STREAM VALLEYS AND THEIR MEANING 475 



ANALYSIS OF THE WORK OF A RIVER IN CARVING ITS VALLEY 



The development of the immediate valley or thalweg of a 

 river is accomplished mainly by a combination of three well-known 

 processes — vertical down-cutting, lateral cutting, and down-valley 

 migration of the curves or meanders, to which Davis has applied 

 the excellent term "sweep." Of these, vertical cutting, when the 

 conditions are favorable, is much the most rapid, because it has 

 gravity as its direct and powerful ally. 



In a normal degrading stream, all three processes are active in 

 shaping the valley, but vertical cutting may be so much more rapid 

 than the others as to mask their effects. When, however, a stream 

 approaches grade and vertical cutting diminishes, one or the other 

 of the two remaining processes advances to prime importance. 



Any one of these processes, if dominant, is capable of impressing 

 upon the valley, which it is helping to shape, certain characteristics 

 of form which are distinctive. An acquaintance with these distinc- 

 tive forms should enable one, on examining a valley, to determine 

 which process took the leading part in its sculpture. If, carrying 

 the analysis back a step farther, we can determine the controlling 

 factors which lead to dominance of a given process, we shall be in a 

 position to read much of the physical history of a region from the 

 form of its valleys. 



DISTINCTIVE FORMS ASSOCIATED WITH EACH OF THE THREE VALLEY- 

 CUTTING PROCESSES 



Dominant down-cutting implies that a stream is sinking its bed 

 vertically at a greater rate than it is cutting laterally or down- 

 valleyward on the bends. The necessary result of such a process 

 is the development of a gorge-valley with narrow bottom, not, as a 

 rule, much wider than the stream at flood, and with sides whose 

 steepness, while usually great, depends largely on the nature of 

 the rock and on the activity of weathering agents. The stream 

 tends to sink itself vertically in whatever course, whether straight 

 or meandering, it may hold at the beginning of down-cutting. 

 Lateral cutting and sweep will modify this original pattern some- 

 what, but not to the extent of destroying the characteristic gorge 

 form. 



