. Elementary physical geography . Fig. 159. in position. The river is eating its way into the floodplainon the concave bank, and depositing upon the convex bank (Figs. 158-160, in whichthe dotted areas repre-sent sand deposits). Thisprocess of change oftencauses the river to cutacross the narrow neck ofland between two partsof the curve, and thusshorten the course andabandon the old delta and floodplainregions, these are knownas oxbow cut-offs () ; and after they areformed, they become crescent-shaped lakes, and sometimesthey are almost complete circles. In the course of time t
. Elementary physical geography . Fig. 159. in position. The river is eating its way into the floodplainon the concave bank, and depositing upon the convex bank (Figs. 158-160, in whichthe dotted areas repre-sent sand deposits). Thisprocess of change oftencauses the river to cutacross the narrow neck ofland between two partsof the curve, and thusshorten the course andabandon the old delta and floodplainregions, these are knownas oxbow cut-offs () ; and after they areformed, they become crescent-shaped lakes, and sometimesthey are almost complete circles. In the course of time theselakes are destroyedby being filled withsediment when thestream is in flood,and when the flood-plain is submergedbeneath the riverwater (Fig. 160). These greatfloodplains are con-stantly beingraised by the de-posit of sediment;and the time of their formation is that of the flood stage ofthe stream, when it is no longer confined to its channel, but. Fig. 160. DELTAS, FLOODPLAINS, WATEBFALLS, ETC. 293 overflows and submerges the great level tracts on either is being deposited from this great expanse of water,because the velocity is decreased in these shallow areas. Itis to prevent this flooding that the levee banks are built onthe margin of the floodplain of the Mississippi. These banksare built to a sufficient height to shut out the high waterfrom the flood-plains. While the streamis constantly atwork building upits floodplain dur-ing floods, by itsmeandering it isconstantly at workremoving por-tions; and so thereis a process of in-termittent move-ment of sediment,from up streamdown toward themouth. It is de-posited duringflood ; later it maybe attacked by the lateral cutting of the stream, and thenit is carried a step down stream, perhaps to be depositedagain, and then after awhile to again start in movement. Upon a floodplain the tributaries to a river enter themain stream at very acute angles. The slope is so gen
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