32 DYNAMICAL GEOLOGY. 



slowly seaward. Through the middle of this compara- 

 tively still water runs the swift current of the river- 



FIG. 11. Ideal section of a flooding river, a, a, a, a, original bed ; 6, 6, flood- 

 plain ; l t I, natural levees ; c, low water ; d, d, d, half flood ; e, e, e, full flood. 



channel. Now, on the two sides of this swift current, 

 just where it conies in contact with the stiller water, and 

 is checked by it, there will be a line of abundant and 

 coarser sediment. 



Artificial Levees. Natural levees can not restrain 

 the floods of rivers, since they are made by such floods. 

 By deposit, the bed of the river, the natural levees, and 

 the back swamp, all rise together, maintaining their rela- 

 tive level. If, therefore, we desire to restrain the floods 



FIG. 12. Ideal section of a river-bed and plain which was built up naturally for a 

 time and then restrained by artificial levees, /, I. 



and reclaim the flood-plain, we must build artificial levees 

 upon the natural ones. This interference modifies greatly 

 the phenomena of deposit. The river continues to build 

 up its bed as before, and would in time again flood as 

 before, if the levees were not built up higher from time 

 to time. The flood-plain, however, no longer receives 

 deposit. Therefore the river-bed being raised by deposit, 

 and the levees by man, the river finally runs on the top 

 of an embankment, which rises ever higher above the sur- 



