164 



EARTH FEATURES AND THEIR MEANING 



FIG. 169. Tree in part undermined 

 upon the outer bank of a meander. 



The scour upon the convex side of a meander causes the river 

 to swing ever farther in that direction, and through invasion of 

 the silted flood plain to migrate across it. Trees which lie in its 



path are undermined and fall out- 

 ward in the stream with tops di- 

 rected with the current (Fig. 169). 

 Whenever the flood plain is for- 

 ested, the fallen trees may be so 

 numerous as to lie in ranks along 

 the shore, and at the time of the 

 next flood they are carried down- 

 stream to jam in narrow places 

 along the channel and give the er- 

 roneous impression that the flood 

 has itself uprooted a section of for- 

 est (see p. 418). 



The cut-off of the meander. - 

 As the meander swings toward its extreme position it becomes 

 more and more closely looped. Adjacent loops thus approach 

 nearer and nearer to each other, but in the successive positions 

 a nearly stationary point is established near where the river 

 makes its sharpest turn (Fig. 170, G, and 

 Fig. 454, p. 417). At length the neck of land 

 which separates* meanders is so narrow that 

 in the next freshet a temporary jamming of 

 logs within the channel may direct the waters 

 across the neck, and once started in the new 

 direction a channel is scoured out in the 

 soft silt. Thus by a breaking through of 

 the bank of the stream, a so-called " cre- 

 vasse," the river suddenly straightens its 

 course, though up to this time it has steadily 

 become more and more sharply serpentine. 

 After the cut-off has occurred, the old chan- 

 nel may for a time continue to be used by the 

 stream in common with the new one, but the advantage in velocity 

 of current being with the cut-off, the old channel contains slacker 

 water and so begins to fill with silt both at the beginning and 

 the end of the loop. Eventually closed up at both ends, this loop 



FIG. 170. Diagrams to 

 show the successive 

 positions of stream 

 meanders and the 

 relatively stationary 

 point near the sharp- 

 est curvature. 



