0.43 meter per year, compared to a 2,54-meter per year average rate for 

 profile stations between 1969 and 1976, Stations where bulkheads had 

 been installed were omitted in the determination of the recent recession 

 rate. Property owners also made a variety of other attempts to reduce 

 erosion at many of the remaining stations, but these efforts apparently 

 had only a minor effect and measurements from such stations were retained 

 in the calculation of the recent recession rate. The 2.54-meter per year 

 rate is, therefore, an estimate of the recent rate recession on a rela- 

 tively unprotected shoreline. The older historic measurements also re- 

 flect natural recession, unaffected by man's interference. 



The recent recession rate of more than six times the historic average 

 reflects the effect of high lake levels in accelerating shore erosion. 

 The 1969-75 period represents the most intense phase of erosion during a 

 lake level cycle, whereas the 119-year rate includes the effects of sev- 

 eral episodes of both high and low levels (Fig. 2). 



Four measurement stations do not constitute a large sample on which 

 to base an estimate of long-term recession rates for this 60-kilometer 

 stretch of shore; however, each measurement does cover a 119-year period, 

 and variations in retreat rate do decrease as the period of observation 

 increases (see Fig. 14). In fact, these four measurements are about as 

 efficient for estimating the long-term mean rate as the larger number of 

 measurements made during this investigation are for estimating the shorter 

 term mean rate. The difference between bluff and shore erosion over the 

 50-year period should be inconsequential compared to the 600-percent in- 

 crease in recession during the recent period of high water. 



The four historic measurements near the present study area, and other 

 available information on historic rates along Lake Michigan's eastern 

 shore are shown in Figure 18. These historic rates are based on net 

 changes in bluff position surveyed in the 1830 's and again in the 1950 's 

 (Powers, 1958). Over such a long period of time the error involved in 

 assuming equilibrium becomes small; i.e., recession of the bluff tends to 

 approach recession measured at any other point on the upper profile in 

 the sense that any differences become small relative to the total dis- 

 placement of the profile (in this case an average of 52 meters) . 



The relatively uniform low rates of historic recession along most of 

 the eastern shore further indicate that the estimate of 0.43 meter per 

 year cannot be too far from the true rate of historic recession for the 

 present study area. 



Changes in the rates of shore retreat between various time intervals 

 at severely eroding localities on Lake Michigan are given by Seibel (1972) 

 and by Hands (1976) . Net changes presented in those references were meas- 

 ured over periods of several years; some periods coincided with episodes 

 of high water, others with episodes of low water. The rates of successive 

 periods at given locations commonly varied by 200 to 600 percent. 



Thus, although the surveys for this study covered only a part of a 

 lake level cycle, sufficient historic evidence is available to indicate 



37 



