levels, and of wave energy variations during the period of adjustment are 

 desirable to refine methods of predicting shore response. In the interim, 

 if it is known that persistent longshore bars have migrated landward 

 faster than the adjacent shore retreated, after some coastal submergence, 

 then it should be assumed that the shore will continue to recede, even 

 after subsidence ceases, until such time as the original spacing between 

 bars and the shore is reestablished. 



5 . Comparison of Recent and Historic Changes . 



The 50-year retreat of the shoreline in the immediate vicinity of 

 Pentwater Harbor was determined by plotting the 1919 shoreline (based on 

 a survey by the U.S. Army Engineer District, Milwaukee) and the 1969 

 shoreline (based on aerial photos) to a common scale using a zoom-trans- 

 fer scope. The average shore retreat was then estimated by planimetering 

 the area between the two shorelines and dividing by shore length (450 

 meters). The 1969 lake level stood 0.46 meter below the 1919 level, so 

 that shore recession was actually greater than shoreline retreat. There- 

 fore, the observed shoreline retreat was reduced by the estimated with- 

 drawal that would have accompanied a decline in water levels to their 

 1919 elevation. The magnitude of such a withdrawal was estimated using 

 the average profile shape at stations 4.5, 5, 6, and 6.5 in this 450-meter 

 stretch (Fig. 17). The estimated 50-year mean rate of shore recession 

 obtained (see inset in Fig. 17) was 0.30 meter per year. During the 1967- 

 76 period of high water the average rate of recession in this area was 

 only 0.25 meter per year (top part of Table 3). Thus, the rate of re- 

 cession for this stretch of shore actually decreased during the recent 

 period of high lake levels. 



As discussed in Section VI, various influences combine to stabilize 

 the shore in the vicinity of the harbor; consequently, recent rates of 

 retreat near the harbor are not typical of retreat on the adjacent unpro- 

 tected beaches. It is interesting to note that if measurements had only 

 been made in the vicinity of the harbor, they would have produced no 

 evidence of the increase in recession rates that actually accompanied 

 recent high lake levels. This may be far from an isolated case, because 

 before the present concern for environmental preservation most studies of 

 long-term beach changes on the Great Lakes were conducted near jettied 

 inlets or at sites of critical erosion where efforts were made to stabi- 

 lize the lakeshore. To the extent that these efforts were effective, 

 they tended to reduce the range of recession rates observed through time 

 and, therefore, to also obscure the correlation between lake levels and 

 shore recession. 



One data set which does not concentrate on areas of critical erosion 

 was compiled by Powers (1958) who resurveyed a part of the shore bluff 

 near section corners along most of the entire perimeter of Lake Michigan. 

 Two of his stations which fall within the present study area are shown as 

 P85 and P86 in Figure 4; two more stations were located just south of the 

 study area, 4 and 15 kilometers, respectively, from station 29. The rates 

 of bluff recession at these four points between 1838 and 1957 averaged 



35 



