DEFINITION OF FLOODPLAIN

Generally flat or low-lying areas that are adjacent

and run parallel to river channels and are covered by

water during flood stages of the river. The floodplain of a

river is built by alluvium carried by the river and deposited in

overbank environments, forming layers of silt, clay, and sand.

Overbank deposits are typically cut by many narrow elongate

channels filled by sands and gravels, marking places where

the river formerly flowed and meandered away during the

course of river evolution. Active and buried channels are typically

separated from the floodplain deposits by a sandy or

gravelly levee deposit, formed during flood stages of the river.

These form because the velocity of floodwater decreases

rapidly as it moves out of the channel, causing the current to

drop heavy coarse-grained material near the river, forming a

levee. Floodplains are also found around some lake basins

that experience flood stages.

Floodplains are being increasingly built upon, creating

potential and real hazards during floods. Floodplains are

characterized by fertile soils and make excellent farmlands,

which are nourished by yearly, decadal, and centurial floods,

whereas buildings, towns, and cities have a much more difficult

time dealing with periodic flooding.

See also FLOOD; GEOMORPHOLOGY; RIVER SYSTEM.

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