Nov., 1907.] Aggradation and Degradation of Valleys. 195 



that it is largely the deposition of overload in the slackwater 

 sides of streams which engenders meanders and promotes their 

 development. Although the changes in course were so great 

 during deposition, after the supply of sediment was cut off, and 

 erosion began, the stream would go for an hour or more with only 

 slight local changes. 



With every swing of the stream from one side of the valley to 

 the other, a thin layre of alluvium was scraped off and a terrace 

 was left. Six distinct terraces were at one time counted on one 

 side of the valley. Terrace fronts were less than half an inch 

 high unless two or more had been combined by cutting out the 

 lower ones. The terraces had a great variety of shapes and 

 directions corresponding to the mean,derings of the stream ; con- 

 sequentlv no tv\'o successive fronts were parallel. 



Slope, waste, and water supply in relation to eroding power. — 

 The slope as noted above was about sixty-six feet per m_ile, and 

 even with this steepness, deposition went on actively as long as 

 the stream's maximum capacity for carrying sediment was 

 taxed, but when the pile of clay was no longer replenished active 

 erosion began. The water supply was kept as nearly the same 

 as possible, and the slope was not increased. Since the amount 

 of sediment in the stream was the only factor changed it follows 

 that any change in the habit of the stream may be intimately 

 related to the load. 



Let us look, however, for a moment at the other factors in the 

 problem. The supply of water has a great effect in determining 

 the power of a stream. Several times during the experiment 

 the volume of the stream was doubled for a few minutes and it 

 was easy to see that the power of the stream, whatever it was 

 doing, was more than doubled. With an unlimited supply of 

 waste, the stream simply carried more and on ihe average, car- 

 ried it farther with the double volume than with the single 

 volume. It did not degrade at all but rather aggraded faster. 

 With no waste at the source, the increased volume accelerated 

 the stream in its degrading; banks were more quickly undercut, 

 and debris was more abundantly and more rapidly hurried down 

 stream. At a time when the supply of clay was limited but the 

 stream was aggrading, a doubling of the volume of water changed 

 the habit of the stream and it began to degrade. In fact, at 

 this time, a volume of water could be used such that the stream 

 neither aggraded nor degraded perceptibly; but that a very 

 slight increase initiated erosion, and a very slight decrease 

 initiated depostion. The balancing of load and power was very 

 accurate and the adjustment so perfect that very slight changes 

 in stream volume were sufficient to unbalance forces and change 

 the stream's habit. 



