6Contrasting river landforms from source to mouth Channel featuresValley featuresLong profile
7Upper Course - Channel features River channel is rocky.Covered with various shapes and sizes of boulder.Discharge is low.Under flood conditions rivers energy is expended on vertical erosion with hydraulic action and corrosion processes at work.Potholes may form.
10Upper Course - Valley features Valley sides are steep and form a ‘V’ shaped cross section.Interlocking spurs.V shaped valleySteep sidesZig-zag bends(interlocking spurs)
11V – shaped valleyForm due to a combination of the following processes:Vertical erosion by the river itself.Physical weathering (eg: frost action) which provides debris to move down slope.Mass movement (inc: soil creep & landslides) to move debris down slope.
19WATERFALLRETREATS . .UPSTREAM . .OVERHANGPLUNGEPOOLUNDERCUTTINGOF SOFT ROCK
20Formation of rapidsResistant rockLess Resistant Rock
21Middle Course - Channel features Channel is now wider and has smoother banks and bed compared to the upper course.
22Middle Course - Valley features River erosional energy is now increasingly expended horizontally rather than vertically.Lateral erosion by the river’s meanders broadens the valley floor into a narrow flood plain.Meanders gradually shift their course downstream.
25Meanders A meander starts as a slight bend: Water flows faster on the outer curve of the bend (more energy), and slowest on the inner curve (less energy).So the outer bank gets eroded while material is deposited at the inner bank.Over time the outer bank gets worn away (river cliff) and the inner one builds up (river beach). The bend grows into a meander.
26Meanders (refer to previous notes and diagrams) Alternating series of irregularities developPools – deeper stretches of slow moving waterRiffles – shallower section of faster flow, flowing above coarser materialRiver develops a winding or sinuous courseFaster flow on outer bend results in erosion and formation of River CliffSlower flow on inside of bend results in deposition and formation of Slip-off Slope
27Meanders Meanders develop and migrate laterally and downstream Helicoidal flow further assists meander formation and transports sediment from river cliff to the slip-off slope on the inside of the next bend.
28MEANDERS Meanders Flood plain Possible break through point Possible ox-bowlakeMost erosionon the outsideof the bend .. Fastest flowFlood plain
31Middle Course - Profile Gradient is now less steep.Overall the profile is smooth – but an outcrop of resistant rock could still cause a waterfall.
32Lower Course - Channel features The channel is now at its broadest and deepest.Bedload is carried entirely in suspension and is solution.Deposition now dominates – particularly during floods.Erosion also occurs – in the formation of meanders
33Lower Course – Valley features Thanks to lateral erosion the valley sides may now be several kilometres away.Typically it may also contain the following features:Floodplain & natural levéesBraided channelsMeandersOxbow lakesEstuaries and deltas
35Natural levéesAs the river floods, sediment is dropped over all the flooded areas but most falls along the river channel itself.This sediment raises the height of the banks is flooding occurs regularlyLevees themselves do not prevent flooding because as the banks are raised, more sediment is dropped on the river bed, raising the water level.
36River is actually flowing above The floodplain !!Gentle valley sidesLevees is oftenartificiallystrengthenedFlat floodplainCoarse materialForms naturalleveesLayers of siltDeposited during floods
37Braided channelsFormed by the choking of the main channel by the deposition of a considerable amounts of the river load.The channel splits into several smaller channels which flow around fresh ‘islands’ of deposited material before rejoining.
40Ox-bow lakesDownstream migration of meanders produce pronounced meander loops whichmay form ox-bow lakes during flood conditions1 Lateral erosion on outside of bends of pronounced meander2 Narrow neck of meander gradually becomes narrower3 Neck is cut through by river during floods and river forms new straighter channel4 Cut-off is sealed by deposition5 Ox-bow lake begins to silt up(Core Higher, P 65, Fig 3.36)
41Oxbow lakesAn Oxbow lake starts as a meander. During a flood the river cuts across the meander, forming a new channel.Sediment is deposited along the sides of the new channel. The loops gets sealed off and an oxbow lake forms.The water in it becomes stagnant. The lake will remain sealed of until either the river floods into it or it dries out.
45Estuaries and deltasAn estuary is where a river meets the sea (it is tidal).Deltas are essentially the seaward extension of the floodplain and form where tides are too weak to remove deposit sediment.