LAND WATERS STREAMS 193 



is likely to go with the decreasing declivity down-stream; but this 

 is partly, or sometimes wholly, counterbalanced by the increasing 

 volume of water. By the exchange of load, therefore, a stream 

 may ultimately sink its channel below the flood-plain which the 

 earlier and perhaps smaller stream had developed. 



(4) Any 'stream which has reached the flood-plain stage is 

 likely to meander. After the flood-plain has become wide, the 

 width of the belt within which the stream meanders is less than 

 the width of its plain. In the Lower Mississippi, for example, the 

 meander belt is often no more than a third to a tenth of the width 

 of the flood-plain. The meanders migrate down the valley. In 

 so doing they depress the meander belt, the tendency being to 

 reduce it to the level of the channel, and, therefore, below the level 

 of the flood-plain. As the meander belt widens, the depression 

 which it develops becomes more and more capacious. Presently 

 it may attain such dimensions as to hold the water of ordinary 

 floods. At this stage, or even before, such parts of the earlier 

 flood-plain as remain are terraces. 



Terraces developed by the normal activities of a stream are 

 always low, and it is improbable that they would ordinarily be 

 conspicuous. 



In conclusion, it may be stated that many river terraces, mostly 

 very low, are normal features of valley development, coming into 

 existence at definite stages in a valley's history. They are generally 

 composed, in large part, of river alluvium. Others result from 

 more or less accidental causes, working singly or in conjunction, 

 and to this class belong all of the more conspicuous terraces devel- 

 oped from flood-plains. 



Discontinuity of terraces. Where a stream's deepened channel 

 is in the middle of its flood-plain, there is, temporarily, a terrace 

 on either side; but wherever the deepened channel is at one margin 

 of its flood-plain, a terrace remains on the other side only. Even 

 where continuous at the outset, terraces soon become discontinuous, 

 for all processes of sub aerial erosion conspire to destroy them. A 

 stream is likely to meander on its second and later flood-plains, 

 as on its first and highest one. Wherever the meanders on its 

 second flood-plain undercut the first terrace, the terrace at that 



