floodplain(redirected from Flood-plain)
Also found in: Dictionary, Thesaurus.
the part of a valley floor that is flooded at high water but is above the low-water stage. A floodplain consists of two parts: a base of channel aluvium underlying a flood alluvial layer formed by the annual or less frequent deposition of silt by floodwater. Occasionally the foundation, composed of bedrock or more ancient alluvium, is exposed.
The most intensive accumulation of coarse-grained alluvium, forming crests and ridges, occurs in the parts of the floodplain closest to the channel. Farther into the floodplain fine-grained alluvium settles. Only silt particles are carried close to the high bank, where the floodplain is lower and swampy (terrace flood-plain).
As sediment accumulates on the surface of the floodplain, the banks of some sections are continuously undercut by the river flow, and bars are built up in other sections. As a result, the contours of floodplain change constantly. On the surface of the floodplain there are numerous hollows, which are traces of meanders (oxbows) and branches of the river that formed cutoffs and alternate with levees. The hollows attest to the channel’s meandering. In the parts of the valley where the alluvium accumulation primarily occurs, that is, in the lower course of the river, a solid channel levee often forms along the channel. The surface of the floodplain is frequently divided by a low scarp into a high floodplain and a lower one.
The floodplain is formed when the valley broadens as the result of lateral shifts in the river channel. Vast floodplains, reaching 20–40 km in width, are characteristic of large, level rivers with uneven flow. If the river flows in a tectonic trough, the width of the floodplain is determined by the dimensions of the trough.
The vegetation plays an important role in determining the relief of a floodplain. It helps prevent erosion and promotes the accumulation of alluvium. The center of the floodplain and the terrace floodplain are usually covered by trees and shrubs. Some floodplains are developed for agricultural use. In such cases, a large part of the land is used for meadows, which are among the best feed lands. The floodplain soils, regularly replenished with organic silts, are very fertile. During the flooded period, the floodplain is a spawning ground for fish.
Placer deposits of such useful minerals as gold, platinum, and cassiterite are found in the alluvial deposits making up the flood-plains of small mountain rivers. The sand and gravel are used as building materials.