Since a stream’s capacity to transport its load depends on its velocity and volume of water, any reduction in either of these elements will naturally cause a stream to deposit at least a part of its load. However, in case a stream is not carrying the maximum load, a reduction in velocity or discharge will not cause deposition.

There are certain places where a stream is bound to deposit its load. Such places are on the inside of the bends or meanders in the stream, where a stream’s velocity is reduced. In addition, flood plains and deltas are the places where stream deposition takes place.

Wherever there is a major break in the slopes of the channel such as, where the stream leaves the mountains and enters into the plain, deposition of stream load is a common feature. Such deposits are widespread along the flanks of high mountains, where they are called alluvial fans.

Actually alluvial fans are fan-shaped deposits of alluvium that range from several hundred meters to tens of kilometers across. Small, swift tributary streams may deposit small alluvial fans and cones in the valleys of larger but more sluggish streams which the main streams cannot carry away.

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Irrespective of the size and shape of the deposited material, the fluvial deposit is called alluvium. There is one distinguishing feature of the alluvium in which a stream deposition is characterized by sorting of materials according to their size.

That is why in alluvium one can see distinct layers of silt, sand and gravel. Reduction in velocity compels a stream to sort out rock pieces by selectively picking on the basis of the size of material as bed load. That is why there are alternating layers of coarse and fine sediments deposited by a stream.