Deepening erosion becomes secondary and the deposition becomes primary activity of the stream. This condition is found when the stream leaves the mountains and enters the plain. The stream tries to bring it in a state of equilibrium. Many landforms are formed in this process. A study of these landforms provides an opportunity to study the nature of the mature stage.
Alluvial Cones and Alluvial Fans. When a stream leaves a mountain gorge or area and enters a plain, its slope is abruptly reduced. This reduces the energy of the stream or in other words its load carrying capacity is reduced abruptly. As the river, which had a high speed while flowing in the mountains, has a lot of sediment. The stream, due to loss of slope, deposits the sediment. This process helps the stream to extend its graded condition upstream. It also means that the stream wants to increase its slope by the deposition of sediment so that its load carrying capacity is increased. The deposited sediment spreads in a semi-circular arc outside the mouth of the gorge. The section of this sediment spreading in a semicircular form is known as Alluvial Fans.
Many times these alluvial fans increase in height by the deposition of the sediment of the fans and their slope becomes steeper. Such formations are called Alluvial Cones. Usually the alluvial cones are found in semi-arid areas.
The alluvial fans have an extent of half a meter to a few km. Sometimes parallel streams form their alluvial fans close to one another. After sometime these fans coalesce (merge) into one another and form a composite fan called Piedmont Alluvial Fan. Sometimes several individual fans merge in a line mountain front to form a bahada (or: bajada - a broad, sloping depositional deposit caused by the coalescing of alluvial fans), which are seen mainly in arid regions.
A flood-plain is the most common depositional feature created by all sizes of river, be they very large or just small brooks. The alluvium in a flood-plain is composed of several kinds of deposit. Principally, the flood-plain is built up laterally by channel deposits in a coalescing series of bars composed of sand and gravel scoured from the outside of meander bends upstream; and it is also built up vertically by aggradation of overbank deposits during flooding. Floods deposit coarse material as levees near the channel, and finer silt and clay over the rest of the flood-plain surface, covering bars previously formed by the river.
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