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I know that when some rivers (such as the Nile) flow out to sea, they break up into several smaller channels. But I was wondering about the necessary features of the land and water for it to occur.

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The reason why some rivers form a delta (Nile, Ganges, Mississippi, etc.) and some other form an estuary (Thames, Seine, etc.) is in the balance between sedimentary deposit from the river flow and sediment removal from the sea waters.

When this balance is positive (the river deposits faster than the sea can remove) a delta is formed, while when the sea removes faster than the river can deposit an estuary is formed.

On the sea side this is influenced mostly by tides and currents, while on the river side this is influenced by all the factor regulating the amount of transported sediments.

I.e. Nile has a delta, but since Aswan dam has been built, the amount of sediments transported to the sea has greatly decreased: in return this has resulted in the delta starting to be eroded by the Mediterranean sea.

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If you mean geographical requirements, then all you need is,

  1. A body of water (preferably salty)
  2. Some land (the soily or sandy type) that is lower or just above sea level (optional)
  3. A river (the fresh kind)

If you put these together, and the river is depositing sediment from its bed faster than the ocean/sea can take it away, then boom, you got yourself one heckuva river delta. Now, if your factors do not overlap correctly, then you may end up with an estuary instead. that, you do not want.

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