204 National Geographic Magazine. 



cycle is a long measure of time in regions of great elevation or 

 of hard rocks ; but whether or not any river ever passed through 

 a single cycle of life without interruption we need not now in- 

 quire. Our purpose is only to learn what changes it would ex- 

 perience if it did thus develop steadily from infancy to old age 

 without disturbance. 



In its infancy, the river drains its basin imperfectly ; for it is 

 then embarrassed by the original inequalities of the surface, and 

 lakes collect in all the depressions. At such time, the ratio of 

 evaporation to rainfall is relatively large, and the ratio of trans- 

 ported land waste to rainfall is small. The channels followed by 

 the streams that compose the river as a whole are narrow and 

 shallow, and their number is small compared to that which will be 

 developed at a later stage. The divides by which the side-streams 

 are separated are poorly marked, and in level countries are sur- 

 faces of considerable area and not lines at all. It is only in the 

 later maturity of a system that the divides are reduced to lines 

 by the consumption of the softer rocks on either side. The differ- 

 ence between constructional forms and those forms that are due 

 to the action of denuding forces is in a general way so easily 

 recognized, that immaturity and maturity of a drainage area cais 

 be readily discriminated. In the truly infantile drainage system 

 of the Red River of the North, the inter-stream areas are so abso- 

 lutely flat that water collects on them in wet weather, not having 

 either original structural slope or subsequently developed de- 

 nuded slope to lead it to the streams. On the almost equally 

 young lava blocks of southern Oregon, the well-marked slopes 

 are as yet hardly channeled by the flow of rain down them, and 

 the depressions among the tilted blocks are still undrained, un- 

 filled basins. 



As the river becomes adolescent, its channels are deepened and 

 all the larger ones descend close to baselevel. If local contrasts 

 of hardness allow a quick deepening of the down-stream part of 

 the channel, while the part next up-stream resists erosion, a cas- 

 cade or waterfall results ; but like the lakes of earlier youth, it is 

 evanescent, and endures but a small part of the whole cycle of 

 growth ; but the falls on the small headwater streams of a large 

 river may last into its maturity, just as there are young twigs on 

 the branches of a large tree. With the deepening of the chan- 

 nels, there comes an increase in the number of gulleys on the 

 slopes of the channel ; the gulleys grow into ravines and these 



