River Landscapes » Meanders, Ox-bow Lakes, Levees and Flood Plains

Meanders

As a river moves into its middle and lower course, it stops flowing in a straight line. It starts to swing from side to side in large bends. These bends are called meanders. You can see them clearly on a map or in a photo taken from above.

A meander is shaped by erosion and deposition working at the same time on opposite sides of the river.

Key terms:

• Meander: a large bend in a river.
• River cliff: a steep bank on the outside of a meander, made by erosion.
• Slip-off slope: a gentle, gravelly bank on the inside of a meander, made by deposition.

How a meander forms

Because the outside bank is eroded and the inside bank is built up, the bend becomes more curved and slowly moves sideways across the valley. The river's channel gets longer and more winding.

Ox-bow lakes

Meanders do not stay the same for ever. The outside bends keep eroding, so two neighbouring bends move closer together. The narrow piece of land between them is called the neck.

An ox-bow lake forms in these steps:

1. Erosion on the outside of the two bends makes the neck narrower and narrower.
2. During a flood, the river breaks through the neck and takes the shortest route.
3. Deposition seals off the ends of the old bend, because the water there is now slow.
4. The old bend is left as a curved lake that is cut off from the river.

Key term: ox-bow lake: a curved lake left when a meander is cut off from the river.

Over many years an ox-bow lake slowly dries out and fills with silt and plants. This leaves a curved marshy area that shows where the river used to flow.

Flood plains

A flood plain is the flat, wide area of land on either side of a river that is covered with water when the river floods. It forms in the lower course in two ways:

• Erosion: as meanders swing from side to side, they wear away the valley sides. This makes the valley floor wide and flat.
• Deposition: every time the river floods, the water spreads out and slows down. It drops the silt and mud it was carrying. Layer upon layer of silt builds up the flat land.

Key terms:

• Flood plain: the flat area next to a river that floods.
• Silt: very fine sediment, smaller than sand.

Flood plains have fertile soil because of the silt left by floods. This is why they are often used for farming.

Levees

Levees are natural raised banks along the edges of a river. They form in the lower course during floods.

1. When a river floods, it spills over its banks and its speed drops.
2. The heaviest material is dropped first, right next to the channel.
3. The lighter silt is carried further away and spreads over the flood plain.
4. After many floods, the ground next to the river is built up higher than the flat land beyond.

The result is a long, low ridge on each side of the river. Levees help to hold the river in its channel, so it takes a bigger flood to spill over.

Estuaries

An estuary is the wide, tidal part of a river where it meets the sea. The river is very slow here, and the tide pushes water back up the river twice a day.

Deposition is very important in an estuary:

• The river slows down, so it drops the fine silt and clay it has carried from upstream.
• The tide also brings in sediment from the sea and drops it as the water slows.
• When the tide goes out, the deposited sediment is left showing as wide areas of mud flats.

The Severn estuary in the south west of the UK is a large example, with wide mud flats at low tide.

Key terms:

• Estuary: the wide, tidal mouth of a river where it meets the sea.
• Mud flats: areas of mud left uncovered at low tide.

Putting it together

Exam-style question

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

(b) Explain how an ox-bow lake is formed. [4 marks]

(c) Explain how levees are formed on a river's flood plain. [3 marks]