THE WORK OF STREAMS 



121 



The fertility of the Nile valley in Egypt is due to the thin layer 

 of silt which is spread over the flood plain each year. If the sedi- 

 ment deposited on a flood plain is coarse, the plain will be infertile. 



Meanders. — After a river has become more sluggish and is con- 

 sequently unable to cut downward, it may undercut its banks on the 

 outside of its curves and thus widen its valley floor. As the outside 

 of a curve is cut away, the inside is filled with sediment to flood level, 

 and a strip of land is thus formed. In this way, as well as by deposi- 

 tion during overflow, a broad, flat valley is developed which, as has 

 been said, is called a flood plain because covered by water during floods. 

 On such a flood plain a river 

 will take a still more winding 

 or meandering course (Fig. 108). 

 The origin of these meanders is 

 easily conjectured. Imagine a 

 perfectly straight stream flow- 

 ing through a level alluvial plain. 

 If, under such conditions, a tree 

 is blown over into the stream, 

 a rock falls from the bank, or 

 a tributary stream forces the 

 current against the opposite bank, 

 or brings in gravel and builds 

 a natural jetty, the current will 

 be deflected a little, the bank 

 will be undercut, and the chan- 

 nel changed at this point (Fig. 

 109). The stream will then strike the opposite bank obliquely 

 a little further down its course, wearing it away at this point, and 

 thus, one after another, new meanders will be formed. A single 

 obstruction may, therefore, affect the oscillations of the current for 

 an indefinite distance down its course. The length of the Mississippi 

 River (Fig. 1 10), from the mouth of the Ohio to the Gulf of Mexico, 

 is 1000 miles, so meandering is its course, although the direct dis- 

 tance is only 600 miles. One of the plans for improving the Missis- 

 sippi is to straighten the channel by cutting off the curves. 



Oxbow Lakes. — Once initiated (Fig. 109), meanders tend to be- 

 come more pronounced in form, changing from an open loop to one 

 which is horseshoe-shaped. The neck of land separating one bend 

 from the next may become more and more narrow (Fig. m) until, 



Fig. 109. — Diagram showing the initiation 

 of meanders. (Modified after Salisbury.) 



