STREAMS AND THEIR WORK 133 



The people living in the flood-plain of the Nile depend 

 upon the deposits made by the river during its annual 

 overflow. As the flood-plain of the Mississippi is 

 enriched in a similar way, it is important as an agri- 

 cultural region. 



Until the use of steam and electricity, streams were 

 the most important means for travel. Even now the 

 courses of rivers are followed in exploration of a new 

 country. It is economical to transport by water many 

 materials when time is not an important element, as 

 transportation by water is cheaper than by railway. 

 In hilly or mountainous regions railroads are always 

 built along the water courses. 



As the population of the country increases, so does 

 the demand for food; this necessitates a larger area 

 for growing crops. Rivers help in making this pos- 

 sible, as they are the main sources for irrigation pur- 

 poses. The semi-arid lands of western United States, 

 where not enough rain falls for agricultural purposes, 

 are irrigated. The Government has under way thirty 

 projects which will irrigate 3,000,000 acres. This is 

 about one-fifteenth of the amount of land that can be 

 so developed. 



Waterfalls and rapids furnish water power that may 

 be used for manufacturing purposes. The available 

 coal supply in the United States is limited. As the 

 age of coal formation is past, it is evident that the 

 supply will some day be exhausted. When this time 

 comes it will necessitate another source of power. At 



