112 



GEOLOGY 



from year to year, or, sometimes, when the stream is in flood, from 

 day to day. The force of streams at such times is often disastrous 

 (Fig. 68). Occasionally they sweep away dams, bridges, and even 

 buildings on their banks. The strong rods and beams of the bridges, 

 and the steel rails of railways are bent almost as if they were twigs 

 by the force of the occasional torrent which follows an exceptional 

 rain, such as a cloudburst (Fig. 69) . 



The source of river water is the rain and snow which fall from 

 the atmosphere. This may be inferred from various familiar 



Fig. 70. Fig. 71. 



Fig. 70. Map showing the many streams of a humid region. Central Ken- 

 tucky. The area is about 225 square miles. 



Fig. 71. Map showing the few streams of an arid region. Northern Ari- 

 zona. The area is as great as that shown in Fig. 70. 



phenomena. Thus (1) streams are more numerous in regions 

 where the rainfall is abundant than in those where it is scarce (Figs. 

 70-71); (2) multitudes of small streams spring into being with 

 each heavy fall of rain and with each period of rapidly melting 

 snow; (3) streams are notably swollen after rains, and most after 

 heavy ones; and (4) many small streams which flow during wet 

 weather dry up in times of drought, while others shrink. It is 

 true that lakes, glaciers, and springs feed the rivers, but the lakes, 

 glaciers, and springs derive their supply of water from precipitation. 



