Meanders and Ox Bow Lakes
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 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 the cross section diagram. 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. In the diagrams, 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 .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.
Landforms of Deposition - Flood plains and Levees
Rivers flood on a regular basis. The flat area over which they flood is known as the floodplain and this often coincides with regions where meanders form.
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 levée. Meandering rivers can contribute greatly to floodplain development by eroding laterally and helping to flatten valley floors.
Levées can be reinforced as a flood prevention measure, and there are many examples of concrete and earth being used to artificially build the height of Levées.
The formation of floodplains and levees
Landforms of Deposition – ESTUARIES
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