Nov., 1907.] Aggradation and Degradation of Valleys. 
'95 
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 
w r as 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 
lov r er ones. The terraces had a great variety of shapes and 
directions corresponding to the meanderings of the stream; con¬ 
sequently no two 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 mile, and 
even with this steepness, deposition w^ent on actively as long as 
the stream’s maximum capacity for carrying sediment w'as 
taxed, but wffien the pile of clay w r as no longer replenished active 
erosion began. The w r ater supply w r as 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 h£.s a great effect in determining 
the power of a stream. Several times during the experiment 
the volume of the stream w^as doubled for a few' minutes and it 
w r as easy to see that the power of the stream, whatever it was 
doing, was more than doubled. With an unlimited supply of 
wmste, the stream simply carried more and on the 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 w r aste 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 dowm 
stream. At a time w r hen the supply of clay w r as limited but the 
stream w r as 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 u r ater 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 pow'er w r as 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. 
