186 THE WORK OF RUNNING WATER 



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

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

 has accomplished its life work, it has borne to the sea all 

 the burden it has to bear, its labors are ended, it has reached 

 old age. 



Rivers in Dry Climates. In a region where the climate 

 is very dry, rivers are often intermittent in their flow. 

 They contain water only after rains. Such rivers may 

 dry up before they reach any other body of water, their 

 water entirely evaporating or sinking into the dry soil. 

 Their development is therefore somewhat irregular. 



If the slopes are steep and there is little vegetation to 

 protect them and hinder the quick run-off of the water, 

 rivers flood very rapidly, eroding their channels and wash- 

 ing away their banks. Where they descend upon level 

 ground they silt up their old courses and acquire new 

 channels. Thus a river which for the larger part of the 

 year is a mere brook may after a rain become a devastat- 

 ing torrent, bursting its banks and carrying destruction 

 to settlements and farm lands along its course. It may 

 even change its entire lower course. 



Accidents in River Development. A river may by some 

 accident, such as the melting of ice during the Glacial 

 Period, have had its supply of sediment greatly increased, 

 causing it for a time to build up its valley floor instead 

 of eroding it, thus forming a filled river valley. When 

 the supply of sediment failed the river began cutting down 

 the filled valley, leaving terraces along the sides to mark 

 the successive levels at which it flowed. Such terraces are 

 often very prominent along our northern rivers. 



The region in which a river is situated may be elevated, 



