360 FIRST YEAR SCIENCE 



work of dissection would have progressed much farther 

 before the river had been able to sink its channel so 

 deep. The water that falls hundreds of miles away is 

 doing a mighty work which the meager rainfall of the 

 region through which it passes cannot supplement. 

 Majestic, awe-inspiring, stupendous, this gigantic trench 

 is but a prank of the river's boisterous youth. 



/Summary. Just as the waves and ocean currents work 

 upon the coastlines, so the rain and the streams are con- 

 stantly wearing down the surface of the land. All streams 

 come from rain or melting snow, which condenses in the 

 air after evaporating from water surfaces. The rainfall 

 varies from nothing at all in some places to over fifty feet 

 a year in others, but in the United States the greatest 

 rainfall is about eighty inches a year. 



Some of the rain evaporates at once after falling ; some 

 flows away on the surface of the land ; some sinks into the 

 ground, to return as springs, wells and geysers. The 

 water which flows along the surface has the greatest effect 

 upon the land. It forms the little streams which remove 

 the surface water, the huge rivers which drain the country 

 and form great arteries of trade, and the beautiful lake- 

 reservoirs which hold back floods and offer easy trans- 

 portation to mighty ships. 



But most important of all is the erosion caused by flow- 

 ing water. It wears down the hills and spreads them out 

 in fertile fields, in deep trenches and broad valleys ; it fills 

 lakes and builds great deltas. By its falls and rapids it 

 furnishes water power for manufactures. 



Rivers that have not yet widened their valleys and still 

 have falls and rapids are called young ; an old river is one 

 whose bed has been worn smooth, and which has built 

 for itself a broad level valley, through which it wanders, 



