THE WORK OF RUNNING WATER 



89 



By the shifting of their courses, as the result of deposition and 

 meandering, streams have affected human interests in many ways. 

 Villages which have grown up on the banks of navigable rivers, be- 

 cause of the river trade, have sometimes been left far inland by 

 changes in the positions of the streams. Such villages usually 

 decay when the streams withdraw their patronage. Other villages 

 built on flood plains have been washed away, while others have 

 been preserved at great expense. Streams are often the boundaries 

 between counties and even states. In such cases, the shifting of the 

 stream introduces many complications into the boundary lines. 

 The case is even more serious where a river forms an international 

 boundary. Thus the shifting of the Rio Grande makes it an un- 

 satisfactory boundarv between the United States and Mexico. 



Fig. 92. Cement-lined canal in connection with the Salt River irrigation 

 project, Arizona. (U. S. Rec. Serv.) 



Fertility of alluvial plains. Alluvial plains are often very fer- 

 tile, and many of them are of great value for farming purposes. This 

 was as true in ancient times as now, for the valleys of the Nile, the 

 Po ; and many of the rivers of southern Asia were garden spots of 



