218 National Geographic Magazine. 



have thus far considered an ideal river. It now seems advisable 

 to introduce a few terms with which to indicate concisely certain 

 well marked peculiarities in the history of actual rivers. 



An original river has already been defined as one which first 

 takes possession of a land area, or which replaces a completely 

 extinguished river on a surface of rapid deformation. 



A river may be simple, if its drainage area is of practically one 

 kind of structure and of one age ; like the rivers of southern 

 New Jersey. Such rivers are generally small. It may be com- 

 posite, when drainage areas of different structure are included in 

 the basin of a single stream. This is the usual case. 



A compound river is one which is of different ages in its differ- 

 ent parts ; as certain rivers of North Carolina, which have old 

 headwaters rising in the mountains, and young lower courses 

 traversing the coastal plain. 



A river is complex when it has entered a second or later cycle 

 of development ; the headwaters of a compound river are there- 

 fore complex, while the lower course may be simple, in its first 

 cycle. The degree of complexity measures the number of cycles 

 that the river has entered. 



When the study of rivers is thus attempted, its necessary com- 

 plications may at first seem so great as to render it of no value ; 

 but in answer to this I believe that it may be fairly urged that, 

 although complicated, the results are true to nature, and if so, 

 we can have no ground of complaint against them. Moreover, 

 while it is desirable to reduce the study of the development of 

 rivers to its simplest form, in order to make it available for in- 

 struction and investigation, it must be remembered that this can- 

 not be done by neglecting to investigate the whole truth in the 

 hope of avoiding too great complexity, but that simplicity can 

 be reached safely only through fullness of knowledge, if at all. 



It is with these points in mind that I have attempted to decipher 

 the history of the rivers of Pennsylvania. We find in the Sus- 

 quehanna, which drains a great area in the central part of the 

 state, an example of a river which is at once composite, com- 

 pound and highly complex. It drains districts of divers struc- 

 ture ; it traverses districts of different ages ; and it is at present 

 in its fourth or fifth degree of complexity, its fourth or fifth cycle 

 of development at least. In unravelling its history and search- 

 ing out the earlier courses of streams which may have long since 

 been abandoned in the processes of mature adjustment, it will be 



