SUMMARY 207 



But most important of all is the erosion caused by flowing 

 water. It wears down the land's surface, bears away and 

 deposits the eroded materials, cuts deep trenches, and forms 

 broad valleys ; it fills lakes and builds great deltas. Falls 

 and rapids furnish 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, doing 

 little if any erosive work. Rivers sometimes develop flood 

 plains through which they wander in S-shaped meanders. 



If the region of a river becomes elevated, the river may be 

 revived, and if it is a meandering river, intrenched mean- 

 ders will be formed. If a river region becomes depressed, 

 the river will be drowned. These drowned river valleys 

 form some of the finest harbors in the world. Many 

 rivers build deltas when they empty into bodies of quiet 

 water. 



Rivers have always played a most important part in 

 history, because river valleys offer the easiest routes of 

 travel and furnish most fertile soils. Even in this day of 

 railways, the largest cities of the world owe their great size 

 to combined railway and water transportation facilities. 

 So important is adequate water transportation that the 

 countries of Europe have developed a wonderful network of 

 artificial waterways and the United States contemplates 

 spending millions of dollars in similar enterprises. 



Springs and shallow wells furnish water to favorably 

 situated rural districts and to some small communities. 

 Most great cities must depend on surface water. Supplying 

 water to populous communities is a most difficult under- 

 taking. Water must be piped to homes and office buildings, 



