The Middle and Lower Course of a River. Starter: Draw a simple diagram of the river cross section at the following areas: Upper course Middle course Now draw on the direction of erosion (lateral or vertical). Fluvial Landforms: Depositional.
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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?
Valley opens out, more gentle slopes, wider valley bottom
First signs of meanders
River channel wider, deeper, greater velocity and discharge
Also known as the ‘Mature’ stage
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
WHAT DO THE ARROWS POINT TO?
WHICH WAY IS THIS MEANDER MOVING?
Point bar deposits on the inner meander bend where there is low energy
Constant erosion (undercutting) erodes the outer (concave) bank of the meander, forming a river cliff
Constant deposition at the outer (convex) bank deposits sediments, forming a river slope
Cut bank erosion (River Cliff)
Point bar deposits
WHERE IS DEPOSTION TAKING PLACE?
A U-shaped body of water formed when a wide meander from the main stem of a river is cut off to create a lake
River constantly erodes the outer banks of two adjacent meanders causing the meanders to move closer together forming a loop
Overtime, the loop becomes more distinct and is separated by a narrow neck of land
As erosion and deposition continue to take place along the inner and outer banks respectively, the two meanders eventually meet
The sediments deposited will eventually dam up the water in the cut off, forming an ox-bow lake
When 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 river
Meander neck becomes smaller
Explain why there is more deposition at ‘A’ rather than at ‘B’.
Sketch this plan view of a meander bend and label the line of highest velocity.
Draw 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’.
X =erosion on the outside of the meander bend
A. 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.
What else can you identify?
How can you tell that this is not the Upper Course of a river?
This is a cross section of a meander bend. Sketch the diagram and mark on the following
Slip off slope
Area of deposition