Description: Deltas are deposits of sediment found in the mouth of a river. Marine deltas are formed where rivers flow into seas and lacustrine deltas form where rivers flow into lakes.
Formation: Rivers deposit large volumes of sediment in the mouth for two reasons:
1. Coming in contact with the sea causes the river to lose energy and slow down. This leaves it unable to carry the large suspended load it gathered during upstream erosion.
2. Flocculation: As fresh water (river) mixes with salt water (sea) a chemical reaction causes sediment particles to stick together and fall onto the riverbed, where they settle.
In some places, the currents and tides are weak and unable to carry away the deposited sediments. When this happens, sediments build up in layers called beds, forming new fertile patches of land called deltas (Fig. 8.21).
The largest and heaviest sediments are the first to be deposited as the river loses energy. They form the topset bed which extends the shortest distance out into the sea or lake.
Medium-grained sediments travel a little further before they are deposited on the foreset bed.
The finest/lightest sediments travel the furthest before they are deposited on the bottomset bed.
As the sediments build up, the river may be forced to split into a number of different channels called distributaries. Each of the three types of marine delta has a different shape, depending on how it was formed.
Land Sea or lake River channel DID YOU KNOW?
The Yellow River in China deposits approximately 1.6 billion tonnes of sediment into its delta each year.
Topset bed
Bedrock Fig. 8.21 Formation of a delta.
Arcuate delta Example: Nile Delta, Egypt
Arcuate deltas are fan-shaped, with the widest part of the fan facing the sea. They form where the river water is as dense as the sea water. Arcuate deltas are made of coarse materials such as gravel and sand and their distributaries form dendritic drainage patterns (Fig. 8.22).
Fig. 8.22(b) Satellite image of the Nile Delta. 117
Foreset bed Bottomset bed Distributary channels Fig. 8.22(a) Arcuate delta.
