144 



very significant fact tliat the former regimen of a stream is revealed in tlii! 

 cliaracter of its valley deposits. If a stream has been in the habit of 

 depositing only very fine silt, the valley deposits (alluvinm) will consist: 

 of fine material only. On the other hand if the stream has been in the 

 habit of depositing coarse material, the valley deposits will reveal this 

 fact. If furthermore a stream is now deiK)siting coarse material where it 

 formerly deposited only fine material, and if this change has come about 

 pari passu with the deforestation of the region, and no other adequate 

 cause can be assigned, it is a fair inference that the deforestation of the 

 region has changed the regimen of the stream. This effect also finds ample 

 illustration in southern Indiana. Torrential streams now emerge on the 

 sides of broad alluvial valleys, building fans of coarse and sterile gravel 

 out over the finer silt of the main stream fiood plane. Deep scouring of 

 fertile valleys by flood waters is only too common. 



Now the importance of this change in stream regimen for the water- 

 supply engineer is two-fcild. First, if floods are notably increased in fre- 

 quency and V()lunu> it will bo necessary to build more massive structures 

 to withstand thcni, aiul it will also be necessary to build large enough 

 reservoirs to hold tiie flood water, since very little catch of water can bo 

 expected in the growing season. Second, the greatly increased erosion of 

 slopes and valleys brings down immense quantities of sediment which tends 

 to silt up reservoirs. Tlie raiiidity and completeness with which reservoirs 

 are silted up, in the soutiiern Appalachian region, as described by Pro- 

 fessor Glenn, almost passes belief.' lie says: "From the slopes along these 

 streams a steadily increasing amount of waste is working its way down 

 the channels, filling the dams and destroying their storage capacity' ; and 

 this loss of storage means a decrease of efliciency that is calculated by the 

 most experienced mill engineers to amount to 'M) to 40 per cent, in plants 

 that have been built especially for storage and a somewhat less marked 

 decrease in other plants, the exact amount depending on the topography 

 of the basin and the regimen of the particular stream on which the plant 

 is located. So universal is this silting of storage basins that a promuient 

 mill engineer of wide experience in his reports on the construction of 

 power plants no longer calculates on power or anything except the flow 

 of the stream, and he has increased his usual estimates by an allowance' 

 for increased storm waters that must be taken care of without endanger- 

 ing the dam or plant. 



' Glenn, loc. cit. 



