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Floodplains

A floodplain is an area of land which is covered in water when the river bursts its banks.

They form due to both erosion and deposition. Erosion removes any interlocking spurs, creating a wide, flat area on either side of the river. During a flood, material being carried by the river is deposited (as the river loses its speed and energy to transport material). Over time, the height of the floodplain increases as material is deposited on either side of the river.

Floodplains are often agricultural land, as the area is very fertile because it's made up of alluvium (deposited silt from a river flood). The floodplain is often a wide, flat area caused by meanders shifting along the valley.

Levees

• Levees occur in the lower course of a river when there is an increase in the volume of water flowing downstream and flooding occurs.
• Sediment that has been eroded further upstream is transported downstream.
• When the river floods, the sediment spreads out across the floodplain.
• When a flood occurs, the river loses energy. The largest material is deposited first on the sides of the river banks and smaller material further away.
• After many floods, the sediment builds up to increase the height of the river banks, meaning that the channel can carry more water (a greater discharge) and flooding is less likely to occur in the future.

Estuaries

An estuary is where the river meets the sea. The river here is tidal and when the sea retreats the volume of the water in the estuary is reduced. When there is less water, the river deposits silt to form mudflats which are an important habitat for wildlife.

Deltas

Deltas are found at the mouth of a river, as it enters a sea or lake. The sediments the river has been transporting by bedload and suspended load are deposited as velocity decreases as the river flows into the sea. Deltas are usually made up of three types of deposit:

• The larger and heavier sediments are the first to be deposited as the river loses its energy. These form the topset beds.
• Medium-sized sediments travel little further before they are deposited, forming the foreset beds.
• The finest sediments travel furthest into the lake before deposition and form the bottomset beds.