62 



WORK OF RUNNING WATER 



and evaporation great. In such cases, evaporation and absorption 

 may dissipate the water gathered above, and the stream disap- 

 pears (PL II). A stream like the St. Lawrence, which carries water 



Fig. 43. Diagram to illustrate the intermittency of streams due to fluctuations 

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

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

 would therefore be oca amd bdb rather than aa and bb. 



from a great lake, does not depend on ground-water for its con- 

 tinuous flow. Again, a stream which carries the water of a melting 

 glacier may be permanent, even though not fed by springs. 



Other modes of valley development. Not all valleys are 

 developed from gullies in the manner outlined above, i. The out- 



Fig. 44 Fig. 45. 



Figs. 44 and 45. Diagram to illustrate one mode of valley lengthening. In 

 Fig. 44 there are two small valleys, a and b, and the former ends at the base of the 

 steep slope. In Fig. 45, valley b is represented as having been lengthened so as to 

 join a, and the two have become one. 



flow of a lake would develop a valley, and the valley might be in 

 process of excavation all the way from the lake basin to the sea at 

 the same time. A valley developed in this manner is not simply 

 a gully grown big by head erosion, and the valley would not pre- 

 cede the stream. 



If a narrow coastal plain is limited landward by a steeper slope, 

 valleys might develop as shown in Figs. 44 and 45. Again, in 

 some mountain regions valleys are formed by the up-folding of 



