2.9.3 Lower Course

In the lower course,

• The river channel is the widest and the deepest.
• It has the largest volume of water and load because of the numerous tributaries flowing into it.
• There is lateral erosion along this coures.

• Levees
• Braided Channels
• Deltas
• Ox-bow Lakes

2.9.3.1 Levees   [fig 2.9.3(a)]
In the winter, when the river volume is at its highest, the river is prone to flooding. As the river overflows its banks, friction with the floodplain slows down the flow. The loss of energy means load is deposited. The deposition is graded - this means that the larger particles are dropped first (being the heaviest) near the river bank edge and the smaller particles are taken further along the floodplain. The larger particles build up over repeated floodings to create a leveé, which increases the capacity of the river.

However, in the summer when the river volume and energy is at its lowest, deposition occurs in the river channel, raising the bed. This means that the capacity of the river is lowered and flooding again will occur in the winter months, creating bigger leveés. This cycle raises the river higher than the landscape over time. The theoretical reason for this is shown at the bottom.

Fig 2.9.3(a)

 

2.9.3.2 Braided Channels   [fig 2.9.3(b)]
Braided channels are formed in the summer months when the river volume and energy are lowest. Deposition occurs in the channel of the load carried and these build up to form obstructions. The river has to divert its flow around these obstructions and this is called a braided channel.

In the winter, when the river is at a higher level, the silt may either be removed or the river will be high enough to flow over the top of the obstructions.

Fig 2.9.3(b)

2.9.3.3 Deltas  [fig 2.9.3(c)]

Deltas are formed when the river meets the sea. The sea does not flow, so the river suddenly stops upon meeting the sea and loses its energy to carry load. The load is then deposited on the sea bed and over time, this builds up to form a new piece of land in the river mouth. The river then has to divert its flow into smaller 'distributaries' to reach the sea again. When it does so, more load is deposited and this builds the new delta out into the sea.

For a delta to survive, the river must bring a constant supply of material to deposit on the delta or it will be eroded away by the sea. Many human-made dams on rivers stop silt getting to deltas in this way.

Fig 2.9.3(c)

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