DRAINAGE. Ill 



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direction in which water will drain from the land 

 will depend on the direction of the slope of the 

 land. If a large area of land so slopes that all the 

 water draining from it collects in streams flowing 

 into the ocean through a common river mouth, 

 and the rainfall on such area is large, the river 

 itself will be large. 



The smaller streams and rivers which collect in a 

 single and larger river, and discharge their waters 

 through a common mouth, are called, collectively, 

 a river system. 



The area of land that drains into a river is called 

 a river basin. 



The size of a river, therefore, depends upon the 

 amount of the rainfall on its basin, and on the size 

 of its basin. 



When the quantity of water discharged into a 

 river is greater than its channel can hold, a flood 

 occurs, or the river is said to inundate its banks. 



A heavy rain-fall does not necessarily produce 

 an inundation. If the character of the river basin 

 is such that a comparatively small part of the rain- 

 fall runs directly off the surface, and a large part 

 sinks into the ground and collects in the reservoirs 

 of springs, and slowly passes through such springs 

 into the rivers, sufficient time may be given for 



