Having studied the characteristics of a river in its upper reaches we now need to follow the river as it enters its middle course. Here 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.
Meander Formation
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 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 alow 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). It is deeper on the outer bend (due to greater erosion) and shallower on the inside bend(an area of deposition).
Over time meanders gradually change shape and migrate across the floodplain. As they do so meander bends becomes pronounced due to further lateral erosion and eventually an ox-bow lake may form.
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 narrower.
- 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.
Visualizing Meander formation:
This excellent animation (from Cleo Net) looks at the process of meander formation and how meander develop overtime becoming more sinuous, resulting in the narrowing of the meander neck and the formation of an ox-bow lake.
Another excellent animation - this one focuses particularly on the development of an ox-bow lake as a meander continues to grow.
Key Terms Check:
- Meander - a bend in a river
- River Cliff - a small cliff formed on the outside of a meander bend due to erosion in this high energy zone.
- Slip off Slope - a small beach found on the inside of a meander bend where deposition has occured in the low energy zone.
- Ox-bow lake - a lake formed when the continued narrowing of a meander neck results in the eventual cut through of the neck as two outer bends join. This result in the straightening of the river channel and the old meander bend becomes cut off forming an ox-bow lake.
- Meander scar - feature left behind when the water in an ox-bow lake dries up.