THE WORK OF RUNNING WATER 207 



drops coarse material to take fine, its channel is degraded unless there 

 is at the same time a great reduction in the stream's energy. Such 

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

 but this is partly, or sometimes wholly, counterbalanced by the increas- 

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



Fig. 195. — Diagram to illustrate the development of river terraces by the widening 

 of a channel or meander belt. The valley flat above might not be called a terrace; 

 but the same plain below, where the meander belt has some width, would be called 

 a terrace. 



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

 and perhaps smaller stream had developed. 



(3) Again, so long as a stream is actively eroding at its head, there 

 is likely to be some aggradation below. At a later stage in the stream's 

 history, when active erosion at the head has ceased because of the reduc- 

 tion of the surface, less material will be carried from the upper part 

 of the A^alley, and the stream on the flood-plain below, formerly loaded 

 with material from up the valley, is now free to take up and carry away 

 material temporarily left on the flood-plain. The result is a deepening 

 of the channel. 



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

 to meander. After the flood-plain has become mde, 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. It has 

 already been pointed out that 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- 



