240 



MODIFICATION OF THE GREAT LAKES 



being the usual month for Ontario, June or Jul}- for Krie, July 

 for Michigan and Huron, and August or September for .Superior. 

 Fig. 4 shows the character of the annual 

 oscillations, as given by averages of long 

 series of years. 



In a wet j^ear more water enters the lake 

 than leaves it, and there is a net rise of the 

 surface; in a dry year there is a net fall. 

 A series of wet years produce exception- 

 ally high water, and a series of dry 3'ears 

 exceptionally low, so that the entire range 

 of water height is considerably greater than 

 the annual range. The recorded range for 

 lakes Superior, Michigan, and Huron is 

 between 5 and 6 feet; for Erie and Ontario, 

 between 4 and 5 feet. 



The accompan3'ing diagram (Fig. 5) of 

 the oscillations of Lake Michigan illus- 

 trates the annual C3"cle and also the pro- 

 gressive changes from 3'ear to 3'ear. Being 

 compiled from monthly means of gage 

 readings, it does not show tides and seiches 

 nor the oscillations of short period. 



These various oscillations of the water, 

 though differing widel3^ in amplitude, rate, 

 and cause, yet coexist, and the3^ make the 

 actual movement of the water surface 

 highly complex. The complexity of move- 

 ment seriousl3^ interferes with the use of 

 the water plane as a datum level for the 

 measurement of earth movements, and a 

 system of observations for that jjurpose 

 needs to be planned with much care. The 

 main principles of such a S3''stem are, how- 

 ever, simple, and may readil3'" be stated. 

 The most important is that the direct 

 measurement of the heights of individual 

 points should not be attempted, but com- 

 parison should alwa3^s be made between 

 two points, their relative height being 

 measured b3^ means of the water surface 

 used as a leveling instrument. 



