322 ADJUSTMENT OF RIVERS 



lake. In this plain the divides between the streams are so wide 

 and flat that water gathers on them after heavy rains, having no 

 reason to flow in one direction rather than another. 



As the river system becomes somewhat older, the stream chan- 

 nels are deepened, the larger ones being cut down to base-level, 

 and if the region be one of considerable elevation, deep gorges 

 and canons are excavated. If the streams flow across strata of 

 different hardness, waterfalls result where a hard ridge crosses 

 them, but in the main stream these cascades and rapids are 

 ephemeral and soon removed by the stream's wearing down the 

 obstacle. On the head-waters of streams, however, waterfalls may 

 persist for a long period. The river valleys are widened out by 

 atmospheric denudation, and channels are formed on their sloping 

 sides, which gradually grow into side valleys. The lakes are for 

 the most part drained or silted up, only the more important and 

 deeper ones remaining, while the system of tributary streams and 

 rills is greatly expanded. A mature river system is characterized 

 by the complete development of its tributaries and drainage, so 

 that every part of its basin is reached by the ramifying channels. 

 The waterfalls have disappeared, except near the stream-heads, 

 and the stream channels have sought out and utilized every 

 weakness in the strata, adjusting themselves to the structure of 

 the rocks and the alternations of hard and soft beds. 



The complete network of streams has enlarged the valley 

 surfaces, which increases the rate of destruction and brings to 

 the river a greater load of sediment to carry. In maturity the 

 river receives its maximum load, sometimes so great that the lower 

 reaches of the main stream are unable to transport it all, and 

 spread the excess out over the flood plain. The channel of an 

 overloaded stream may be so raised and banked in by its own 

 deposits, that some of the tributaries are deflected and made to 

 run for some distance parallel to the main stream, perhaps even 

 reaching the sea independently. An example of this is the Loup 

 Fork of the Platte in Nebraska. " The Platte flows there upon a 

 ridge of its own creation. The Loup comes down into its valley 

 and flows parallel with it for many miles." (Gannett.) 



