GEOGRAPHY RIVERS

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  • Created on: 23-04-13 19:44

Card 1 - What processes and factors are responsibl

Slope Processes = Transfer material downslope to the river they include : 

SLOW MOVEMENTS

  • Soil Creep = individual soil particles are pushed in to the surface by wetting , heating or freezing of water. They move at right angles to the surface , they then fall down under the influence of gravity , once the particles have cooled , dried or thawed - net movement is downslope 
  • Rain-splash erosion = raindrops compact the soil and dislodge partical equally in all directions. On steep slopes the downward movement is more effective than upward slpash and so erosion downslope increases with slope angle. 

FLOW MOVEMENTS 

  • Surface wash = occurs when soil infiltration capacity has exceede and can for gullies - occurs in ENgland during winter as water drains accross saturate or frozen ground (heavy rain/snow)
  • Sheet wash = unchannelled flow of water over a soil surface it can transport materials which have been dislodged by rain-splash
  • Throughflow = is water moving down through the soil - channeled into natural pipes in the soil (small channels) - this is enough energy to to transport material 
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Fast Mass Movements / Factors Affecting Slopes

Fast Mass Movements 

  • Slides = sliding material maintains its shape until it impacts at the bottom of the slope and leads to large slumpes terraces - can be small or large 
  • Falls = occurs on slopes above 70 degrees , initial cause of the fall may be weathering e.g. freeze-thaw weathering - once the rock is detached they fall under the influence of gravity 
  • Slumps = occur on weaker rocks (especially clay) and have a rotational movement along a curved slip plane . Clay absorbs water and becomes saturarted and exceeds liquid limit , it then flows along slip plane - and is therfore heavy e.g. Holbeck Hotel , Scarborough 

Factors affecting slopes : 

  • Climate = in general slopes in cold (temperate) environments are rounder due to chemical weathering as slopes in arid areas are jagged or straight due to mechanical weathering and over-land run off 
  • Rock Type and Structure = River Tee divide Britian into soft rock /hard rock - north west are hard rock , and resistant rocks such as granaite as south and east are young weak rocks such as chalk and clay 
  • Aspect= (the direction a slope faces) in UK north facing slopes remain in shade rarley rise above freezing as south facing slopes go through cycles of freeze-thaw- solifuction and run off lower the level of slope 
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Weathering

Weathering = the decomposition and disintergration of a rock in situ ( decomposition = chemical weathering / disintergration = mechanical weathering producing small angular fragments) 

Mechanical (physical weathering): 

  • Freeze thaw = when water in joints crack and freezes and expands by 10% - most effective when there are frequent fluctuations up and below freezing e.g. alpine regions
  • Salt Crystal Growth = in areas where tempreature fluctuates around 26-28 degrees or when water evaporates , salt crystals may be left behind to attack the structure
  • Disintergration = in hot desert areas where there is a large temperature change - rocks heat up in day and contract at night - outer layer of rock peels (exfoliation) due to rock being a poor conductor of heat
  • Pressure Release = overlying rocks removed by erosion causing underlying ones to expand

Chemical Weathering 

  • Carbonation Solution = occurs on rocks with calcium carbonate e.g. chalk - rainfall and dissolved co2 reacts with calcium carbonate and forms calcium bicarbonate which is soluble 
  • Hydrolysis = on rocks with ORTHOCLASE e.g. granite which then reacts with acid water to form kaolinite , silic acid and potassium and hydroxyl. - the acid and hydroxyl are removed leaving china clay as product 
  • Hydration = certain minerals absorb water expand and change 
  • Oxidation = when iron compunds react with o2 to produce a reddish brown coating
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Factors affecting weathering

Factors affecting Weathering : 

  • Climate = Frost shattering increases as the number of freeze-thaw cycle increases . Chemical Weathering incraeses with moisture and heat  (increaes 2 to 3 times for every increase of 10 degrees) 
  • Geology= chemical composition , the nature of cements in sedimetary rock , joints and bedding planes e.g. limestone consists of calcium carbonate and is therfore carbbonation solution happens 
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factors affecting river flow and velocity

