REVIVAL OF RIVERS 137 



lated by the tides, and they deposit all the debris furnished to them. 

 They are justly called old rivers, though their old age is premature, 

 and they may be situated in young valleys. Partial drowning re- 

 sults when, through damming by drift, a portion of a river valley is 

 converted into a lake, as was the case in the upper Genesee River. 

 (Fairchild-13.) 



Revival and Rejuvenation of Rivers. Rivers which have be- 

 come prematurely old, or which have grown old in the normal cycle 

 of development, may be revived or rejuvenated by an uplift of the 

 land or by a removal of the cause of senescence. The overloaded 

 stream is most easily revived by the removal of the cause of senes- 

 cence, i. e., the excessive supply of material. This is better spoken 

 of as a recovery. Drowned coastal rivers may be revived by eleva- 

 tion until they have again reached the condition prior to drowning. 

 If further elevation occurs they will become rejuvenated and enter 

 upon a new cycle of erosion as youthful streams. Partly or wholly 

 drowned rivers, through damming by drift or otherwise, may be re- 

 vived by removal of the dam, or. what is more often the case, 

 by the cutting of a new outlet. This will result in a complex stream, 

 in which the outlet portion is youthful, while the rest is mature or 

 older. The narrow gorges with rapids and waterfalls connecting 

 the present upper and lower Genesee valleys are examples of 

 young connecting portions interpolated between revived mature 

 portions. 



Rivers which in the course of their normal development have 

 reached a state of old age, may be rejuvenated by a favorable 

 change of the environment, through an uplift of the land, or a low- 

 ering of the base-level of erosion, ?'. e., the local level to which ero- 

 sion proceeds; or by an infusion of new life, in the case of certain 

 rivers, through a change in climatic conditions, which will greatly 

 increase the amount of water, and hence the power of work of the 

 stream. The Great Falls of the Missouri in eastern Montana occur 

 in a rejuvenated valley, i. e., a valley now in its second or later 

 cycle (n"^ -[- I cycle, where n may be the first or any later cycle). 

 (Davis.) The rivers of the Piedmont district of Virginia, and all 

 rivers which incised themselves into uplifted peneplains which 

 they had helped to make, are examples of rejuvenated rivers, now 

 in their n**^ -)- i cycle. These rivers are generally characterized by 

 well-entrenched meanders, which differ strikingly from the crooked 

 curves of young streams. Such meanders are shown by the Seine in 

 the Normandy upland, the Moselle and the Rhine in the old Rhine 

 uplands of western Germany, the Susquehanna in the old Appa- 

 lachian upland, and by many other streams. 



