368 STUDIES FOR STUDENTS 



At times when for any reason, for example a climatic change, the fine 

 material becomes deficient or the river volume becomes augmented, 

 the river with the same grade may pick up the coarse material pre- 

 viously laid down and transport it with comparative rapidity to a 

 lower portion of its course. 



Third, Daubree showed that upon revolving coarse and fine rock 

 material in a barrel the coarser fragments, after being rounded, are 

 reduced in size very slowly, so that a journey of several hundred miles 

 would be required to reduce a pebble of 2 inches in diameter to i inch 

 diameter. This may seem at first thought contradictory to the obser- 

 vations on river transportation, but the explanation is doubtless as 

 follows: — The material which is of such a size that the current, w^hich 

 is already partially loaded with finer detritus, is just able to move it, 

 is only moved with a fraction of the speed of the slightly smaller 

 material. The result is that in being moved a mile down stream it 

 may have moved ten or fifty miles relatively to the finer detritus. In 

 so far as the rubbing and attrition are concerned, it has accomplished 

 a journey many times the length of the actual distance moved. This 

 lagging, of the hardest coarse material under .stable river-- conditions 

 will be such that the added attrition due to the lagging is able to reduce 

 it to that slightly smaller size which may be just handled by the 

 slightly decreased velocity of the next lower portion of the river. With 

 all but the hardest it appears, however, as previously stated, that the 

 pebbles go to pieces faster than is called for by the necessities of trans- 

 portation, so that in the lower portions of the valley there is an abnor- 

 mal proportion of fine material — material which can be carried forward 

 by a sluggish current — with the result that the condition for equilibrium 

 is a flattened slope. 



Fourth, the slope of a graded river is known to be in delicate adjust- 

 ment between the volume of water, the quantity and fineness of load. 

 These relations briefly stated are: (i) A more voluminous stream 

 tends to flow down the same slope with greater velocity, the frictional 

 resistence of the bed bearing a smaller ratio to the moving force. 

 To remain in equilibrium upon such a slope the stream must carry 

 more waste or coarser waste. Otherwise it will tend to load up its 

 current by cutting down in the upper part of its course, building up 

 in its lower until the grade is flattened, the velocity diminished and 



