The international geography . he sandand mud in banks or bars, as in the Thames or Tay, or spread it overso great an expanse as to produce no obstruction, as in the Firth ofForth. But all great rivers which enter a lake and many which enterthe sea deposit their sediment in the form of a delta, which growsgradually seaward, and the water crosses it in many and variable channels(Figs. 362 and 441). The margin is often lined with lagoons separated fromthe sea by bars of mud; but the delta itself is a flat expanse of veryfine soil. The effect of floods in rivers flowing over a nearly flat plainis
The international geography . he sandand mud in banks or bars, as in the Thames or Tay, or spread it overso great an expanse as to produce no obstruction, as in the Firth ofForth. But all great rivers which enter a lake and many which enterthe sea deposit their sediment in the form of a delta, which growsgradually seaward, and the water crosses it in many and variable channels(Figs. 362 and 441). The margin is often lined with lagoons separated fromthe sea by bars of mud; but the delta itself is a flat expanse of veryfine soil. The effect of floods in rivers flowing over a nearly flat plainis to cause a deposit of alluvium along the sides of the stream, and aconsequent silting-up of the bed, which results in the river flowing atlast along an embankment above the general level of the plain andsloping gently on both sides down from the river. When a flood occursthe banks are apt to burst, and the river descends upon the low groundwith tremendous force, often forming a new channel for itself to the sea. Land Forms 57. Fig. 33.—r/je Alluvial Fan of tlie 111opposite Leiik in the Rhone at every 100 feet. This frequently happens on a small scale ir. the lower Mississippi, andto a far greater degree in the Hwang-ho (Fig. 264). The flood-plainsand deltas of great rivers in latitudes which ensure a genial climate arcthe most fertile lands in the world, and have been the cradles of all thegreat nations of the ancient East—Assyria, Eg\^pt, China, and India. When a stream from a mountain valley flows out on to a plain, ora flat-floored longitudinal valley, thesudden change of slope causes the depo-sition of the detritus it carries down inthe form of a fan of alluvial soil, overwhich the stream usually flows in severalbranches. The alluvial fan is a form ofaccumulation intermediate between thedelta laid down in still water and thescree or talus of detached rock fragmentswhich grows, sometimes as a magnificentsweep of boulders, at the base of a lineof cliffs. I
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectgeography, bookyear19
