118 



GEOLOGY 



underground from places where the water-table is higher to those 

 where it is lower. In these and other minor ways the ground- 

 water surface is depressed. 



A well sunk to such a level as to be supplied with abundant 

 water in a wet season may go dry during a period of drought be- 

 cause the ground-water level is 

 depressed below its bottom. 

 Thus either well shown in Fig. 

 76 will have water during a 

 wet season when the water- 

 level is at a; but well No. 1 

 will go dry when the water 

 surface is depressed to 6. 



These principles are as ap- 

 plicable to valleys as to wells. 



Fig. 76. Diagram illustrating the 

 fluctuation of the ground-water 

 surface; a = wet- weather ground- 

 water level; 6 = ground- water 

 level during drought. Well No. 

 1 will contain water during the 



wet season, but will go dry in 

 times of drought. Well No. 2 will 

 be permanent. 



When a valley has been deep- 

 ened until its bottom is below 

 the ground-water surface, water 



seeps or flows into it from the sides. The valley is then no 

 longer dependent on the run-off of showers for a stream. 

 When the bottom of a valley is below the ground-water level 

 of a wet season, without being below that of a dry one, it will 

 have an intermittent stream. If the rainfall of the year were 

 concentrated in a single wet season, the intermittent stream would 

 flow not only during that season, but for so long a time afterward 

 as the ground-water level remained well above the valley bottom. 

 In regions subject to frequent and short periods of precipitation 

 alternating with droughts, the periods of intermittent flow may 

 be many and short. Many valleys are now in the stage of devel- 

 opment 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. 77). Since a valley normally develops head \vanl. 

 its lower and older portion is likely to acquire a permanent stream 

 while its upper and younger part has only an intermittent one. So 

 soon as a valley gets a permanent stream, the process of valley- 



