72 



GEOLOGY, 



long a time afterward as the ground-water level remained well above 

 the valley bottom. In regions subject to frequent and short periods of 

 heavy precipitation, alternating with droughts, the periods of intermit- 

 tent flow may be many and short. Since the precipitation of many 

 regions varies greatly from year to year, it follows that a stream may 

 flow continuously one year and be intermittent the next. Many val- 

 leys in various parts of the earth are now in the stage of development 

 where their streams are intermittent. 



As a valley containing an intermittent stream becomes deeper, the 

 periods when it is dry become shorter, and when it has been sunk below 

 the lowest ground- water level, it will have a permanent stream (3, Fig. 53). 



Fig. 53.-^Diagram to illustrate the intermittency of streams due to fluctuations of 

 the groutid-water level. The water level aa would be depressed next the valley 

 2-2 by th€ flow of the water into the valley. The profile of the ground-water sur- 



I face woiild therefore be aca rather than aa. 



Since a valley normally develops headward, its lower and older portion 

 is likely to acquire a permanent stream, while its upper and younger 

 part has only an intermittent one (Fig. 47 and Fig. 1, PI. Ill, near 

 Anthony, Kan. The intermittent part of the stream is indicated by the 

 dotted blue line). For the same reason the head of a stream is likely to 

 be farther up the valley in wet weather than in dry. So soon as a valley 

 gets a permanent stream, the process of enlargement goes on without the 

 interruption to which it was subject when the supply of water was inter- 

 mittent. 



In general a permanent stream at one point in a valley means a con- 

 tinuous stream from that point to the sea or lake which the valley joins; 

 but to this rule there are many exceptions. They are likely to arise 

 where a stream heads in a region of abundant precipitation, and flows 

 thence through an arid tract where the ground- water level is low, and 

 evaporation great. In such cases, evaporation and absorption may dis- 

 sipate the water gathered above, and the stream disappears (Fig. 2, 

 PL III, near Paradise. Nev.). 



