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NEW YORK STATE MUSEUM 



123,330 cubic feet per second instead of 265,000 cubic feet per 

 second, as determined by the Lake Survey. These figures, while 

 not conclusive, are suggestive, so much so, indeed, that taking into 

 account all the conditions it seems clear that in a series of years of 

 minimum rainfall the runoff of the Great Lakes, tributary to 

 Niagara river, may be as low as from 6 to 9 inches a year on the 

 catchment. At the former figure the mean discharge would be 

 about 177,700 cubic feet per second. 1 



As an additional source of loss from the Great Lakes the diver- 

 sion of 10,000 cubic feet per second through the Chicago drainage 

 canal to the headwaters of Illinois river may be referred to. 

 Thus far the discussion of such loss has been mainly conducted 

 on the supposition that the mean discharge of the Great Lakes at 

 Niagara was about 265,000 cubic feet per second. If this were 

 true the injurious effect of such diversion could only appear dur- 

 ing a series of extremely dry years. The writer can not but think 

 that this whole question of the runoff of Niagara river has become 

 fogged by a discussion based thus far pjurely on averages. What 

 we really want to know is the runoff of a cycle of dry years. With 

 such data we can compute the effect of a given diversion more 

 satisfactorily than when dealing with means. 



With a cycle of rainfall years, either high or about the average 

 very little effect from such diversion will be observed, the con- 

 sensus of opinion at the present time apparently being that it will 

 not exceed about 0.3 to 0.4 foot in depth over the areas affected. 

 Owing to the balancing of conditions due to the pondage of the 

 Great Lakes, and which requires years in order to complete a 

 cycle, it is uncertain whether the abstraction of 10,000 cubic feet 

 per second at Chicago would be specially detrimental at Niagara 



*By way of illustrating further the probable inaccuracy of the Lake Survey 

 figures, it may be pointed out that if the determination of evaporation from 

 the water surfaces at 101,890 cubic feet per second and runoff at 265,000 

 cubit feet per second for the year 1868 is correct, the total outgo from these 

 two sources was 308,800 cubic feet per second, leaving the land evaporation 

 for that year at 156,330 cubic feet per second, or at 0.9 foot over the 

 catchment. 



By studying the evaporation of the Upper Mississippi reservoirs, the 

 Desplaines and Muskingum rivers, and other streams herein referred to, 

 it will readily be seen that it is exceedingly improbable that a land evapo- 

 ration as low as 0.9 foot ever occurred over the whole catchment of the 

 Great Lakes. 



