measurements from common survey periods; the lines do not necessarily re- 

 flect actual shoreline position between measurement stations. 



Large positive ordinate values in Figure 8 indicate points where the 

 shore retreated a large distance. Extreme negative values indicate where 

 the shoreline advanced a large distance lakeward. Vlhere data points 

 cluster closely, the shore remained relatively stable. 



The net retreat of the shoreline between 1969 and 1975 averaged 18 

 meters or 2.9 meters per year, but as Figure 8 shows, longshore varia- 

 bility in retreat was extreme, ranging from 0.3 to 5.7 meters per year. 

 Because of the large variation observed between adjacent stations, knowln 

 edge of the 6-year retreat at a single point by itself would be of little 

 help in estimating the rate of retreat at another point a kilometer away. 

 This is not to disparage the calculation of an average rate based on 

 several measurements up and down a particular stretch of shore. Confi- 

 dence in such an estimate of the mean can be increased without limit by 

 increasing the number of measurements. The uniformly small rates measured 

 at four stations adjacent to the Pentwater jetties indicate that partic- 

 ular stretch of shore suffered less retreat than did the surrounding 50 

 kilometers. In fact, the shore experienced a net advance at two stations 

 in Mears State Park just north of the jetties and nowhere else except at 

 station 17, on the tip of Little Sable Point (see Fig. 4). 



The shoreline remained remarkably stable at stations 24 and 12. At 

 station 9 (about 2 kilometers in a northerly direction from station 12) , 

 there was negligible net retreat, but this resulted from early retreat 

 being compensated by progradation sometime during the last 13 months of 

 study. 



Falling lake levels and progradation during the last year of the study 

 advanced the shore at 24 of 30 stations. Though the additional beach 

 width gained during this last year was considerable, only three stations 

 (4.5 and 5, just north of the jetties, and 17 on the tip of Little Sable 

 Point) advanced enough to regain their 1969 shoreline. The average gain 

 (each station given equal weight) between 1975 and 1976 was 4.3 meters 

 or about 20 percent of the retreat which had occurred during the previous 

 6 years. 



2. Recession . 



The effect of declining water levels and of sediment deposition was 

 discussed previously as both contributing to the advance of the shore and 

 the partial recovery of former beach widths during the last 13 months of 

 study. However, the relative importance of the two distinct processes 

 was not identified. Progradation refers to displacement of a certain 

 topographic contour toward the lake; recession refers to displacement of 

 that contour toward the land. The exact magnitude of recession at the 

 specified datum will depend on the elevation of the datum specified as 

 well as the position along the shore where the measurement is made. 



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