FISHERIES, GAME AND FORESTS. 385 



the last tabulations, streams vary greatly in water yield in different years, such variation 

 being in some degree proportional to rainfall. It is also proportional in different 

 catchment areas to the ground water storage capacity of the soil. Sand areas and 

 other porous soils hold back the water which falls upon the surface, while impervious 

 clay soils allow it to quickly run off after each rainfall. Forest coverings also retard 

 the run-off, producing a more equable flow. 



THE DATA NEEDED. 



In order to develop the storage capacity of any given stream to the greatest possible 

 degree, it is necessary to have enough rainfall records, covering the precipitation at 

 points either within or in the vicinity of the catchment basin of the stream, to insure a 

 precise statement of the average monthly and annual rainfall of the basin. It is also 

 indispensable to have a run-off record determined from several years' gagings. In 

 order to place the determination of the flow line beyond all question the gagings 

 should cover at least fifteen years. Whenever we work with much less length of 

 gagings than about fifteen years there is always a chance for more or less serious error. 

 Having the foregoing data as well as the area of the catchment basin, which can only 

 be satisfactorily determined from a topographical map, we are prepared to compute 

 the average line of flow to which the stream can be maintained, provided its upper 

 reaches contain the required storage ground. There is no way of determining the 

 height of dam for full development storage until these data are at hand. 



RESULTS ON GENESEE AND HUDSON RIVERS. 



By way of illustrating practical results in this direction, it may be pointed out that 

 on Genesee River, with rainfalls and run-offs as per tabulations herein contained, and 

 with a drainage area of 1,000 square miles above the point where the proposed 

 storage is to be made, the mean flow fixed upon at Rochester, where the drainage 

 area is 2,365 square miles, is 1,080 cubic feet per second during the months from May 

 to November, inclusive, and 1,000 cubic feet per second for the balance of the year, 

 the additional 80 cubic feet of the summer months being the amount of water required 

 by Erie Canal during that period. The proposed regulation, therefore, is on the basis 

 of 1,000 cubic feet per second for the entire year for the manufacturing interests, or, 

 taking into account that the minimum flow of the stream in its present, natural, 

 unregulated condition is 200 cubic feet per second, the result of constructing the 

 proposed storage will be to increase the minimum flow permanently to five times the 

 present figure. It is estimated that this result can be attained on Genesee River for an 

 initial expenditure of $2,600,000. The capacity of the storage reservoir to be 

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