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Landforms - Meanders, Ox-bow Lakes, Floodplains and Levees

4. Meanders Floodplains by rgamesby
 
In contrast to the upper reaches of a drainage basin where the rivers start, the middle reaches are characterised by more gentle relief, erosion and deposition processes and wider valley floors (due to lateral erosion)
Meanders across a floodplain

Think about it

1) Try this hot potatoes task on Meanders

2) Fully explain how rivers can transform the v-shaped valleys of the upper valley into broader flatter valleys.

3) Describe the map below that shows an abandoned meander

Meanders

Meanders occur in the middle valley and are the result of erosion AND deposition processes on a river.

In this section of the valley the river erodes laterally and migrates across the valley floor over time, widening the valley.  Within the river itself, the fastest current is found on the outside of a bend and the slowest current on the inside of the bend, this can be observed on diagram A. This is because the depth of the water on the outside of the bend is deeper, so there is less friction and hence higher velocities.

Over time, this means that erosion occurs on the outside of meander bends and deposition occurs on the inside. This process can lead to formation of one of Geography's classic landforms, Ox bow lakes.  It is claimed that nearly everyone in the UK can explain how ox bow lakes form, surely a useless bit of knowledge!

Diagram A) a cross section showing the speed or velocity of water through a meander

A cross section of a meander

Diagram B) a labelled cross section to show the key features of the meander.  Note that on the shallower inside of the bend sediment can accumulate to form a slip off slope, whereas on the deeper outside of the bend where the current is faster and erosion operates, a river cliff forms.

MEANDER CROSS SECTION 2

Ox Bow Lakes

In the diagrams below, erosion of the outside of the meander means that the neck of land becomes narrower and narrower over time.

On the inside of the bend the slow flow encourages the deposition of beaches.

After a long time the neck of land gets totally cut through by erosion processes such as hydraulic action and abrasion (watch an animation of this).

This cuts off the meander bend which is totally isolated by deposition leaving an Ox bow lake, which is a crescent shaped lake that will eventually fill with reeds and sediment over time.

Meander formation

 


Flood plains, Levees and Yazoo streams

Rivers flood on a regular basis. The area over which they flood is known as the floodplain and this often coincides with regions where meanders form.

When rivers flood in the middle valley the cover an area of land known as the flood plain.

When they flood velocity is slowed and deposition of any rocks being transported is encouraged.

This deposition leaves a layer of sediment across the whole floodplain.

After several floods there are several layers of sediment (rocks) deep on the flood plain.

In addition, the largest rocks and most deposition occurs next to the river channel.  This leaves a ridge of higher material next to the river channel on both banks of the river known as a levee.





Floodplains block diagram
 
Map of Abandoned Meander  

Click here for full screen version