ANTECEDENT RIVERS 127 



to show its general features. From this will be seen the extent to whicli 

 this stream has monopolized the drainage of a wide region. From the 

 head of its delta to the lower end of its interior basin the length of the 

 river is nearly three times that of any other river between the Alsek and 

 Cook inlet, but beyond this point it has an equal length to its main 

 head, and several of its tributaries are longer than any stream which 

 flows from the Chugach range directly into the sea. Moreover, the di- 

 vide between its southern tributaries and the streams of the Pacific slope 

 is nearer the coast than to the valley of their master stream. These 

 facts, taken with the rugged character of the topography in the inter- 

 montane portions of the Copper River system, tend to show that this 

 river has not gained its mastery of the region by headwater erosion and 

 piracy, but that its features are inherited from a mature system which 

 drained an equal or greater watershed previous to the general elevation 

 of the region. From analogy with the Alsek river as well, it seems that 

 the Copper is antecedent in origin, for the two wings of the Saint Elias 

 range were in all probabilit3^ raised more or less synchronously, and 

 the history of the drainage features in the two regions is likely to have 

 been the same. On the whole it may be fairly inferred that the Copper 

 river is antecedent to the uplift of the Chugach range. 



The intermontane portions of the rivers which traverse the coastal 

 mountains of British Columbia and Alaska lie from 6,000 to 10,000 feet 

 below the level of the plateau-like summits of the mountainous belt, 

 while the valleys of their upper portions are incised from 1,500 to 5,000 

 feet beneath the surface of the adjacent plateau. The greater depth of 

 the valleys within the mountainous belt is their only point of distinc- 

 tion aside from modifications due to ice-work. In both regions the 

 rugged topography near the streams testifies to the recency of the work 

 of excavation. 



The conception of the antecedent character of the rivers whose origin 

 has been discussed above, together with the other that the plateaus both 

 of the interior and coastal belts are uplifted peneplains, suggests at once 

 that the mature drainage systems from which the present rivers have 

 inherited their courses were developed upon the peneplains prior to the 

 regional elevation of the northern Pacific province. Previous to the 

 initiation of this upward movement of the land, the precursors of 

 the present coastward streams flowed upon a surface of low relief, 

 which probably sloped uniformly from the neighborhood of the present 

 locus of the continental divide toward the Pacific ocean, and, extending 

 throughout the region now occupied by the Pacific Mountain system, 

 had counterparts in adjacent plains of a similar character, whose slopes 

 were dependent upon the direction of their respective drainage systems. 



