CHAPTER XXIII 

 STREAMS AND THEIR WORK 



Introduction. Rain that falls on the earth is dis- 

 posed of in three different ways : part may evaporate ; 

 part may immediately run off into streams ; part 

 may sink into the ground. Of that which sinks into 

 the ground a portion may gradually rise to the sur- 

 face by capillary action and then evaporate ; another 

 portion may come to the surface as springs; still 

 another portion may never appear again on the sur- 

 face, but may find its way to the sea by underground 

 passage. 



Streams and stream erosion. Streams receive their 

 water from several sources : rainfall, lakes, ponds, and 

 melting snow and ice from mountain tops, as well as 

 ground water. 



We are all aware of the fact that streams carry 

 sediment ; in fact, it frequently appears that the princi- 

 pal work of a stream is to carry land into the sea. 

 All streams are incessantly at this task of lowering 

 their valleys to the lowest possible level. We can thus 

 understand why streams are important factors in modi- 

 fying the topography of the land. 



The erosive power of a stream depends upon four' 



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