Types of Flow 

  • Turbulent Flow = turbulence provides the upward motion in the flow that allows the lifting of fine particles which will contribute to depositional landforms further down the river....Turbulent flow happens because: 
  • complex channels shapes such as meandering channels and alernating pools (deep parts of rivers on outside of meander) 
  • high velocities
  • cavitiation where pockets of air explode under high pressure 
  • Laminar Flow = movement of water in a series of sheets , not that common in rivers but can occur in bed in the lower course - best conditions for laminar flow are shallow channels, smooth straight channels , low velocities - if only laminar flow sediment would remain on bed
  • Channel Shape;
  • Efficency of strems shape is HYRAULIC RADIUS = CROSS SECTIONAL AREA/WETTED PERIMETER(higher ratio more efficent & smaller frictional loss) - ideal channe shape = semi-cricular 
  • Channel Roughness
  • Causes friction that slows down the velocity of water . Friction is caused by contact between water and riverbed and bank or vegetation boulders etc. 
  • Further DOWNSTREAM bed becomes less turbulent so bed becomes smoother and tehrfore velocity increases 
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Transport

  • Suspension = smallest particles (silts & clays) are carried on suspension as the suspended load 
  • Saltation = larger particles bouncing or hopping along the river bed (gravels)
  • Solution = minerals dissolved in water such as those in limestone rock 
  • Traction= larfer pebbles and boulders roll or slide along the river bede 

Sediment yield : the amount of material carried away by water. Very high sediment yields are found in Medditerian areas and East Africa as: 

  • high relief
  • tectonic activity 
  • limited vegetation cover

River bank erosion is related to river discharge - river bank erosion is an important source is an important source of suspended sediment in a river

The load of a river varies with discharge and velocity the relationship between them is shown in a HJULSTROM CURVE 

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Erosion

There are different types of erosion in a river: 

  • Abrasion: is the wearing away of the bed and bank by the LOAD carried by the water
  • Attrition: is the wearing away of the load carried by the river- creates smaller round particles 
  • Hyrdraulic Action: the force of air and on the sides of rivers and in cracks and occurs as river tumbles over a waterfall and crashes onto and erodes rock below
  • Soloution: removal of chemical ions especially calcium - very effective on chalk

Factors affecting erosion rates:

  • load = greater load and heavier it is the greater potential of erosion 
  • velocity= greater velocity , greater potential of erosion 
  • gradient = incraesed gradient increases rate of erosion 
  • gelogy= weak rocks such as sand are more easily eroded than consilidated rocks
  • pH = rate of soloution are increased when the water is more acidic 

Deposition (this occurs when a river LOOSES energy) due to

  • a shallowing of gradient which decreases velocity and energy 
  • a decrease in the volume of water in the river 
  • an increase in friction between water and the channel  - human obstructions e.g. dams 
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Land-forms produced by EROSION

V-SHAPED VALLEYS: (the angle of the V-shape depends on)

  • the rate of DOWNWARD EROSION by the river
  • the RESITANCE of the rocks to weathering , erosion 
  • climate
  • location on river (upper course tend to have steeper V shape)

WATERFALLS AND GORGES : 

  • resistant rock , underlain by weaker rock is under-cut by HYDRAULIC ACTION
  • Abrasion enlarges the plunge pool 
  • collapse and retreat then occurs 
  • waterfalls initiated due to faulting , drop in sea level (or tributaries) 
  • waterfalls move upstream due to undercutting and collapse leavign behind a gorge (narrow , steep-sided valley)

POTHOLES: - base of river formed by abrasion by pebbles 

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Land-forms produced by erosion and DEPOSITION

MEANDERS; 

  • meanders develop as this is the only way the stream can use up energy it now posses 
  • they are deeper on the outside bank and shallower on the inside bank 
  • they develop by pools and riffles and the THALWEG begins to flow from side to side (the line of maximum velocity in a river) 
  • HELICOIDAL FLOW (cork-screw motions that occurs in a meander) where surface water flow towards the OUTSIDE BANKS while the bottom flow is towards the INNER BANK 
  • This causes variation in erosion and deposition which give rise to river cliffs (steep sided slopes on outer bend of meander) and slipp off slopes (gentle slope seposited on the inner bend of meander) on the inner bank 
  • meandering is more pronounced when bed load is varied 
  • meandering best devlops near bankfull stage (river unable to cope with more water)

OXBOW LAKE 

  • the result of erosion and deposition 
  • lateral erosion is concentrated on the outer and deeper bank of a meander 
  • in time of flooding erosion incraeses causing the river to break through to create a new channel 
  • old meander is closed off to form oxvow lake 
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Land-forms produced by DEPOSITION

BRAIDED CHANNELS: 

