LAND WATERS STREAMS 



119 



enlargement goes on without the interruption to which it was sub- 

 ject when the supply of water was intermittent. 



Fig. 77. Diagram to illustrate the intermittency of streams due to fluctua- 

 tions of the ground-water level. The water level aa would be depressed 

 next the valley 2-2, by the flow of the water into the valley. The profile 

 of the ground-water surface would therefore be oca rather than aa. 



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

 a continuous 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, evapora- 

 tion and absorption may dis- 

 sipate the water gathered 

 above, and the stream disap- 

 pears (PL II). 



Other modes of valley de- 

 velopment. Valleys are not 



always developed from gullies Fig ' 1^~ Dia f am to show how a val- 

 ley may be developing all the way 



the manner outlined above. from a water-filled basin (lake) to 



the sea at the same time. Small 

 valleys leading to the lake are also 

 developing. The black area = 



The outflow of a lake would 

 develop a valley, but the val- 

 ley might be in process of 

 excavation all the way from the lake basin to the sea at the same 

 time (Fig. 78). A valley developed in this manner is not simply 



the sea. 



