SURFACE WATER, DRAINAGE, AND IRRIGATION 307 



reaches (Figure 262) . 

 However, streams are 

 always at work to 

 remove such features 

 after they are made 

 and to reduce their 

 beds to an even grade. 

 Thus old rivers are 

 devoid of rapids and 

 falls. 



-.''".. FIG. 262. Niagara Falls. 



In climates of mod- 

 erate or heavy rainfall, the erosion of. the land is usually 

 rapid enough to produce wide valleys ; but in regions 

 of scant rainfall, the streams cut their beds much 



faster than the 

 adjacent land is 

 eroded, and deep 

 gullies or canons 

 result. 



The Colorado is 

 the best example 

 of such a river 

 (Figure 263). It 

 flows hundreds 

 of miles through 

 canons varying in 

 depth from a few 

 hundred feet to 

 several thousand 

 feet. The veloc- 

 ity of this river 

 is so great that 



U. S. Geological Survey. 

 FIG. 263. Grand Canon, Arizona. the Sand and 