  • when a main channel seperates into a number of smaller interlocking channels 
  • braiding occurs when the river does not have the capcity to transport its load in a single channel 

A number of conditions lead to braiding; 

  • Channel gradient that is slightly steeper than the meandering stream 
  • a load that contain a large proportion of coarse material 
  • a highly variable discharge 

FLOOD PLAINS; 

  • increased discharge lead to the bankfull stage the channel them becomes unable to cope with the additional water and therefore the river spills out into surrounding area 
  • repeated flooding and deopsition leads to build up of alluvium (fine sediments of silts and clays transported by suspension)and forms a floodplain - due to low gradients meanders are a charecteristic of flood plains
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Land-forms produced by DEPOSITION

LEVEES

  • raised ridges running parallel to edge of the channel and are formed by repeated flooding - floodwaters spill out of the channel and tehrfore the heaviest and coarsest sediment is deposited first and over time will build up

DELTAS - for deltas to form the river needs to: 

  • carry a large volume of sediment 
  • enter a still body of water such as the sea or lake 
  • it is then seposited 
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CASE STUDY : MANAGING THE THAMES , OXFORD

The flood hazard - Oxford is prone to flooding for a number of reasons

  • it is low lying
  • water from two large areas drains into the Thames and Cherwell which meet at Oxford
  • River Cherwell flood more now as in valley trees and hedges are cut down so reducing interception and increasing overland flow
  • much of area is clay which is impermeable so water flows over surface and into rivers
  • housing development has increased the area covered in impermeable concrete so there is less infiltration and increase vol. of water entering storm sewers and drains greater speeds
  • agricultural -led to reduction in tree cover and increase amount of water flow into river
  • bridges hold back water because debris gets trapped in river channel so less room for water to flow
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CASE STUDY : MANAGING THE THAMES , OXFORD

Current Flood Management Strategies:

  • Land Use Zoning - areas close to river and liable to flood are given to farming , recreation (Port Meadow) , and sports grounds and allotments, where it causes economic disruption but does not lead to loss of life or damage to property.
  • Flood relief schemes- flood relief channels have led to declines in flood levels and length of flood periods (& levees and channel scour have been used which has caused large reduction in average peak flow floods in the Thames between Oxford and London)
  • Urbanisation - though it increased rate of run-off but it has also improved land drainage has the potential to increase and decrease flood levels. If run-off is increased less water remains in the soil and the potential for the soil to become saturated is reduced
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CASE STUDY : MANAGING THE THAMES , OXFORD

Proposed Flood Management Strategies

  • Flood relief channel -the most effective solution to the flooding problem in Oxford is an 8km , 25m wide flood relief channel from the River Thames at Bisney to Sanfod Lock- its too cost £100million. However a flood relief channel has potential impact on the Oxford Meadows Special Area of Conservation and Iffey Meadows Site of Special Scientific Intrest - however Environmental Agency now not considering it
  • Water Storage Areas - 4 large water storage areas beside the Thames in North Oxford- consist of large areas of farmland allowed to flood- very expensive 
  • Otmoor near oxford RSPB have raised water levels on 267 heactres formerly arbable land and also created 22 hecatres of reed-bed resevoir - can hold half a million cubic metre of winter rain 
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CASE STUDY : THE TEES

It is located in North-West England , the length of the river is 100km , the river starts in the Moorlands in Cumbria, in a wet and muddy bog. The bog lets the water trickle in to a stream when the bog cant hold anymore water. Near the source the stream channel is very narrow and the bed is large , with semi-angular rocks. Near source water is fast and water is turbulant , material bouces and hits sides causing a V-shaped valley. There are resevoris which control how much water is released to rest of the river (artificial). The most water in the river is in March as it has spring rain and snow melt. Much of the land use is pasture farming as there is better growing conditions. There is a waterfall made out of whin stone which is a igneous rock and very hard , the river cant erode this rock but below is sedimentary rock which is soft and can be eroded , so therefore it undercuts and causes collapse and retreats upstream. As you get nearer the mouth flood pain becomes wider and meander here are bigger (max erosion outer curve) , however the lower section can only deposit small material as there is little energy (river bed = estuary mud) , industries are located here as there is a big location which is flat and near the sea e.g. iron works and good transport links. 

