Even in the usual simplified model (Fig. 5) the magnitude of recession 

 depends on the elevation where the recession is measured. Shore recession 

 by definition is measured at the elevation of the final water level, be- 

 tween the points where the final water level intersects the initial and 

 the final profiles. In the model, recession also occurs at all other 

 elevations where the initial bottom sloped at a greater angle than the 

 effective angle of ■pvofile adjustment; i.e., where the slope was greater 

 than the ratio of vertical to horizontal displacement of the idealized 

 profile (Fig. 5). At those elevations where the bottom sloped at an angle 

 less than this effective angle, the contours would move lakeward, even as 

 the whole profile and features on it move up and landward. In nature, 

 shore profiles are not smooth and do not always increase in depth lake- 

 ward. Thus, progradation may occur at several elevations, while recession 

 (net erosion) occurs elsewhere and the overall profile migrates landward. 

 The profile shape can also change as the shore recedes. The degrees to 

 which these natural complications increase the variability in measurements 

 of contour migration is shown in Figure 9. 



In the first plot at the top of Figure 9, the progressive recession 

 of the 176.92-meter contour (at the average elevation of the lake surface 

 during the 1975 survey) can be read on the vertical axis. Between 1969 

 and 1976, the average net recession at 176.92 meters was 10.5 meters and 

 the maximum net recession was 28 meters at station 14. Progradation 

 occurred at stations 4.5, 5, and 17. The 176.92-meter recession is very 

 similar to that of total shore retreat (see Fig. 8). The total shore 

 retreat is, of course, a little larger (averaging 12 meters and with a 

 maximum of 34 meters) because it includes transgression resulting from 

 0.2 meter of submergence. The areal patterns of recession and retreat 

 are, however, virtually identical. At slightly lower elevations (shown, 

 in succeeding plots in Fig. 9) the overall pattern of recession remains 

 much the same, though the magnitudes of recession progressively depart 

 from the magnitudes of shore retreat. This simply means that the overall 

 pattern of shore retreat, which could theoretically have been obtained 

 from aerial photos, reflects the overall pattern of actual recession of 

 the upper beach face, which could not be obtained without repeated ground 

 surveys . 



If progressively lower elevations are observed in Figure 9, the sim- 

 ilarity between recession and retreat deteriorates rapidly. Recession at 

 an elevation of 176.33 meters (the level of the lake surface during the 

 1967 survey) is shown at the top of the second coltimn of plots in the 

 figure. The spread of recession values encountered at the different sta- 

 tions has increased, but the same overall pattern remains recognizable; 

 zones of maximal net recession occurred at the south end of the study 

 area, around Little Sable Point and at two points a few kilometers north 

 and south of the harbor. At still lower elevations, the increased long- 

 shore variability overwhelms similarities between recession and shoreline 

 retreat. Not only would the magnitude of recession change drastically if 

 measured at slightly different elevations on the lower beach face, but 

 even the longshore pattern (i.e., the area of most and least severe 

 erosion) would be obscured in measurements made only at these lower 



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