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FLOOD PLAIN
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See also:FLOOD See also:PLAIN , the See also:term in See also:physical See also:geography for a plain formed of sediment dropped by a See also: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 See also:lower portion of the river valley becomes filled with alluvial deposits; and since in times of flood the See also:rush of See also:water in the high regions tears off and carries down a greater quantity of sediment than usual, the river spreads this also over the lower valley where the plain is flooded, because the rush of water is checked, and the stream in consequence drops its extra load. These flood plains are sometimes of See also:great extent. That of the See also:Mississippi below See also:Ohio has a width of from 20 to 8o in., and its whole extent has been estimated at 50,000 sq. m. Flood plains may be the result of planation, with aggradation, that is, they may be due to a graded river working in meanders from See also:side to side, widening its valley by this See also:process and covering the widened valley with sediment. Or the stream by cutting into another stream (piracy), by cutting through a barrier near its See also:head See also:waters, by entering a region of looser or softer See also:rock. and by glacial drainage, may See also:form a flood plain simply by filling up its valley (alluviation only). Any obstruction across a river's course, such as a See also:band of hard rock, may form a flood plain behind it, and indeed anything which checks a river's course and causes it to drop its load will tend to form a flood plain; but it is most commonly found near the mouth of a large river, such as the See also:Rhine, the See also: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 See also:bed above the surrounding plain. Occasional breaches during floods cause the overloaded stream to spread in a great See also:lake over the surrounding See also:country, where the silt covers the ground in consequence. Sections of the See also:Missouri flood plain made by the See also:United States See also:geological survey show a great variety of material of varying coarseness, the stream bed being scoured at one See also:place, and filled at another by currents and floods of varying swiftness, so that sometimes the deposits are of coarse See also:gravel, sometimes of See also:fine See also:sand, or of fine silt, and it is probable that any See also:section of such an alluvial plain would show deposits of a similar See also:character. The flood plain during its formation is marked by meandering, or anastomosing streams, ox-See also:bow lakes and bayous, marshes or stagnant pools, and is occasionally completely covered with water. When the drainage See also:system has ceased to See also:act or is entirely diverted owing to any cause, the flood plain may become a level See also:area of great fertility, similar in See also:appearance to the See also:floor of an old lake. The flood plain differs, however, inasmuch as it is not altogether See also:flat. It has a See also:gentle slope down-stream, and often for a distance from the sides towards the centre. Additional information and CommentsThere are no comments yet for this article.
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