AGE OF RIVERS 135 



quired a meandering course, while the peneplain was low, and this 

 will be in a large measure retained, so that the inherited stream will 

 be characterized by deeply incised meanders, whereas the new con- 

 sequent will generally be straight. A more pronounced complexity 

 is produced when by warping, partial damming and division a river 

 enters locally on a new cycle, while other parts remain compara- 

 tively unchanged. xA.n example of such a river is the Genesee, which 

 at present occupies the valleys of two formerly distinct streams of 

 diverse origin. This is really the case with the St. Lawrence drain- 

 age system as a whole, the affluents of the lakes, which belong to 

 this system, having a varied history, as already noted in the case 

 of the Genesee. 



Relative Ages of Rivers and River Systems. In discussing 

 the relative ages of rivers and river systems, we must distinguish 

 between the age of the river or river system and the age of the dis- 

 trict it inhabits. Thus a young stream may exist in a young country, 

 as in the case of new streams on a young coastal plain, or it may 

 exist in a mature or old country. The age of the land must be 

 considered distinct from the age of the river. (Johnson-21.) 

 Only the latter will be considered here. Streams are classed as 

 young, mature and old, according to the degree of their develop- 

 ment. No conception of actual time is involved, for of two rivers 

 born at the same time one may reach maturity while the other is still 

 very youthful. Young rivers are characterized by rapids and falls, 

 and by steep-sided, narrow gorges. Lakes may further interrupt 

 the course of the drainage. A river is mature when it has destroyed 

 its lakes and falls, and reduced its valley sides to graded slopes, and 

 broadened its floor so that it may swing upon it in great meanders. 

 A mature river is able to carry away all the load that it receives, 

 but generally has no power to do further erosion. A river system 

 is mature when all the branches have become mature, but generally 

 youthful branches are found long after the trunk stream has become 

 mature. Old rivers are those which move sluggishly along across 

 a flat, much-encumbered valley bottom. Their grade is so low that 

 they cannot carry away the rock waste, and so they aggrade their 

 beds, or stagger about in them by avoidance of the debris. In a 

 river system the main stream may grow old, while its branches are 

 still mature or even youthful. 



Aging of Rivers by Accident. Rivers may grow old rapidly 

 through various accidents. Among these are mutilation, sudden 

 overloading, and drowning. The first of these produces perma- 

 nent senescence, and may be compared to the permanent aging of 

 an animal body through a severe illness or accident. The second is 



