. Elementary physical geography . Fig. 158. so rich that the floodplain regions are important agricul-tural districts. The main stream meanders through theplain in great swinging curves (Figs. 135,172, and 157-160),so that its course is sometimes greatly increased in the Mississippi, a steamer is often within a few hundredyards of a portion of the river which can be reached by wateronly by a sail of several miles. These, which are known asoxbow curves, are constantly changing in form and hence 292 PHYSICAL Fig. 159. in position. The river is eating its way into the floodp
. Elementary physical geography . Fig. 158. so rich that the floodplain regions are important agricul-tural districts. The main stream meanders through theplain in great swinging curves (Figs. 135,172, and 157-160),so that its course is sometimes greatly increased in the Mississippi, a steamer is often within a few hundredyards of a portion of the river which can be reached by wateronly by a sail of several miles. These, which are known asoxbow curves, are constantly changing in form and hence 292 PHYSICAL 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
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