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. 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 may form a flood plain simply by filling up its valley (alluviation only).
Any obstruction across a river's course, such as a band of hard rock, may form a flood plain behind it, and indeed anything that checks a river's course and causes it to drop its load will tend to form a flood plain. Still, flood plains are most commonly found near the mouth of a large river, such as the
Rhine, the
Nile, or the Mississippi, where there are occasional floods and the river usually carries a large amount of sediment. "Levees" are formed, inside which the river usually flows, gradually raising its bed above the surrounding plain. Occasional breaches during floods cause the overloaded stream to spread in a great lake over the surrounding country, where the silt covers the ground in consequence.
Sections of the
Missouri flood plain taken by the
United States geological survey show a great variety of material of varying coarseness, the stream bed being scoured at one place, and filled at another by currents and floods of varying swiftness, so that sometimes the deposits are of coarse gravel, sometimes of fine sand or of fine silt, and it is probable that any section of such an alluvial plain would show deposits of a similar character.
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.
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