A. EROSIONA river can erode material from its bed andbanks Fluvial erosion occurs where a river isflowing fast, so it has a lot of energy to erodethe bed and banks of the river. This mainlyoccurs in the upper portion of the river. Thisoccurs in 3 different ways:AbrasionHydraulic Action andAttrition
• Abrasion – Moving water throws particles it is carrying against the bed and banks of the river which then dislodges more material• Hydraulic Action - The sheer force of the water pounding into the bed and banks can dislodge material• Attrition - Particles being carried downstream knock against each other, wearing each other down. This results in smaller, rounder particles as you move downstream
B. Methods of TransportationFluvial transportation mainly occurs in the midsection of the river. Rivers transport largermaterial (e.g. rocks) where they have the mostenergy and smaller material (e.g. ash0 wherethey have the least energy. This occurs in fourmain ways:SolutionSuspensionSaltationTraction
• Solution - Some minerals (particularly in limestone areas) dissolve easily in water and are not visible to the naked eye and are carried ‘in solution’ downstream• Suspension – As the speed or velocity of a river increases, it is able to pick up and carry larger and larger particles in its flow. Where particles are carried along in the flow and are not in contact with the river bed, they are said to be travelling in suspension.
• Saltation - Heavier particles may not be held in the flow all the time but may be bounced along the bed• Traction - The heaviest particles (eg boulders) cannot be carried so are rolled along the river bed. Such particles may only be moved when the river has a large volume of water in it
C. DepositionThis occurs in a river when the kinetic energy isLOW. This means the river speed is slow andtherefore the flow cannot carry the particles anylonger. This takes place mainly at the base ofthe river where it is wide and slow.
Landforms CreatedThere are different landforms created at eachsection of the river, as different processesdominateUPPER COURSE: Narrow channel, fastmoving, EROSION dominatesMIDDLE COURSE: Medium channel, medium speed, TRANSPORT DOMINATESLOWER COURSE: Wide channel, slowmoving, DEPOSITIONdominates
Upper Course LandformsHere the river is fast flowing but there is little waterand load so does not have the power to erode thehillsides. Instead it erodes downwards to create v-shaped valleyse.g. Whakapapaiti River
Middle Course LandformsHere the river is wider and less steep. This meansthere is a greater volume of water allowing theriver to erode sideways. This creates meanderswhich can change the course of the river and ox-bow lakes.
Lower Course LandformsThe river is at its widest and slowest at this point.This drop in energy means that depositiondominates forming braided channels and deltas.Braided channel Delta
It’s your turn … • Draw a simple diagram to represent each process within the river • Solution = • Suspension = • Saltation = • Traction =
River Features• Rivers are eroding, transporting and depositing constantly within the drainage basin system.• The river can be divided into 3 sections• Upper Course at the Source,• Middle Course• Lower Course at the Mouth of the river.• The river displays different characteristics at each section
Fluvial (River) Processes1. Fluvial processes create landforms.2. These processes are affected by specific factors.
Also known as ‘Torrent’ or ‘Youth’ stages Interlocking Spurs In the Upper Course, the river is fast flowing, but there is little water and load. The river is often called a stream and does not have the erosive power to remove the hillsides (spurs), but erodes downwards instead. EROSION TYPE: Vertical and Headward
Pothole sEROSION TYPE: Vertical ( by EDDY CURRENTS ) Boulders broken off by erosion that sit on the river bed create swirlingeddy currents as the water flows past as the river is not strong enough yet to move the boulders by TRACTION. These eddies swirl the boulder round and erode a pothole in the river bed by ABRASION.
Waterfall and Gorge 2(PROFILE VIEW) EROSIONTYPE: Verticaland Headward
Also known as the ‘Mature’ stage Meanders 1 (Aerial View) Meanders are formed because the current swings to the outside of a bend and concentrates the erosion there. Deposition occurs on the inside of the bed where there is not enough energy to carry load. EROSION TYPE: Lateral
Meanders 2 (Profile View / Cross Section X - Y) EROSION TYPE: Lateral This cross section clearly shows the eddy current (near ’X’) formed by the velocity of the river being concentrated on the outside of the bend. These UNDERCUT the bank causing the formation of a RIVER CLIFF. On the inside (NEAR ‘Y’), a SLIP-OFF-SLOPE is formed where current is too slow to carry any load.
Ox-Bow Lake 1 (Aerial View) Ox-bow lakes are formed when two meander RIVER CLIFFS are being eroded towards each other. These will eventually meet, causing the river to thenEROSION TYPE: Lateral flow across the bottom
Braided Channels (Oblique Side View) DEPOSITION FEATURE: no erosion in the Lower CourseIn the Summer months, load is dropped by the low volume of low-energy water in the river. These build up to form obstructions in the river and it divides up to flow around them. In the winter, it is likely that the river volume will increase and remove these obstructions.
This deposition feature is oneDelta (Aerial of the largest. When the View) flowing river hits the non- flowing sea, energy is suddenly lost. This causes all of the load in the river to drop in the river MOUTH. This builds up over time to create a delta – an area of land. The river divides into DISTRIBUTARIES to continue to the sea, which is now some way away from its original meeting point.
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