River Landforms » Lower Course River Landforms

The lower course: a wide, slow river with a heavy load

In the middle and lower course the valley is wide and flat. The river is deep and carries a large load of fine material. The main change from the upper course is that the river now erodes sideways more than downwards, and it deposits more often. You met these ideas in the lessons on how rivers change downstream and on river erosion, transportation and deposition. Here we use them to explain the landforms.

Key terms:

• Meander: a large, curving bend in a river.
• Thalweg: the line of fastest flow in a river, where the water is deepest. It swings from side to side along the channel.
• River cliff: a steep, eroded bank on the outside of a meander bend.
• Slip-off slope: a gentle slope of deposited sand and gravel on the inside of a meander bend.
• Oxbow lake: a curved lake left behind when a meander is cut off from the river.
• Floodplain: the flat, wide area of land either side of a river that is covered by water when the river floods.
• Levee: a natural raised bank along the edge of a river channel, built from deposited material.
• Delta: a low, flat area of land built from deposited sediment where a river meets the sea or a lake.

Meanders

A river in its lower course does not flow in a straight line. It swings from side to side in bends called meanders. The thalweg moves towards the outside of each bend, so the water there is faster and deeper. On the inside of the bend the water is slower and shallower.

How to explain it in the exam, as numbered steps:

1. The thalweg swings towards the outside of a bend, so the water is fastest and deepest there.
2. Lateral erosion undercuts the outside bank, which collapses to form a steep river cliff.
3. On the inside of the bend the water is slower and shallower, so material is deposited.
4. The deposited material builds up as a gentle slip-off slope.
5. Over time the bend becomes larger and more curved.

Sketch labels: a curving river, an arrow for the thalweg on the outside, "river cliff, steep, erosion" on the outside bank, "slip-off slope, gentle, deposition" on the inside bank. A cross-section through a meander is lopsided: deep and steep on one side, shallow and gentle on the other.

Oxbow lakes

A meander can grow until it almost loops back on itself. The narrow strip of land between two bends is the neck. An oxbow lake forms when the river takes a shortcut across the neck.

1. Lateral erosion on the outsides of two bends makes the meanders bigger.
2. The neck of land between them becomes narrower and narrower.
3. During a flood the river breaks through the neck and takes the shorter, straighter route.
4. Deposition seals off the ends of the old bend, because the water in it is slow.
5. The old meander is left as a curved lake, an oxbow lake. Over time it can dry out into a marsh.

Sketch labels: the main river now straight, the neck where it broke through, the curved oxbow lake, and the deposited material sealing each end.

Floodplains

A floodplain is the wide, flat valley floor beside the river. Two things build it:

• Lateral erosion: as meanders swing sideways across the valley they cut away the valley sides and widen the valley floor.
• Deposition: each time the river floods, it leaves behind a layer of fine silt and mud (alluvium). Layer upon layer builds up the flat surface.

Floodplains have fertile soil, which is why many people farm them.

Levees

Levees form on the edge of the channel when the river floods.

1. The river floods and water spills over its banks.
2. The water spreads out and slows down suddenly, so it loses energy.
3. The heaviest, coarsest material (sand and gravel) is deposited first, right next to the channel.
4. Finer silt and clay are carried further across the floodplain.
5. After many floods the coarse material builds up into raised banks along the river, called levees.

Sketch labels: river channel in the middle, a raised bank on each side labelled "levee, coarse material", and the flat floodplain beyond with "fine silt".

Deltas

A delta forms where a river flows into a standing body of water, a sea or a lake. The river slows down as it enters, loses energy, and deposits its load. The sediment builds up faster than waves and tides can remove it, so the land grows outwards.

1. The river reaches the sea or a lake and its flow slows.
2. The river deposits its load, with the coarsest material dropped first.
3. Deposits build up and block the channel, so the river splits into many smaller channels.
4. Over many years the sediment builds up above water level to form a low, flat delta.

Two delta shapes you should know, in brief:

• Arcuate: a curved, fan-like shape, like the Nile delta.
• Bird's-foot: long fingers of land reaching out into the sea, like the Mississippi delta.

Exam-style question

Fig. 1.1 is a made-up sketch of a river in its lower course. It shows a meander with a steep bank on the outer bend and a gentle bank on the inner bend. A smaller curved lake lies beside the river, and the land either side of the river is flat.

(a) Identify the landform on the inside of the meander bend. [1 mark]

(b) Describe two features of a delta. [2 marks]

(c) Explain how levees are formed on a floodplain. [3 marks]

(d) Explain how a meander develops and how it can later become an oxbow lake. [6 marks]