mean lake level remained essentially stable in 1974 (i.e., repeated the 

 sequence of record high monthly means set in 1973), then began dropping 

 slowly in 1975. During the last half of 1976, precipitation in Lakes 

 Superior and Michigan basins was down 40 and 45 percent, respectively, 

 from their long-term averages, and Lake Michigan levels began to fall 

 rapidly. 



The relationship between shore retreat and lake level changes is in- 

 dicated in Figure 16. The ordinate value of each point is the difference 

 between the daily mean lake levels from one survey to the next, several 

 years later; the abscissa is the distance the shore retreated between 

 surveys. The general tendency of shore retreat to be proportional to the 

 change in water level and the deviation of individual measurements from 

 this trend are evident . 



-0.75 



-0.50 0.25 



Submergence 



0.25 



Emergence 



Figure 16. Submergence (the rise in water level) versus retreat (the 

 landward migration of the shoreline). Data on emergence 

 were obtained between 1975 and 1976, several years after 

 the lake levels began to decline slowly. Outliers far 

 from the evident linear trend are identified by station 

 number . 



As expected, local variations in wave exposure, in the nature and 

 orientation of the shore, in offshore topography, etc., cause the indi- 

 vidual measurements to scatter widely about the mean predicted solely on 



32 



