In this stage the river channel has become much wider and deeper as the channel has been eroded and the river has been fed by many tributaries upstream. Consequently, despite the more gentle gradient the velocity of flow may be as fast as in the uplands. As well as changes in the river channel, its surrounding valley has also become wider and flatter in cross-section with a more extensive floodplain. One of the most distinctive features of the river in the middle course is its increased sinuousity. Unlike the relatively straight channel of the upper course, in the middle course there are many meanders (bends) in the river.
A meander in general is a bend in a sinuous watercourse. A meander is formed when the moving water in a stream erodes the outer banks and widens its valley. A stream of any volume may assume a meandering course, alternatively eroding sediments from the outside of a bend and depositing them on the inside. The result is a snaking pattern as the stream meanders back and forth across its down-valley axis. When a meander gets cut off from the main stream, an oxbow lake is formed. Over time meanders migrate downstream, sometimes in such a short time as to create civil engineering problems for local municipalities attempting to maintain stable roads and bridges.
Meanders form due to the greater volume of water carried by the river in lowland areas which results in lateral (sideways) erosion being more dominant than vertical erosion, causing the channel to cut into its banks forming meanders.
1. Water flows fastest on the outer bend of the river where the channel is deeper and there is less friction. This is due to water being flung towards the outer bend as it flows around the meander, this causes greater erosion which deepens the channel, in turn the reduction in friction and increase in energy results in greater erosion. This lateral erosion results in undercutting of the river bankand the formation of a steep sided river cliff.
2. In contrast, on the inner bend water is slow flowing, due to it being a low energy zone, deposition occurs resulting in a shallower channel. This increased friction further reduces the velocity (thus further reducing energy), encouraging further deposition. Over time a small beach of material builds up on the inner bend; this is called a slip-off slope.
Ox-Bow Lake formation
An oxbow lake is 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. This landform is called an oxbow lake for the distinctive curved shape, named after part of a yoke for oxen. In Australia, an oxbow lake is called a billabong, derived from an indigenous language. The word “oxbow” can also refer to a U-shaped bend in a river or stream, whether or not it is cut off from the main stream.
An oxbow lake is formed when a river creates a meander, due to the river’s eroding the banks through hydraulic action and abrasion/corrosion. After a long period of time, the meander becomes very curved, and eventually the neck of the meander will touch the opposite side and the river will cut through the neck, cutting off the meander to form the oxbow lake.
Here is a Diagrammetic Representation of Ox-Bow Lake Formation:
- As the outer banks of a meander continue to be eroded through processes such
as hydraulic action the neck of the meander becomes narrow and
- Eventually due to the narrowing of the neck, the two outer bends
meet and the river cuts through the neck of the meander. The water now
takes its shortest route rather than flowing around the bend.
- Deposition gradually seals off the old meander bend forming a
new straighter river channel.
- Due to deposition the old meander bend is left isolated from the main
channel as an ox-bow lake.
- Over time this feature may fill up with sediment and may gradually dry up
(except for periods of heavy rain). When the water dries up, the feature
left behind is knwon as a meander scar.
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