THE WORK OF RUNNING WATER 71 



Streams which are fed by lakes, and streams which have their 

 sources in snow and ice fields which last from year to year, do not 

 depend on ground-water, though they often receive it. 



Valleys are not all grown-up gullies. Not all valleys were 

 formed by the growth of gullies. A vast area in the northern part 

 of North America, for example, was once covered by a great sheet 

 of snow and ice. The rivers which had existed in this area ceased 

 to flow, for the most part, while the ice lay on the land. Many of 

 their valleys were filled, at least in places, by the debris (drift) which 

 the ice left when it finally melted. The result was that great areas 



Fig. 68. Diagram showing tributaries in an early stage of development. 



were left without well-defined valleys. The melting ice, however, 

 supplied great quantities of water, and this water flowed along the 

 lowest lines of descent which it could reach, and developed valleys 

 along their courses. Valleys developed by such waters may have 

 permanent streams at the start, since they do not depend on ground- 

 water. 



Again, the melting of the ice left many lakes on the surface of 

 the land it had covered, and the rainfall of the region was great 

 enough to make many of them overflow. When a lake overflows, 

 the out-going water follows the lowest line of descent, and develops 

 a valley in the way just outlined. In these ways, rivers were soon 

 re-established on the surfaces from which the ice melted. 



Growth of tributaries. Most valleys are joined by many 

 smaller tributary valleys. The reason is readily understood. The 

 erosion of the slopes by the water flowing from them is greater along 

 some lines than others, and tributary gullies are started (Fig. 68), 

 and grow in the same way as the valleys from which they develop. 



