IRfvere anD ffloofcs, 159 



about September of each year. If the Nile 

 should dry up, or if the annual floods should 

 materially change in height, it would make a 

 desert region of all that portion of Egypt now 

 so productive. 



The great rivers of China, the Yang-tse- 

 Kiang and the Hwangho, are also tropical 

 rivers and have an annual flood. Sometimes 

 the rise is as much as fifty-six feet. These 

 annual floods are also caused by the monsoon 

 winds that carry moisture from the ocean, 

 which is condensed and precipitated in the 

 mountains of central Asia. The conditions 

 are substantially the same as those which pre- 

 vail at the sources of the Nile in Africa. 



Rivers are produced from all sorts of causes, 

 some of them flowing only during the rainy 

 season, while others are fed by melting snow 

 from the higher mountains, and as the snow is 

 rarely melted away entirely during the sum- 

 mer, in the high mountains, there is a con- 

 tinual flow from this source. The snow forms 

 a system of storage, so that the water is held 

 back and is gradually given up as it melts. If 

 this were not true mountainous regions would 

 be subjected to disastrous floods. If the pre- 

 cipitation were always in the form of rain it 

 would immediately run off instead of being 

 distributed over a whole season. The Platte 

 is an instance of a river largely fed by the 

 melting snows of the Rocky Mountains. 



