STORAGE RESERVOIRS. 103 



"The maximum and minimum discharges, except for Niagara, show a much 



, maximum discharge 



greater divergence, the ratios of . . - for 1883 being as follows: 



minimum discharge 



"Niagara, 1.19; Ohio, 28.22; Missouri, 29; and Upper Mississippi, 10.29. 



"This striking dissimilarity in the regimen of streams of similar magnitude, and, 

 with one exception, of similar climatic conditions, is entirely due to the reservoir action 

 of the Great Lakes. Of that portion of the St. Lawrence drainage-basin which lies 

 above Niagara Falls, viz., 265,095 square miles, 87,400 square miles, or almost one- 

 third, is made up of the water-surfaces of Lakes Superior, Michigan, Huron, and Erie. 

 One foot upon this immense area represents 2,436,000,000,000 cubic feet greater than 

 the excess of the late Mississippi River flood at Cairo above the bankful stage. 



" The mean annual fluctuation of Lake Superior, based upon twenty -five years' 

 observation (1871-1895), is 0.93 foot; of lakes Michigan and Huron, i foot; of Lake 

 Erie, 1.16 feet. This fluctuation represents an annual storage of 2,419,000,000,000 

 cubic feet of water, equivalent to about 153,000 cubic feet per second for a period of 

 six months. The maximum annual fluctuation during the above period is just about 

 twice the above mean, and of course represents twice as much water stored. 



" In addition to the annual fluctuation, there is constantly going on a periodic 

 change which often requires several years to complete the cycle. As an illustration 

 of this characteristic of the Great Lakes take the period of eight years from 1872 to 

 1879, inclusive, during which the mean annual level of the four upper lakes rose for a 

 period of four years and fell during the following three years. The rise in mean level 

 was, for Lake Superior, 1.03 feet; for Lakes Michigan and Huron, 2.02 feet; and for 

 Lake Erie, 1.97 feet. The total storage represented by this rise of mean level was 

 4,000,000,000,000 cubic feet. The fall in mean level following the rise was, for Lake 

 Superior, 1.63 feet; for Lakes Michigan and Huron, 1.46 feet; and for Lake Erie, 1.17 

 feet equivalent to 3,627,000,000,000 cubic feet. After this fall the mean level began 

 to rise again. 



"The foregoing figures convey some faint idea of the magnitude of the storage 

 of the Great Lakes, and of the way in which it operates to preserve a balance not only 

 between the wet and dry seasons of each year, but between those cycles of wet and dry 

 years which are continually recurring. These reservoirs absorb the flood-waters of 

 spring and pay them out in the following dry season, thus preventing floods on the one 

 hand and low water on the other. And while these seasonal changes are going on the 

 lakes respond to the varying conditions of longer periods, levying upon years of more 

 than average precipitation in order to maintain a flow in the outlets during. the years 

 of deficiency which are certain to follow. 



" The result of this storage action of the Great Lakes is to produce a river system 

 radically different in its general characteristics from nearly all other streams. Such 

 conditions as high and low water, as elsewhere understood, are here entirely unknown. 

 Commerce pursues its way through these lakes and rivers without serious hindrance 



