The Rivers and Valleys of Pennsylvania. 205 



into side valleys, joining their master streams at right angles (La 

 ISToe and Margerie). With their continued development, the ma- 

 turity of the system is reached ; it is marked by an almost com- 

 plete acquisition of every part of the original constructional sur- 

 face by erosion under the guidance of the streams, so that every 

 drop of rain that falls finds a. way prepared to lead it to a stream 

 and then to the ocean, its goal. The lakes of initial imperfection 

 have long since disappeared ; the waterfalls of adolescence have 

 been worn back, unless on the still young headwaters. With the 

 increase of the number of side-streams, ramifying into all parts of 

 the drainage basin, there is a proportionate increase in the surface 

 of the valley slopes, and with this comes an increase in the rate 

 of waste under atmospheric forces ; hence it is at maturity that 

 the river receives and carries the greatest load ; indeed, the in- 

 crease may be carried so far that the lower trunk-stream, of gentle 

 slope in its early maturity, is unable to carry the load brought to 

 it by the upper branches, and therefore resorts to the temporary 

 expedient of laying it aside in a flood-plain. The level of the 

 flood-plain is sometimes built up faster than the small side-streams 

 of the lower course can fill their valleys, and hence they are con- 

 verted for a little distance above their mouths into shallow lakes. 

 The growth of the flood-plain also results in carrying the point of 

 junction of tributaries farther and farther down stream, and at 

 last in turning lateral streams aside from the main stream, some- 

 times forcing them to follow independent courses to the sea 

 (Lombardini). But although thus separated from the main 

 trunk, it would be no more rational to regard such streams as 

 independent rivers than it would be to regard the branch of an 

 old tree, now fallen to the ground in the decay of advancing age, 

 as an independent plant ; both are detached portions of a single 

 individual, from which they have been separated in the normal 

 processes of growth and decay. 



In the later and qiiieter old age of a river system, the waste of 

 the land is yielded slower by reason of the diminishing slopes of 

 the valley sides ; then the headwater streams deliver less detritus 

 to the main channel, which, thus relieved, turns to its postponed 

 task of carrying its former excess of load to the sea, and cuts ter- 

 races in its flood-plain, preparatory to sweeping it away. It does 

 not always find the buried channel again, and perhaps settling 

 down on a low spur a little to one side of its old line, produces a 

 rapid or a low fall on the lower slope of such an obstruction 

 (Penck), Such courses may be called locally superimposed. 



