THE WORK OF STREAMS 



183 



FIG. 191. Diagram of high alluvial 

 terraces. 



Alluvial terraces. Under new conditions, a river which 

 has been depositing may find itself underloaded. The 

 change may be due to a movement of its valley resulting 

 in a steeper gradient, to an increase of volume, to a decrease 

 in the amount of sediment 

 received from its head- 

 waters, or to still other 

 causes. Whatever the 

 cause, the river, if greatly 

 underloaded, sinks its 

 channel rapidly. The 

 remnants of the old flat 

 then stand as alluvial terraces on one or both sides of the 

 valley (Fig. 191). After the river has opened out a new flood 

 plain at a lower level, for some reason it may again degrade 

 actively, leaving a second set of terraces. Indeed, this pro- 

 cess may be repeated a number of times. If a river which 

 has been aggrading is able, under changed conditions, to de- 

 grade, but remains nearly loaded, it may shift from side to 

 side of its valley while it slowly lowers its channel, and by 

 this means form a series of terraces. This is illustrated in 

 Figure 192, where a stream is supposed to have filled its valley 



to the level A-D-B, and 

 to occupy a position near 

 the left edge of its flood 

 plain, at A . If the stream 

 now shifts toward the op- 



FIG. 192. Diagram to illustrate the , -j < <i n 



formation of terraces by a river which is P oslte slde of the valley, 



degrading slowly, and shifting from side meanwhile degrading, it 



to side of its valley at the same time. MI .1 ,1 



will occupy presently the 



position C. Should movement to the right stop there, be- 

 cause of contact with a projection of the valley wall, or 

 for some other reason, and the river return toward the left 

 side of the valley, a remnant of the old flood plain, C-D-B, 

 would remain as a terrace. In similar manner, should 

 the river fail to reach the left side of its vallev on the re- 



