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very significant fact that the former regimen of a stream is revealed in tho 

 character of its valley deposits. If a stream has been in the lialiit of 

 depositing only very line silt, the valley deposits (alliivinm ) will consist 

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

 liabit of (U'liositing coarse material, the valley deposits will reveal this 

 fact. If fnrthermore a stream is now deiM)siting coarse material where it 

 fornierly depo.sited only fine material, and if this change has come about 

 Itaii ijdssu with the deforestation of the region, and no other adequate 

 canse can be as.sigued, it is a fair inference that the deforestation of the 

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

 iUnstration 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 flood 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-fold. First, if floods are notably increased in fre- 

 (juency and volume it will be necessary to build more massive structures 

 to withstand them, and it will also be necessary to build large enough 

 reservoirs to hold tlie flood water, since very little catch of water can be 

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

 slopes and valleys brings down innuense quantities of sediment which tends 

 to silt up reservoirs. The rapidity and completeness with which reservoirs 

 are silted up., in the southern Api)alachian region, as described by Pro- 

 fessor Glenn, almost passes belief.^ He says : "From the slopes along these 

 streams a steadily increasing amount of waste is woi-king its way down 

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

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

 most experienced mill engineers to amount to 80 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 tliis silting of storage basins that a promineiit 

 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 lie has increased his usual estimates l)y an allowance 

 f<ir increased storm waters that nnist be taken care of without endanger- 

 ing the dam or jiiant. 



' filfiin. lot: (it. 



