FLOOD PLAIN, the land-form term for a plain made of sediment deposited by a river. When the gradient of a river becomes very slight, it is unable to carry all the material brought downstream and consequently the lower valley becomes filled with alluvial deposits; in times of flood the river torrent tears off and transports an unusually large quantity of sediment ; in the flooded lower valley the rush of water is checked, and the stream in consequence drops its extra load. These flood plains are some times of great extent. That of the Mississippi below Ohio has an occasional width of 8o m. with a total area estimated at 5o,000 sq.m. Flood plains may be caused by a graded river meandering from side to side, and thus widening its valley and ultimately covering the widened valley with sediment. Any sudden increase in the river's velocity and volume such as by cutting into another stream (piracy) or by cutting through a barrier near its head waters, may result in a flood plain. Any obstruction across a river's course, such as a band of hard rock, or ice flow, may form a flood plain behind it, but it is most commonly found in the lower reaches of such large rivers as the Rhine, Nile or Mis sissippi, where there are occasional floods and usually large amounts of sediment. The flood-plain is usually bounded by levees (q.v.). Sections of the Missouri flood plain made by the United States Geological Survey show a great variety of material of vary ing depths and of varying coarseness, from heavy gravel through fine sand, to fine silt, and it is probable that any section of such an alluvial plain would show similar deposits.
The flood plain during its formation is characterized by marshes, meandering or anastomosing streams, oxbow lakes and bayous (q.v.), and is occasionally completely drowned. When the drain age system is entirely diverted owing to any cause, the flood plain may become a smooth area of great fertility, similar in appearance to the floor of an old lake, from which it differs, however, inas much as it is not quite flat. It has a gentle slope along the original down-stream course, and also usually from the edges towards the centre.