RELATION OF LAKES TO CLLMATIC CONDITIONS. 



59 



The mean elevation of the surfaces of the Lanrentian lakes above the 

 sea, their maximum depth, etc., as shown by soundings, are as follows : 



]\Ieax Elevation' and ^Lwimtm Depth, etc., of the LAinEXTiAN Lakes. 



The average discharge of the lakes is stated ])y Schermerhorn to be 

 double that of the Ohio and nearly equal to one half the discharge of the 

 Mississippi. The area of the Laurentian basin is a third larger than the 

 hydrographic basin of the Ohio, or about a fifth of the combined areas 

 of the bi\«ins of the Mississippi and its affluents. The outflow of the St. 

 Lawrence basin is slightly less than half its rainfall, while on the ]\Iissis- 

 si[)pi and Ohio the discharge is about a fourth of the rainfall. If the 

 average discharge of the Laurentian lakes passed through a river one mile 

 wide with a mean velocity of one mile per hour, such a river would have 

 a depth of 40 feet from shore to shore. 



The volume of water in the Laurentian lakes is about 6,000 cubic 

 miles, of which Lake Superior contains somewhat less than one half. 

 Perhaps a better idea of this volume may be obtained Avhen it is .said that 

 it is sufficient to sustain Niagara falls in their present condition for about 

 100 years. 



The mean annual rainfall of the St. Lawrence basin is about 31 

 inches ; aud the mean depth of water evaporated from the surfaces of the 

 lakes, between 20 and 30 inches.^ The amount of precipitation on the 

 water surface is, therefore, nearly compensated by the amount evaporated 

 from the same area. 



Clieini.stry of the waters of the St. Lawrence. — The composition 

 of the waters of the Laurentian lakes is shown with approximate accuracy 



1 Thoina.s Kussell, "Deptli of Evaporation in the United States," Montlil}- AVeather 

 Koport, r. S. Signal Oftice, Sept. 1888. 



