Middle course landforms
Oxbow lakes and meanders are found in the middle course of the river (long profile) and are formed as a result if erosion and deposition.
The formation of meanders leads eventually to the development of oxbow lakes
Video showing the features of the middle course of the River Tay –
http://www.bbc.co.uk/learningzone/clips/river-tay-middle-course/4312.html
Video on meanders –
http://www.bbc.co.uk/learningzone/clips/rivers-meanders/403.html
Video on oxbow lakes –
http://www.bbc.co.uk/learningzone/clips/rivers-oxbow-lakes/405.html
Picture of a meander on the left and an ox-bow lake on the right
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Meanders
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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 bank and the formation of a steep sided river cliff. 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 or point bar. The water in a meander flows in a corkscrew like movement as it moves from the inside of the bend towards the outside of the bend. This is called helicoidal flow.
Remember – a meander is asymmetrical in cross-section (see diagram to the below). It is deeper on the outer bend (due to greater erosion) and shallower on the inside bend (an area of deposition).
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Oxbow lakes
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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 narrower.
Eventually due to the narrowing of the neck, the two outer bends meet and the river cuts through the neck of the meander usually during a flood event when the energy in the river is at its highest. 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 known as a meander scar.