Because of the large distances between lines and the uniqueness of each 

 line, they cannot be used to measure changes in the alongshore direction. In 

 fact, Birkemeier (1980) found significant variations in bluff recession at 

 points just 30 meters apart. The profile lines do, however, document in 

 detail the temporal changes which occurred at each line. 



Important in describing erosion along the Great Lakes are the primary 

 processes involved. These include variations in lake level, wave action, ice 

 cover, and slope failure. The actual effect of each process or of combina- 

 tions of the processes varies depending on the profile. 



1. Lake Level. 



The 1.43-meter increase in mean annual lake level from 1964 to 1973 was a 

 primary reason for the initiation of this study. After peaking at 176.92 

 meters above the International Great Lakes Datum (IGLD) in 1973, the lake 

 level stabilized in 1974. The variation in mean annual lake level from 1950 

 to 1974 is shown in Figure 5. Long-term fluctuations in water level correlate 

 well with precipitation though there is some phase lag (Seibel, 1972). Be- 

 cause the long-term changes are not cyclic, they are difficult to predict. 

 Cohn and Robinson (1976) attempted to predict lake levels through Fourier 

 analysis of historic records between 1860 and 1970. They were able to 

 determine prominent cycles of 1, 8, 11, 22, and 36 years. The model correctly 

 predicted the rise in lake level between 1970 and 1975 and forecasted a 

 general decrease in levels between 1975 and 1980. 



- 580 



Figure 5. Annual average of Lake Michigan water level as recorded 

 at Ludington, Michigan, from 1951 to 1978 (IGLD). 



Seasonal fluctuations are more regular, varying about 0.34 meter from a 

 winter low level to a peak level in the summer. Monthly lake level changes 

 during the study, as well as the maximum and minimum daily levels recorded 

 each month at Ludington, Michigan, are shown in Figure 6. Lake level varia- 

 tions cause an immediate movement of the waterline by either "drowning" or 

 uncovering the beach which, depending on beach slope, can have an important 

 effect on beach width. Moreover, higher lake levels permit wave action to 

 reach higher elevations and to undercut the bluff. 



14 



