192 The Ohio Naturalist. [Vol. VIII, No. 1, 



Aggrading and Degrading. — When the first experiment began, 

 the screw was raised about one inch, which gave a slope of one 

 inch to thirteen feet, or thirty-four feet per mile, equivalent to 

 about one-third of one degree. This is a very steep slope for a 

 stream but the smallness of our stream made it necessary, and 

 even yet the larger particles of clay were only moved a short 

 distance from the heap and that with marked difficulty. Except 

 near the clay pile almost no aggrading at all was done. So the 

 screw was turned up about one-half inch higher which increased 

 the slope to fifty feet per mile. This seems like an enormous 

 grade yet the stream still aggraded its lower valley with diffi- 

 culty. In fact the stream increased its own slope to more than 

 two inches in thirteen feet by distributing its sediment thicker 

 near the pile of clay. If we could have given the stream more 

 time, of course it would ultimately have aggraded its whole 

 course without either uplift. 



The supply of clay was repeatedly replenished that the 

 stream might be constantly taxed to its utmost, and aggradation 

 proceeded. The clay was considerably sorted b}' the stream. 

 With an uniform slope and a shallow stream, as an aggrading 

 river must be, the larger particles suffered many halts as they 

 were rolled along or dragged on the bottom. While stopped, 

 they served as catchers for other particles which would often 

 accumulate around them. Most of the larger pieces found per- 

 manent lodgment before they attained half the journey to the 

 lake and hence the delta built during aggradation consisted 

 largely of very fine material. 



The group of particles temporarily or permanently retained 

 in the channel developed into a sandbar dividing or deflecting 

 the water, and causing it constantly to seek new courses. Six 

 observations of the stream were made in less than one hour and 

 'during each interval decided changes occurred. Figs. 3-8. It 

 thus appears that a stream engaged primarily in aggrading its 

 valley is shallow, constantly filling its own channel and breaking 

 over its banks. 



Aggradation went on until the upper valley was filled even 

 above the rock walls but the lower valley did not fill quite so full. 

 In the hope of more nearly filling the lower valley an increasingly 

 higher perforated plug was repeatedly substituted for the first 

 one, which made the water deeper in the lake. The lower valley 

 then filled satisfactorily. 



Upon the completion of the filling process, the heap of clay 

 was no longer replenished but other conditions were left un- 

 changed in order to see whether the stream would cut down into 

 its deposits without further change. Erosion l)cgan at once and 

 the sediments were picked up and pushed or rolled along to the 

 lake. Degradation proceeded perceptibly faster in the upper 



