a few profile lines need to be resurveyed after an elapse of many years 

 to provide an estimate of the long-term retreat rate, which is equally as 

 efficient as an estimate of a short-term rate based on a larger number of 

 profiles. The 119-year rate of recession based on four stations origi- 

 nally surveyed in 1838 was 0.43 meter per year. The rate of recession 

 between 1969 and 1975 (based on measurements at 20 stations) was more than 

 five times greater than this historic average. This acceleration of re- 

 cession was brought on by the recent high water levels. 



The correlation between water levels and recession rates is poorly 

 defined at localities where sfiore protection measures are adopted at times 

 of greatest potential loss; however, data from the relatively undisturbed 

 stations monitored in this study show that shore recession was roughly 

 proportional to the increase in water levels. Although local variation 

 was considerable, the shore retreated on the average of 4 meters for each 

 0.1 meter of submergence. 



Surprisingly, this 40 to 1 ratio also gives a good approximation to 

 the average advance of the shoreline as water levels declined during the 

 last year of the study. Encroachment of water on the shore as lake levels 

 rise causes only a small part of the total retreat of the shoreline. Ero- 

 sion and accretion are nearly an order of magnitude more important than 

 encroachment in terms of how far the shore is actually displaced. The 

 period of adjustment following a change in the mean water level elevation 

 may last for several years depending on the magnitude of the water level 

 change, the type of beach material, the geomorphology of the shore, and 

 the availability of wave energy to redistribute material. The capability 

 to generalize recession predictions will improve when the balance of sedi- 

 ment volumes shifting back and forth over the entire active profile is 

 better understood. 



Recession of a particular contour is one convenient way of expressing 

 the amount of shore erosion. The actual contour or elevation selected, 

 however, will affect the outcome, and all contour changes do not give 

 equally representative estimates of the regional recession. In this 

 study, all recession lines significantly above the lowest water level 

 gave relatively good indications of at least the vegvonal pattern of shore 

 recession; however, measurements at the higher elevations more efficiently 

 estimated the actual mean recession for a stretch of shore. Recession 

 lines near and below lake level not only were inefficient as estimates of 

 the mean recession for the area, but also failed to reveal even the gen- 

 eral pattern of regional shore retreat. This is because lower contours 

 may prograde lakeward while the higher beach face is eroding. In general, 

 to obtain stable and reliable estimates of recession from a few measure- 

 ments, the measurements should be taken where the beach profile slopes 

 steeply so that small changes in elevation do not cause large changes 

 in contour position. 



The effect of water level changes on recession must be considered if 

 historic changes .in the rate of shore retreat are to be properly ascribed 

 to other causes. A graph of the retreat that accompanied sumbergence is 



40 



