A flood plain can form next to any river or stream.
A floodplain is a natural valley floor composed of sediment that is subject to flooding from an adjacent river or stream. The body of water may be of any size. However, areas near large rivers have larger floodplains. The Mississippi River, for instance, has an enormous floodplain area at nearly 30 million acres.
Flooding
Flooding is the result of rainfall that exceeds the absorptive capacity of soil and the flow capacity of a channel. Terms like 5-year flood or 25-year flood describe the frequency that a flood of a certain size will usually appear. Streams will generally exceed the mean annual flood level once every 2.33 years.
Deposit Types
Deposits are usually laid either laterally or vertically. A lateral accretion is formed when deposits migrate to the sides of the channel. On the other hand, sediment may be deposited vertically by overbank flooding. The difference of building deposits up or forming laterally is usually the difference between aggradation and erosion.
Aggradation
An aggradational flood plain is the most common type. It is formed when thin layers of sediment from other sources are laid down upon the valley floor. The deposits are typically loose and unconsolidated particles of smaller material like silt and clay.
Erosion
Another way of forming a floodplain is through erosion, which is the process when rivers and streams cut laterally into the banks, carving out material and redepositing it downstream. The rate at which erosion occurs may result in a very narrow or broad floodplain.
Significance
Floodplains can absorb water, which keeps rivers and streams from overflowing and reduces the speed of water that may otherwise cause erosion and flooding downstream. Floodplains may also minimize water pollution and provide a suitable habitat for a variety of plants and animals.
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