The flowing water (generally rivers and streams) carries significant amount of sediments with it. This sediment is obtained as a result of erosion by rapidly moving water (a typical feature of upper reaches of a river), which is sufficient to keep the sediments suspended in water. Over time, the water loses much of its speed, due to lack of gradient, too much sediment load, etc. When this happens, the sediment mass and friction is enough to overcome the flow and sediment deposition takes place. When the water body is entering a larger water body, this deposition is commonly known as Delta. When the flowing stream suddenly changes slope (mountainous stream flowing on flat plains), sediment deposition forms alluvial fans.
The deltas are extremely rich in nutrients and are highly productive areas. They cause the river to split into a number of smaller streams, each headed for the ocean.
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