Deposition
When a river loses energy, it will drop or deposit some of the material it is carrying.
Deposition may take place when a river enters an area of shallow water or when the volume of water decreases - for example, after a flood or during times of drought.
Deposition is common towards the end of a river's journey, at the mouth.
Deposition at the mouth of a river can form deltas - for example, the Mississippi Delta.