340 



FIRST YEAR SCIENCE 



A river is said to be mature when it has reduced its 

 valley to grade and is able to meander freely upon its 

 flood plain, its side streams having appropriated all the 

 undrained upland which they are able to obtain. The 

 river is now carrying off in the easiest and most effective 

 way the drainage which falls upon its drainage basin. 



When a river has graded itself and built its flood plain, 



AN OLD RIVER. 

 This river has done its work and has completed a cycle of erosion . 



its own active work consists largely in carrying off the 

 materials brought to it by its side streams. Although 

 these are now able to appropriate no new territory they 

 continue to wear down the country and round off the 

 divides till the whole region, unless reelevated, is reduced 

 to an almost level plain with its entire drainage -system 

 nearly at grade. Most of the material now carried by the 

 river is in solution, and there is but little erosion. The 



