the basis of the lake level change. Those retreat values that deviate 

 most from the linear relationship" are identified by station number. These 

 stations are mostly located either in the immediate vicinity of the 

 Pentwater jetties or on Little Sable Point--areas of anomolous recession 

 as already pointed out in the discussion of spatial variations in retreat. 

 The points to the right of the vertical axis in Figure 16 all represent 

 changes during the last year of study and just before the rapid fall in 

 lake levels. Note that the shoreline at most stations shows an advance 

 for the first time during the period 1975 to 1976. Apparently the recent 

 cycle of accelerated shore retreat was complete, or nearly so, given the 

 lower water levels in 1976. 



Because water levels are rarely stable for long periods of time, 

 almost any increase or decrease in erosion measured on the Great Lakes 

 may be partially attributable to a difference in water levels. In many 

 instances, the data shown in Figure 16 could serve as a basis for esti- 

 mating how much of the recession could have been caused by water level 

 changes and how much must be due to other causes. Examples of this appli- 

 cation are given in Appendix A. 



4. The Timelag Between Lake Level Perturbation and the Reestablishment 

 of Profile Equilibrium . 



A tentative model describing the general response of the shore to 

 rising lake levels was proposed by Hands (1976) . Based on what appeared 

 to be a more rapid adjustment of the offshore bathymetry, and an over- 

 steepening of the upper profile in surveys through 1971, Hands suggested 

 that the lakeshore would continue to recede about another 11 meters before 

 regaining equilibrium, even if the lake level stabilized at the 1971 

 elevation. 



Actual lake level fluctuations were such that annual surveys beginning 

 in 1973 would have precisely identified any lag in shore response that 

 occurred after the end of the rise in lake levels. Unfortunately, the 

 stations were not surveyed until 1975 (Fig. 15). The measurements that 

 were collected, nevertheless, have some bearing on the proposed lag be- 

 tween lake level changes and shore adjustment. The data points in the 

 emergence region of Figure 15, based on the changes that occurred between 

 August 1975 and September 1976, reveal that shore retreat had abated by 

 that time. Since progradation was not occurring everywhere (see Fig. 8) 

 the shore was probably still in a transition stage in 1976, though it is 

 possible that most of the recession may have occurred shortly after the 

 waters peaked in 1973. Given the available data, the critical question 

 in this regard is whether the recession rates between 1971 and 1975 show 

 a significant reduction below the 1969 and 1971 rates. If so, this would 

 indicate the profiles had nearly regained equilibrium and the hypothesized 

 lag in shore response would probably be shorter than 2 years (the time 

 between the 1973 peak and the 1975 survey) . 



Given the great variation in rates of recession at different sites, 

 any examination of changes with time should be based on measurements from 



33 



