When the flow of a river slows down, the loose soil carried by the water begins to pile up on the bottom of the riverbed. As a result, the river becomes shallower. This can cause rivers to flood. 

When water from a river floods over its banks, the loose, rich silt that is carried by the water also spreads to the surrounding plains. As a result, the soil of the surrounding region gains nutrients and becomes more suitable for agriculture. This is why people have inhabited flood plains since the dawn of history.

When a river meets the sea, its flow stops. All the materials that the river water has carried settle on the bottom of the seashore, forming a delta.

Gradually, the materials carried by the river to the delta form layers. These layers slowly accumulate and grow, until they finally reach the water's surface. This results in the formation of islands and sandbanks, as the shoreline moves further towards the sea. 

River deltas, like flood plains, are very suitable for agriculture due to the high nutrient concentration of their soil.

A satellite image: The delta of the river Danube is the second largest in Europe. The Danube carries large amounts of earth to the Black Sea.
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