W-4


 * __What landforms result from both erosion and deposition?__**

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 - [] Video on meanders - [] Video on oxbow lakes - []

Photograph of a river meander and an oxbow lake
 * __Formation of a meander__**

Picture of a meander:
 * 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**.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**.

Remember - a meander is asymmetrical in cross-section (see diagram above). It is deeper on the outer bend (due to greater erosion) and shallower on the inside bend (an area of deposition). [|**http://www.cleo.net.uk/resources/displayframe.php?src=309/consultants_resources%2F_files%2Fmeander4.swf**]
 * __Formation of an oxbow lake__**
 * __Oxbow lake picture/diagram of formation__**
 * 1) 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.
 * 2) 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.
 * 3) Deposition gradually seals off the old meander bend forming a new straighter river channel.
 * 4) Due to deposition the old meander bend is left isolated from the main channel as an ox-bow lake.
 * 5) 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.
 * __Key terms__**


 * Meander**: a bend or curve in the river cannel, often becoming sinuous where the loops are exaggerated


 * Oxbow lake**: a horseshoe or semi-circular area that represents the former course of a meander. Oxbow lakes are cut off from a supply of water and so will eventually become dry.