206 National Geographic Magazine. 



It is only during maturity and for a time before and afterwards 

 that the three divisions of a river, commonly recognized, appear 

 most distinctly ; the torrent portion being the still young head- 

 water branches, growing by gnawing backwards at their sources ; 

 the valley portion proper, where longer time of work has enabled 

 the valley to obtain a greater depth and width ; and the lower 

 flood-plain portion, where the temporary deposition of the excess 

 of load is made until the activity of middle life is past. 



Maturity seems to be a proper term to apply to this long en- 

 during stage ; for as in organic forms, where the term first came 

 into use, it here also signifies the highest development of all func- 

 tions between a youth of endeavor towards better work and an 

 old age of relinquishment of fullest powers. It is the mature 

 river in which the rainfall is best lead away to the sea, and which 

 carries with it the greatest load of land waste ; it is at maturity 

 that the regular descent and steady flow of the river is best de- 

 veloped, being the least delayed in lakes and least overhurried in 

 impetuous falls. 



Maturity past, and the power of the river is on the decay. 

 The relief of the land diminishes, for the streams no longer 

 deepen their valleys although the hill tops are degraded ; and 

 with the general loss of elevation, there is a failure of rainfall to 

 a certain extent ; for it is well known that up to certain consider- 

 able altitudes rainfall increases with height. A hyetographic and 

 a hypsometric maj) of a country for this reason show a marked 

 correspondence. The slopes of the headwaters decrease and the 

 valley sides widen so far that the land waste descends from them 

 slower than before. Later, what with failure of rainfall and de- 

 crease of slope, there is perhaps a return to the early imperfection 

 of drainage, and the number of side streams diminishes as branches 

 fall from a dying tree. The flood-plains of maturity are carried 

 down to the sea, and at last the river settles down to an old age 

 of well-earned rest with gentle flow and light load, little work re- 

 maining to be done. The great task that the river entered upon 

 is completed. 



16. Mutual adjustment of river courses. — In certain structures, 

 chiefly those of mountainous disorder on which the streams are at 

 first high above baselevel, there is a process of adjustment ex- 

 tremely characteristic of quiet river development, by which the 

 down-hill courses that were chosen in early life, and as we may 

 say unadvisedly and with the heedlessness and little foresight of 



