the river, they tend to be more efficient.• The wetted perimeter is proportionally smaller than the volume of water flowing in the channel. Therefore, there is less friction to reduce velocity. Although the turbulent flow of mountain streams might appear faster than that of the gently meandering downstream channel, average velocity is actually slower.• This is because so much energy is expended overcoming friction on the uneven channel bed in the upper course, whereas in the lower course there is little to disrupt water flow.
  18. 18. • A high hydraulic radius means that the river is efficient. This is because the moving water loses proportionally less energy in overcoming friction than when the ratio between the cross-sectional area and the wetted perimeter is low.• Larger channels tend to be more efficient; area increases to a greater degree than wetted perimeter.
  19. 19. As a river flows from its source to its mouth anumber of changes take place in itsmorphology. These changes affect the shapeand size of the channel and result indistinctive landforms along its course. Someof these landforms are the result of erosion,some are the results of deposition and someare the consequence of both.
  20. 20. Waterfalls and rapids occurwhen there is a sudden changein the gradient of the river as itflows downstream. Waterfallsare more dramatic featuresthan rapids and may be theresult of:A resistant band of rock occurringacross the course of the riverThe edge of a plateauThe rejuvenation of the area, givingthe river renewed erosional power assea level falls.
  21. 21.  The river falls over a rock edge into a deep plunge pool at the foot of the fall, where the layers of weak rock are eroded more quickly than the overlying resistant rock. The force of the swirling water around the rocks and boulders enlarges and deepens the plunge pool by hydraulic action and abrasion. This undercuts the resistant rock above. Eventually the overhanging cap rock collapses and the waterfall retreats upstream, leaving a gorge ahead of it.
  22. 22. In upper Teesdale an outcrop of an igneous rock called Whin Sillcauses the formation of the High Force waterfall. The Whin Sill isthe resistant cap rock which overlies softer sandstone,limestone, shales and coal seams. These are eroded morequickly, leaving the overhang of High Force. The waterfallcreated is 22 meters high- the tallest in England. Ahead of it liesa gorge stretching over 500 meters down stream.
  23. 23. • Potholes are cylindrical holes drilled into the rocky bed of a river by turbulent high-velocity water loaded with pebbles. The pebbles become trapped in slight hollows and vertical eddies in the water are strong enough to allow the sediment to grind a hole into the rock by abrasion (corrasion). Attrition rounds and smoothes the pebbles caught in the hole and helps to reduce the size of the bedload.• Potholes can vary in width from a few centimetres to several metres. They are generally found in the upper or early-middle course of the river. This is where the valley lies well above base level, giving more potential for downcutting, and where the river bed is more likely to be rocky in nature.
  24. 24. o Braiding occurs when the river is forced to spilt into several channels separated by islands.o It is a feature of rivers that are supplied with large loads of sand and gravel.o It is most likely to occur when a river has variable discharge. The banks formed from sand and gravel are generally unstable and easily eroded.o As a consequence, the channel becomes very wide in relation to its depth. The river can become choked, with several sandbars and channels that are constantly changing their locations.
  25. 25. Braiding also occurs in environments in which there are rapidly fluctuating discharges:• Semi-arid areas of low relief that receive rivers from mountainous areas• Glacial streams with variable annual discharge.• In spring, meltwater causes river discharge and competence to increase, therefore the river can transport more particles.• As the temperature drops and the river level falls, the load is deposited as islands of deposition in the channel.
  26. 26. Meanders are sinuous bends in a river.In low flow conditions straight channels are seento have alternating bars of sediment on theirbeds and the moving water is forced to weavearound these bars.This creates alternating shallow sections (riffles)and deeper sections (pools).The swing of the flow that has been induced bythe riffles directs the maximum velocity towardsone of the banks, and results in erosion byundercutting on that side.
  27. 27. An outer concave bank is therefore created. Deposition takes place on the inside of the bend, the convex bank. Consequently, although the river does not get any wider, its sinuosity increases.The cross section of a meander is asymmetrical. The outer bank forms a river cliff or bluff with a deep pool close to the bank. This bank is undercut by erosion, particularly abrasion and hydraulic action. The inner bank is a gently sloping deposit of sand and gravel called a point bar.
  28. 28. Once they have been created, meanders areperpetuated by a surface flow of water acrossto the concave outer bank with acompensatory subsurface return flow back tothe convex inner bank.This corkscrew-like movement of water iscalled helicoidal flow.
  29. 29. Eroded material from the outer bank istransported away and deposited on the innerbank.Modern research suggests that the flow israrely strong enough for the river to transportmaterial across to the point bar on the oppisitebank.Point bars are most likely to be maintained bysediment from erosion at the bluff of themeander upstream on the same side of thechannel.
  30. 30. The zone of greatest erosion is downstream ofthe midpoint in the meander bend, because ofthe strongest current does not exactly matchthe shape of the meander.As erosion continues on the outer bank, thewhole feature begins to migrate slowly, bothlaterally and downstream.
