More study is needed on profiles returning to equilibrium under constant 

 water levels to establish procedures for estimating the adjustment time, 

 and also to establish the outer limit of the responding profile as this 

 factor controls the physical work required to adjust the profile after 

 perturbation by a lake level change. 



VI . CONCLUSIONS 



When water levels rise or a coast subsides, shorelines tend to re- 

 treat. Retreat in response to submergence is particularly important on 

 the Great Lakes where climate and hydrologic variations cause significant 

 water level fluctuations. The process of shore retreat and eventual sta- 

 bilization is examined by using beach profiles obtained at 34 stations 

 surveyed in 1967i 1969, 1971, 1975, and 1976 on the eastern shore of 

 Lake Michigan. 



The annual mean elevation of Lake Michigan during this century has 

 gone through several cycles during which water levels rose for several 

 years in succession (e.g., 1964-73), and then declined for a similar 

 period. However, the net rate of shore recession during the last 100 

 years is small relative to the rates measured during the end of the re- 

 cent rising phase. Landward sand transport and shoreline accretion during 

 the intervening years of declining lake levels cause the shore to advance, 

 thus lowering the overall historic recession rate. The mean water level 

 elevation is the principal factor establishing a potential erosion rate 

 for a given shore type; the extent of erosion actually realized will then 

 depend on the available energy. The actual retreat of the shore can be 

 divided into two components: (a) encroachment of the water due to sub- 

 mergence of the beach, and (b) recession due to erosion as the beach ad- 

 justs to the new water surface elevation. Given a change in water level, 

 the encroachment can be predicted exactly; the recession, which may be 

 several times more important in terms of ultimate shore retreat, can only 

 be crudely predicted at present. 



After a rise of 0.8 meter in annual mean lake level between 1967 and 

 1973, recession rates remained well above the historic average through 

 1975. By the fall of 1976, however, shore erosion had ceased at most 

 survey stations. After retreating an average of 24 meters from 1967 to 

 1975, the shore may have finally regained approximate equilibrium with 

 the (by then) slowly falling mean water surface. 



Large variations in retreat were observed at adjacent stations. 

 Areas on Little Sable Point and at the south end of the study area suf- 

 fered the greatest net retreat; the area in the immediate vicinity of the 

 Pentwater jetties suffered the least. Less loss around the jetties re- 

 flects the effects of various shore protection measures employed there. 

 An explanation for the generally high rate of retreat in the other areas 

 is not evident at this time. 



The spread of retreat rates among the different stations decreased as 

 time progressed through the study period. This trend shows that only 



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