Flood Plain

In geography, a flood plain is a plain formed of sediment, typically dropped by a river.

When the slope down, which a river runs has become very slight, it is unable to carry the sediment brought from higher regions nearer its source, and consequently the lower portion of the river valley becomes filled with alluvial deposits. Since, in times of flood, the rush of water in the high regions tears off and carries down a greater quantity of sediment resulting in planation, with aggradation.
10-Year Floodplain Diagram
That is, they may be due to a graded river working in meanders from side to side, widening its valley by this process and covering the widened valley with sediment. Or the stream -- by cutting into another stream (piracy), by cutting through a barrier near its head waters, by entering a region of looser or softer rock, and by glacial drainage -- may form a flood plain simply by filling up its valley (alluviation only).

Formation & Action of Water
The flood plain during its formation is marked by meandering or anastomosing streams, ox-bow lakes and bayous, marshes or stagnant pools, and is occasionally completely covered with water. When the drainage system has ceased to act or is entirely diverted for any reason, the flood plain may become a level area of great fertility, similar in appearance to the floor of an old lake. The flood plain differs, however, inasmuch as it is not altogether flat. It has a gentle slope down-stream, and often, for a distance from the side towards the center.

The action of water on the land and interaction of water with vegetation produce floodplains, which differ appreciably from one another and from uplands in their soils, drainage systems and vegetation. Beaches and small river valleys are usually easily recognizable as floodplains to people with a trained eye. Less obvious floodplains occur in dry washes and on alluvial fans in arid parts of the western United States, around prairie potholes, in areas subject to high groundwater levels, and in low lying areas where water may accumulate.

Flood Area & Length Between Floods
Floodplains are low areas subject to flooding from time to time. Most floodplains are adjacent to streams, lakes or oceans although almost any area can flood under the right circumstances. The amount of land inundated by a flood depends on the flood's magnitude. Approximately 7% of the nation's land area - an area almost as big as Texas - is subject to severe flooding.

Floodplains are designated by the size of the flood that will cover them. For example, the 10-year floodplain is the land covered by the 10-year flood and the 100-year floodplain is the land covered by the 100-year flood. The likelihood of property being flooded varies depending on how high it is above the stream. Buildings on the 10-year floodplain can be expected to flood on the average of once every 10 years while buildings on higher ground will be flooded less often.