Presentation on theme: "The Middle and Lower Course of a River"— Presentation transcript:
1The Middle and Lower Course of a River Starter:Draw a simple diagram of the river cross section at the following areas:Upper courseMiddle courseNow draw on the direction of erosion (lateral or vertical).
3Fluvial Landforms: Depositional Today we will study the main landforms formed by deposition.These landforms will tend to be found in the middle and lower course of a river.What do you think the relief be like in the lower course?
5Discussion How does the gradient of a river change along its course? How does the balance between erosion and deposition change?How does the size of material change along the profile?What are the 4 processes of transportation?
6How is material transported downstream? TractionSaltationSuspensionSolution
8Middle course, R. TeesValley opens out, more gentle slopes, wider valley bottomFirst signs of meandersFloodplainRiver channel wider, deeper, greater velocity and discharge
9What landforms are found in the Middle Course of a River? BraidingMeandersOxbow Lakes
10BraidingBraiding occurs when a river is a river deposits all or some of its load when there is too much material to carry.This is likely to happen when extra material is added into the river and/or when the river loses velocity.Braiding occurs when the river splits into two or more channels due to the deposition. The ‘islands’ formed are called eyots.eyots
11MEANDERS A stream consisting of successive Meanders A meander in general is a bend in a sinous watercourse.
12MeandersMeander bends occur in the middle section when the river has a shallower slope (gradient) so its power does not erode vertically, but laterally.Simulations with laboratory models of rivers have shown that, above a critical minimum velocity, meanders will form. Friction with the channel bed and banks causes turbulence in the water flow, which promotes the development of alternating bars of sediment along the channel.These bars of sediment are called rifflesAlternating with the riffles are pools of deeper water.The riffles disrupt the water flowing over it, swinging the water towards the outer banks.A meander starts to form at the deeper side of the river where it flows faster and has more power to erode sideways.
16FLOW IN MEANDERSOnce the meander starts to develop, helicoidal flow (corkscrew shaped flow) is established, with the water surface being elevated on the outer bank of each curve, and return currents at depth directing the flow towards the opposite bank.The outer bank is eroded as a result of the higher flow velocity, whereas material is carried by the flow and deposition takes place on the inner bank, and forms a point bar.It is the major flow in the meander bends.
20Middle Course: Meanders Also known as the ‘Mature’ stageMeanders 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
21Meander, R. Lavant, Chichester WHAT DO THE ARROWS POINT TO?WHICH WAY IS THIS MEANDER MOVING?FloodplainSlip-Off SlopeRiver CliffPoint bar deposits on the inner meander bend where there is low energy
22MeandersConstant erosion (undercutting) erodes the outer (concave) bank of the meander, forming a river cliffConstant deposition at the outer (convex) bank deposits sediments, forming a river slope
40Ox-bow LakeA U-shaped body of water formed when a wide meander from the main stem of a river is cut off to create a lake
41Ox-bow LakeRiver constantly erodes the outer banks of two adjacent meanders causing the meanders to move closer together forming a loopOvertime, the loop becomes more distinct and is separated by a narrow neck of land
42Ox-bow Lake River Landforms As erosion and deposition continue to take place along the inner and outer banks respectively, the two meanders eventually meetThe sediments deposited will eventually dam up the water in the cut off, forming an ox-bow lake
43Meander neck becomes smaller Oxbow LakesWhen the river floods it breaks through the thin meander neck and the river takes the easier, straight course. This leaves the meander loop ‘cut off’ as an oxbow lake. Over time, the oxbow lake will become colonised by vegetation.new course of the riverMeander neck becomes smalleroxbow lake
45Meander Bend on the River Conwy Explain why there is more deposition at ‘A’ rather than at ‘B’.
46Sketch this plan view of a meander bend and label the line of highest velocity.
47Draw the following four diagrams in the correct order of oxbow lake formation. Match 3 of the diagrams with the most appropriate explanation from ‘A’, ‘B’ and ‘C’.4.3.2.X = erosion on the outside of the meander bend1.XXXXXA. The river floods and erodes through the thin meander neck. The river now takes the easier, straight course. The meander loop is left as an oxbow lake.B. The river meanders. The fastest flow is therefore on the outside of the bend. This fast current erodes the meander neck through the processes of erosion.C. The meander neck is further eroded until only a thin piece of land separates the two channels.
48Name the river landform shown in this aerial photograph. What else can you identify?How can you tell that this is not the Upper Course of a river?
49This is a cross section of a meander bend This is a cross section of a meander bend. Sketch the diagram and mark on the followingRiver CliffFastest velocitySlip off slopeUndercuttingArea of deposition
50Questions What is braiding? Explain how a meander may form and then migrate along the course of a river across . Use the following words:RifflesPoolsHelicoidal flowPoint BarCliffSlip off SlopeErosionDepositionConvexConcaveUsing a series of diagrams, explain the formation of ox-bow lakes.
51To finish…We will now play a short game to see how many of the key terms we can remember.You will be given a card with a key term written at the top.Under the key term, write down 5 other key words you would use to help you explain what that term is
52To finish…Now that the cards are finished, we will play a game of taboo.You have to explain what the term is on your card, without using any of the words on the card!We will split into two teams to do this…