  31. 31. • Oxbow lakes are features of both erosion and deposition.• An oxbow lake is a horseshoe-shaped lake separated from an adjacent river. The water is stagnant (still), and in time the lake gradually silts up, becoming a crescent-shaped stretch of marsh called a meander scar.• An oxbow lake is formed by the increasing sinuosity of a river meander. Erosion is greatest on the outer bank, and with deposition on the inner bank, the neck of the meander becomes progressively narrower.• During times of high discharge, such as floods, the river cuts through this neck, and the new cut eventually becomes the main channel. The former channel is sealed off by deposition.
  32. 32. • In its middle and lower courses, a river is at risk from flooding during times of high discharge.• If it floods, the velocity of the water falls as its overflows the banks.• This results in deposition, because the competence of the river is suddenly reduced. It is usual for the coarsest material to be deposited first, forming small raised banks (levees) along the sides of the channel.
  33. 33. • Subsequent floods increase the size of these banks and further deposition of the bed of the river also occurs. This means that the river, with the channel sediment build up, now flows at a higher level than the floodplain.• For this reason, the authorities sometimes strengthen levees and increase their heights.
  34. 34. • Floodplains are created as a result of both erosion and deposition, although they accumulation of river deposits suggests that they are predominantly depositional features.• They are the relatively flat areas of land either side of the river, which form the valley floor in the middle and lower courses of the river.
  35. 35. • They are composed of alluvium - river-deposited silts and clays. Over time, a floodplain becomes wider and the depth of sediment accretions increases.• The width of the floodplain is determined by the amount of meander migration and lateral erosion that has taken place.• Lateral erosion is most powerful just downstream of the apex (tip/point) of the meander bend.• Over time, this results in the migration of meanders, leaving their scars clearly visible on the floodplain.
  36. 36. • Interlocking spurs are eventually removed by lateral erosion leaving behind a bluff line and widening the valley.
  37. 37. • Feature of deposition• located at the mouth of a river• Deposition occurs as the velocity and sediment- carrying capacity of the river decrease on entering the lake or sea, and bedload and suspended material are dumped.• Flocculation occurs as fresh water mixes with sea water and clay particles coagulate(mix/join) due to chemical reactions. The clay settles on the river bed.
  38. 38. Deltas form only when the rate of deposition exceeds the rate of sediment removal. In order for a delta to form the following conditions are likely to be met:• The sediment load of the river is very large.• The coastal area into which the river empties its load has a small tidal range and weak currents. This means that there is limited wave action and therefore, little transportation of sediment after deposition has taken place.
  39. 39. 3 Types Of Deposit• 1- The larger and heavier particles are the first to be deposited as the river loses its energy. These form the topset beds.• 2- Medium graded particles travel a little further before they are deposited as steep-angled wedges of sediment, forming a foreset beds.• 3- The very finest particles travel furthest into the lake before deposition and form the bottomset beds.
  40. 40. Deltas can be described according to thier shape:• The most commonly recognised is the characteristic arcuate delta, for example the Nile Delta, which has a curving shoreline and a dendritic pattern of drainage.• Many distributaries break away from the main channel as deposition within the channel itself occurs, causing the river to braid. Longshore drift keeps the seaward edge of the delta relatively smooth in shape.
  41. 41. • The Mississippi has a birds foots delta. Fingers of deposition build out into the sea along the distributaries channels, giving the appearance from the air, of a birds claw.• A cuspate delta is pointed like a cup or tooth and is shaped by gentle, regular, but opposing, sea current or longshore drift.
  42. 42. • Rejuvenation occurs when there is either a fall in sea level relative to the level of the land or a rise of the land relative to the sea.• This enables a river to renew its capacity to erode as its potential energy is increased.• The river adjusts to its new base level, at first in its lower reaches and then progressively inland. In doing so, a number of landforms may be created: knick points, waterfalls & rapids, river terraces and incised meanders.
  43. 43. A knick point is a sudden break or irregularity in the gradient along the long profile of a river.
  44. 44. • The river gains renewed cutting power (in the form of vertical erosion), which encourages it to adjust its long profile.• In this sense the knick point is where the old long profile joins the new.• The knick point recedes upstream at a rate which is dependent on the resistance of the rocks, and may linger at a relatively hard outcrop.
  45. 45. A river terrace is a remnant of a former floodplain, which has been left at a higher level after rejuvenation of the river.• Where a river renews its downcutting, it sinks its new channel into the former floodplain, leaving the old floodplain above the level of the present river.• The terraces are cut back as the new valley is widened by lateral erosion.• The river Thames has created terraces in its lower course by several stages of rejuvenation.
  46. 46. • If a rejuvenated river occupies a valley with well developed meanders, renewed energy results in them becoming incised or deepened.• The nature of the landforms created is largely a result of the rate at which vertical erosion has taken place.• When incision is slow and lateral erosion is occurring, an ingrown meander may be produced.• The valley becomes asymmetrical, with steep cliffs on the outer bends and more gentle slip-off slopes on the inner bends.• With rapid incision, where downcutting or vertical erosion dominates, the valley is more symmetrical, with steep sides and a gorge-like appearance. These are described as entrenched meanders.
  47. 47. • The valley becomes asymmetrical, with steep cliffs on the outer bends and more gentle slip- off slopes on the inner bends.• With rapid incision, where downcutting or vertical erosion dominates, the valley is more symmetrical, with steep sides and a gorge-like appearance. These are described as entrenched meanders.