THE LIFE HISTORIES OF RIVERS 161 



For several reasons such basins in the course of a stream are rela- 

 tively short lived (Chapter XXX), and they disappear with the 

 earlier stages of the river history. 



The meshes of the river network. From the continued throw- 

 ing out of new tributaries by the streams, the meshes in the 

 river network draw more closely together as the stages of its his- 

 tory advance. The closeness of texture which is at last developed 

 upon the upland is in part determined by the quantity of rainfall, 

 so that in New Jersey with heavy annual precipitation the meshes 

 in the network are much smaller than they are, for example, 

 upon the semiarid or arid plains of the western United States. 

 Its design will, however, in either case more or less clearly express 

 the plan of rock architecture which is hidden beneath the surface 

 (Chapter XVII). 



The upper and lower reaches of a river contrasted. From 

 the fact that the river progressively invades new portions of the 

 upland and lays the acquired sections under more and more 

 thorough investment, it has near its headwaters for a long time 

 a frontier district which may be regarded as youthful even though 

 the sections near its mouth have reached a somewhat advanced 

 stage. The newly acquired sections of river valley may thus 

 possess the steep grade and precipitous walls which are charac- 

 teristic of early gullies and canons and are in contrast with 

 the more rounded and flat-bottomed sections below. Lateral 

 streams, from the fact that they are newer than the main or trunk 



FIG. 167. Characteristic longitudinal sections of the upper portion of a river 

 valley and its tributaries (after scaled sections by Nussbaum). 



stream to which they are tributary, likewise descend upon somewhat 

 steeper grades (Fig. 167). 



The balance between degradation and aggradation. We have 

 seen that the power to transport rock fragments is augmented at 

 a most surprising rate with every increase in the current velocity. 

 While the lighter particles of rock may be carried as high up as 

 the surface of the water, the heavier ones are moved forward 

 upon the bottom with a combined rolling and hopping motion 

 aided by local eddies. Those particles which come in contact 



