THE LIFE HISTORIES OF RIVERS 



165 



FIG. 171. An oxbow lake in the flood 

 plain of a river. 



or " oxbow " is entirely separated from the new channel, and 

 once abandoned of the stream is transformed into an oxbow 

 lake (Fig. 171 and p. 415). 



Meander scars. Swinging as it occasionally does in its 

 meanderings quite across the flood plain and against the bank of 

 the earlier degrading river in 

 this section, the meander at 

 times scours the high bank 

 which bounds the flood plain, 

 and undermining it in the same 

 manner, it excavates a recess 

 of amphitheatral form which is 

 known as a meander scar (Fig. 

 1 72) . At length the entire bank 

 is scarred in this manner so as 

 to present to the stream a series of concave scallops separated by 

 sharp intermediate salients of cuspate form. 



River terraces. Whenever the river's history is interrupted 

 by a small uplift, or the base level is for any reason lowered, the 

 stream at once begins to sink its channel into the flood plain. 

 Once more flowing upon a low grade, it again meanders, and so 

 produces new walls at a lower level, but formed, like the first, of 

 intersecting meander scars. Thus there is produced a new flood 



plain with cliff and ter- 

 race above, which is 

 known as a river terrace. 

 A succession of uplifts 

 or of depressions of the 

 base level yields terraces 

 in series, as they appear 

 schematically represented 

 in Fig. 172. Such ter- 

 races are to be found well developed upon most of our larger 

 rivers to the northward of the Ohio and Missouri. The highest 

 terrace is obviously the remnant of the earliest flood plain, as the 

 lowest represents the latest. 



The delta of the river. As it approaches its mouth the river 

 moves more and more sluggishly over the flat grades, and swings 

 in broader meanders as it flows. Yet it still carries a quantity 



FIG. 172. Schematic representation of a series 

 of river terraces, a, b, c, e, successive terraces 

 in order of age. d, d, d, d, terrace slopes formed 

 of meander scars. 



