THE WORK OF RUNNING WATER. 



79 



gully is opened. While the main gully is developing into a ravine, and 

 the ravine into a valley, the tributary gulhes are likewise developing 

 into maturer stages. Tributary to a young valley, therefore, there may 

 be gulhes near its head, ravines in its middle course, and small valleys 

 along its oldest portion. It is not to be understood, however, that the 

 oldest tributaries are necessarily the largest, for because of more favor- 

 able conditions for growth the younger tributaries often outstrip the 

 older. 



The position of tributaries with reference to their mains is worthy 

 of note. The water flowing down a slope follows 

 the hne of steepest descent. A gully is usually 



wider at its lower end, and narrower at its upper. /j\ y. 



Wherever this is true the line of steepest descent 

 down its side is not a line perpendicular to its 

 axis, but a hne shghtly oblique to it {ef, Fig. 62), 

 and oblique in such a direction that it meets the 

 axis with an obtuse angle below and an acute 

 angle above. It is in the direction corresponding 

 to this line that tributary gullies tend to develop, d^ G5 C 

 Thus at the inception of its history a tributary ^ig. 62. — Diagram to 



n . VI 1 i • • -j^ • -^1 1 illustrate the oblique 



gully is likely to jom its mam with an angle 



If the tribu- 



position of a tributary 

 gully at its inception, 

 and its later normal 

 change of direction. 



shghtly acute on the up-stream side. 



tary did not begin until after its main was farther 



advanced this tendency would be less and less 



pronounced. Inequalities of material or slope would often counteract 



this tendency, which, at best, would cause the courses of tributaries 



to depart but little from perpendicularity to their mains. 



After the head of a tributary has worked back from the immediate 

 slope of its main every condition which determines the course of a gully 

 is likely to affect it, and it is by no means certain that it will continue to 

 lengthen in the direction in which it started. Since the general slope of 

 the surface into which the tributary works is likely to be seaward, more 

 water is likely to enter from the landward than from the seaward side 

 of its head, so that, except where there are notable irregularities of slope, 

 its tendency will be to turn more and more toward the direction of its 

 main {efg, Fig. 62). 



In depth the tributary is always limited by its main. The principles 

 which determine the length and width of a main valley determine also 

 the length and width of a tributary (see p. 67 et seq.). 



