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ALLUVIUM
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ALLUVIUM , See also:soil or See also:land deposited by See also:running See also:water. All streams, from the tiniest rill to the greatest See also:river, are continually engaged in transporting downstream solid particles of See also:rock, the product of weathering agencies in the See also:area which they drain. Since the capacity of a stream to carry See also:matter in suspension is proportional to its velocity, it follows that any circumstance tending to retard the See also:rate of flow will induce deposition. Thus a fall in the gradient at any point in the course of a stream; any snag, See also:projection or See also:dam, impeding the current; the reduced velocity caused by the overflowing of streams in See also:flood and the dissipation of their See also:energy where they enter a See also:lake or the See also:sea, are all contributing causes to alluviation, or the deposition of stream-See also:borne sediment. It is evident from the foregoing remarks, that while even the smallest stream may make deposits of alluvial See also:character it is in the flood-plains and deltas of large See also:rivers that the See also:great alluvial deposits are to be found. The finer material constituting alluvium, often described as " silt," is See also:sand and mud. Although it may be exceedingly See also:fine-grained, there is usually very little See also:clay in alluvium. The larger materials include See also:gravel of all degrees of coarseness; carbonaceous matter is often an important See also:element. The amount of solid matter borne by large streams is enormous; many rivers derive their names from the See also:colour thereby imparted to the water, e.g. Hwang Ho = Yellow river, See also:Missouri =Big Muddy, the Red river, &c. It has been estimated that the See also:Mississippi annually carries 4o6} million tons of sediment to the sea; the Hwang Ho 996 million tons; the Po67 million tons. Many shallow lakes have been completely filled with alluvium and their sites are now occupied by fertile plains; this See also:process may be seen in operation almost anywhere; a See also:good See also:illustration is the See also:delta of the See also:Rhone in Lake See also:Geneva. Alluvial deposits may be of great See also:size. The flood-See also:plain of the Mississippi has an area of 50,000 sq. m.; the great delta of the See also:Ganges and See also:Brahmaputra has an area of about 6o,000 sq. m.; that of the Hwang Ho reaches out 300 M. into the sea and has a coastal border of about 400 M. Old alluvial deposits are See also:left high above the existing level of many rivers, in the See also:form of " terraces " of gravel and See also:loam, the streams to which these owe their existence having modified their courses and cut deeper channels; such are the alluvial gravels and See also:brick-earths upon which much of " greater See also:London " is built. In some regions alluvial deposits are the resting places of gemstones and See also:gold, See also:platinum, &c.; it is from these deposits that the largest nuggets of gold have been obtained. Alluvial soils are almost invariably of great fertility; it is due to the alluvial mud annually deposited by the See also:Nile that the dwellers in See also:Egypt have been able to grow their crops for over 4000 years without artificial fertilization. Additional information and CommentsThere are no comments yet for this article.
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