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NEW YORK STATE MUSEUM 



The tabulation shows that during the years 1892 to 1895, 

 inclusive, the mean runoff of the Upper Mississippi area was 

 only 3.61 inches on the total catchment. These figures, however, 

 are subject to correction because the state of the reservoirs at 

 the beginning and ending of the four-year period is not given in 

 the report of the United States engineers, from which these data 

 are taken. This correction, however, can not be very large, 

 because the reservoirs are so operated as to be emptied, generally 

 speaking, each year. In considering the runoff of these Upper 

 Mississippi reservoirs, due consideration should be given to the 

 fact that the water area of the reservoirs is 585 square miles, or 

 nearly 18 per cent of the whole. For Lakes Superior, Michigan, 

 Huron, St Olair, and Erie we have a total water surface of 

 87,895 square miles, with a total catchment area, including the 

 surface of the lakes, of 265,095 square miles. The water surface 

 of these several lakes is, therefore, about 33 per cent of the entire 

 area of the basin, or nearly double the relative area of water 

 surface and catchment area for the Upper Mississippi reservoirs. 

 With other conditions the same, this fact would probably lead 

 to a somewhat greater proportion of runoff from the Great Lakes. 

 The Upper Mississippi reservoirs are in a forested region, and it 

 is interesting to consider what the runoff will be after the forests 

 are removed. Taking into account results in other places, it is 

 probable that the runoff, under conditions of deforestation, will 

 not exceed an average of about 2 inches per year. 



Runoff of Desplaines river. By way of further illustrating the 

 yield of streams in the vicinity of the Great Lakes area, we will 

 refer to the runoff of the Desplaines river, as given in table No. 40. 

 This stream has been measured by the Chicago Drainage Com- 

 mission, with certain intermissions as shown since January, 1886, 

 the catchment area above the point of measurement being 633 

 square miles. The catchment comprises a long and narrow flat 

 region extending from near Chicago to a few miles north of 



