DRAINAGE. 109 



XI. DRAINAGE. 



THE rain that falls on the earth either runs 

 directly off the surface or sinks into the ground. 



The part that runs directly off the surface col- 

 lects in small streams that discharge through a 

 river, either into a lake or into the ocean. 



The part which sinks into the ground collects in 

 pockets or places below the surface, called reser- 

 voirs. As a rule, the water escapes from these 

 underground reservoirs by coming out at the sur- 

 face at some lower level, as a spring. During most 

 of the time the now of water in a river is kept 

 up by the springs pouring their waters into the 

 many streams that empty into the river channel. 



The water, therefore, that falls from the sky as 

 rain, flows directly from the earth's surface into a 

 river, or first collects in a reservoir, from which it 

 afterwards flows into a river. 



The running of the water from the level where 

 the rain fell to a lower level is called drainage. 



There are two kinds of drainage : 

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