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CASE STUDY : THE TEES

CROSS PROFILES: 

  • Barnard Castle on the river lie near to meander , the town itself sits confortably on the terrace 
  • the river is primarily cutting down (vertical erosion) we know this as there is high potential energy causedf by a long fall to the base level and also with much discharge
  • river cuts down and deepens valley , gravity de-stabilises slope and weathered rock debris tumbles down to river , landslips and mud flow speed up proccess
  • river winds around base of valley slopes
  • tributaries joined the main stream so there is an increase in volume of water, so therefore has started to meander , water causes lateral erosion 

LONG PROFILE: 

  • abovee waterfall is a new resevoir which stores winter flood water from the Pennines so there is now little erosion due to lack of energy. The waterfall can no longer retreat upstream , however Cow Green Resevoir is now base level for upper stream so there is liitle deposition 
  • Migration of knick-point upstream de-stabilises banks of the channel and mass movement increased
  • waterfall high force is the erosion feature caused by lowering of the long profile downstream on sill
  • overgrazing and loss of vegetation has effect on sedimetn carried - uses energy and reduces erosion
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CASE STUDY : Zambezi

Its located on the border of Zambia and Zimbabwe. The creation of the falls were due to tectonic plates . The Falls are 2km wide and up to 108m deep (5million m3 pass over thr falls in rainy season a minute) 

Zambezi Long Profile: 

  • tough igneous basalt and less resitiant limestone meet has caused the falls
  • a fall in base level is extra energy which allows the river to cut down to its exisitng valley floor - which leaves terraces 
  • a rise in base level energy to the river is reduced which leads to deposition within the channel causing it to spilt into more efficent narrower channels-braiding
  • As river nears base level discharge and load increases - most energy used to deal with this - deposition occurs in channel as rivers energy is not sufficent  
  • Dams of Kariba and Cahora Bassa have created a new base level so have reduced the potential energy in the upper zambia. The energy budget in this part is now defecit and the force of the water over the falls is reduced- so return to graded profile will take longer - convex profile -(dramatic titlting of plateau where zambezi meets sends water down a new course)
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CASE STUDY; Mekong as a multi-use resource

It is located in South-East Asia and is the largest river in Sout-East Asia (8th in the World). Six countries share the Mekong and its resources (Thailand , Laos , Vietnam) , it is 5000km long and 55 million live on its river basin

Transportation:

  • it is limited as river is not navigable past Phom Penh as there is obstacles for ships as in the dry season there are reefs which are hazardous as it is shallow, however in the wet season there are rapids such as Si Phan Don 

Industrial & Economic Development 

  • rapidly accelerated recently - China and Vietnam keen to develop national economy 
  • power to be exported to other countries (80 million dollars to government) by the dams and sell electricty to neighnouring countries (cost 1.5 billion for dams in Laos)

Recreation & Leisure

  • one of the fastest growing tourist destinations - however expansion of tourism has potential to lead to conflict between local residents and tradtitional activities such as fishing
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CASE STUDY; Mekong as a multi-use resource

Residential Development: 

  • pop. predicted to rise - further demands on water supply and infrastructure
  • raw sewage is a problem as much ends up in the river
  • dams uses local land and forces villages to locate somewhere new (have no say where but may recieve luxuries such as TV's) - however they rely on natural environment for income 

Energy Devlopment: 

  • hydroelectrical potential on the Mekong is huge due to steep relief of the area and large volumes of water transported by the rivers
  • dams generate valuble electricty and regulate flooding 
  • dams damage Mekongs valuable resource its fish as disturb natural eco-system 
  • dams starve other countries from water

Farming and Water supply 

  • farming has been hampered by seasonal floods 
  • however 80% of rice production in the Lower Mekong depend on nutrients and silts provided by flooding 
  • dams affect natural flood cycles - mean less frequent flooding -floods in Vietnam & Cambodia will be resuced so soil wont be as fertile and crops wont be as good quality 
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CASE STUDY; Mekong as a multi-use resource

Fishing & Water Supply 

  • mekong yield more fish than anyother river system -annual harvest 2 million tonnes
  • home to over 1200 species of fish 
  • over 1 million peopel in cambodia depend on fishing for a living
  • bigger the flood, greater leaves on offer so fatter and more numerous the fish become 
  • more dams mean smaller floods
  • fish cant swim past dams - fish = 80% of peoples daily protein 
  • Mekong cat fish may die out - no floods less nutirents for fish
  • 40% of cambodias fish wiped out if dams are built 
  • irawaddy dolphin are endangered only 100 left , dams in mekong would wipe them out - tourists come to see dolphins - which will affect people as well 
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