DRA IN A GE MOD I PICA TIONS 5 69 



(2) CLASSES OF DRAINAGE ADJUSTMENTS. 



Neglecting the catastrophic effects of glaciation and volcanic 

 action, drainage modifications may be divided into three classes : 

 ( I ) Changes marking the progress of a river through a normal 

 cycle of development. (2) Adjustments due to character of 

 rocks and geologic structure. (3) Rearrangements caused by 

 local uplifts or depressions of the earth's crust. 



Class I . Changes during the ?iormal development of a stream. — 

 The modifications falling within this class are well understood, 

 having been thoroughly worked out by Davis and others, but for 

 the sake of clearness will be briefly restated. 



A cycle in river development, according to Davis, ^ consists 

 of the interval of time during which a river reduces the land 

 within its watershed to baselevel. Let us for a moment glance 

 at the process by which this is accomplished. In its youth the 

 grade of the stream is necessarily great, since the surface of the 

 land must be far above baselevel. With a steep gradient the 

 current is everywhere rapid, and all of the refuse worn from the 

 land by weathering and corrasion is carried by the force of the 

 current into the sea. The stream so loaded becomes an abrading 

 instrument of great power, and its channel soon partakes of the 

 character of a rocky gorge or canyon. That portion of its course 

 which is nearest the mouth of the stream will first be reduced 

 approximately to the baselevel of erosion. As soon as that is 

 accomplished the gradient becomes so low that the stream can 

 no longer carry its burden of waste to the sea. A portion of its 

 load will be dropped in the channel deflecting the current from 

 its original course and causing it to cut away its banks on the 

 side toward which the current sets. As the current is lessened 

 by these meanders, its carrying power is diminished and it is 

 forced to give up more of its load which is added to the barrier 

 already existing and which tends continually to deflect the 

 stream into broader and broader meanders. Thus in progressing 

 from youth to old age, the stream changes its appearance in accord- 

 ance with the changed conditions which surround it. Conse- 



' The Rivers and \'allevs of Pennsylvania, Nat'l. Geog. Mag., Vol. I, p. 203. 



