VIII RIVERS AND LAKES 331 



to prevent rivers from following their natural 

 law of oscillation. But rivers are very true to 

 their own laws, and a change at any part is 

 continued both upwards and downwards, so 

 that a new oscillation in any place cuts its 

 way through the whole plain of the river both 

 above and below. 



The curves of the Mississippi are, for in- 

 stance, for a considerable part of its course 

 so regular that they are said to have been 

 used by the Indians as a measure of dis- 

 tance. 



If the country is flat a river gradually 

 raises the level on each side, the water which 

 overflows during floods being retarded by 

 reeds, bushes, trees, and a thousand other 

 obstacles, gradually deposits the solid matter 

 which it contains, and thus raising the sur- 

 face, becomes at length suspended, as it were, 

 above the general level. When this elevation 

 has reached a certain point, the river during 

 some flood bursts its banks, and deserting its 

 old bed takes a new course along the lowest 

 accessible level. This then it gradually fills 

 up, and so on ; coming back from time to 



