92 



PHYSIOGRAPHY 



to hold the waters of the wet seasons so that they may be drawn 

 out and used on the lands below, during the growing season. The 

 sites selected for dams are usually narrow places in the mountain 

 valleys. The distribution of the lands now irrigated, or soon to 

 be irrigated by the government, is shown in Fig. 95. 



Although alluvial plains are generally fertile, they are not 

 without their drawbacks as farming regions, for the floods to which 

 they are subject are often disastrous both to life and to property. 



Fig. 96. Diagram illustrating changes in the course of the Yellow River. 

 The shaded area represents t^e area subject to flooding by the main 

 stream and its tributaries. (Richthofen.) 



Some parts of the rich flood plain of the Mississippi, used for farming, 

 are so subject to floods that all buildings connected with the farms 

 are placed above the flat. 



The destruction caused by floods is not confined to farms, but 





