THE WORK OF RUNNING WATER 



51 



water, the water of lakes, and the ice of the mountains, all have their 

 sources in rain and snow; so that rivers, like springs and wells, 

 depend on moisture from the atmosphere for their supply of water. 

 A direct connection between rainfall and rivers may be inferred 

 from various familiar facts: (1) Streams are more numerous in 

 regions where rain is plentiful (Fig. 39) than in those where it is 



Fig. 39. Map showing the many 

 streams of a humid region. 

 Central Kentucky. The area 

 is about 225 square miles. 



Fig. 40. Map showing the few 

 streams of an arid region. 

 Northern Arizona. The area 

 is about 225 square miles. 



scarce (Fig. 40) . (2) Multitudes of small streams spring into being 

 with each heavy fall of rain, to disappear again soon after the rain 

 ceases, or after the snow is gone. (3) Streams are swollen after 

 rains, and swollen most after heavy rains. (4) Many small streams 

 which flow during wet weather disappear in times of drought. 



If a slope of land were perfectly even, like the slope of a smooth 

 roof, the immediate run-off would flow in a sheet. There are slopes 

 so even that therr immediate run-off moves in this way; but on most 

 slopes, even those which appear to be regular, there are some un- 

 evennesses, so that, although the run-off which follows a rain may 

 start as a sheet, it is soon gathered into rills and streamlets which 

 follow the depressions of the surface. The smallest streamlets unite 

 to form larger ones, and the little rills, after many unions with one 



