581 

 580 



Figure 5. Hydrograph showing change in lake levels hetween survey 

 periods on Lake flichigan. 



All shore profiling was done with an automatic engineer's level. Dis- 

 tances were determined in previous years by a rodman carrying one end of a 

 marked measuring wire which was spooled out and read from the reference 

 monument; the standard three-wire reading method was used in 1976. Shore 

 monuments at each profile station were tied to one another, to surrounding 

 bench marks, and to second-order geodetic monuments surveyed by National Ocean 

 Survey (NOS) in 1973. Vertical reference was supplemented during profiling 

 operations using a system of water level recorders, water surface rod- 

 readings, and a portable stilling well which was placed near the shoreline at 

 the station being sounded. 



As mentioned, use of the leveling cart limited coverage to depths of less 

 than 5 meters in 1967. The outer limit was extended to 11, 16, and 21 meters 

 in 1969, 1971, and 1975, respectively. No echo sounding was done in 1976; the 

 shore profiles terminated in about 1.5 meters of water. 



III. PROFILE CHANGES 



1, 



Shore Retreat. 



The annual mean surface elevations of Lake Michigan rose 1.4 meters from a 

 record low in 1964 to a record peak for this century in 1973. The earliest 

 shore profiles in the study area were surveyed in 1967 after the water level 

 rise was well underway. The rates of shore retreat from 1967 through the peak 

 water year, and for 3 years thereafter, are contrasted with historic retreat 

 rates by Hands (1979). The average rate of shore retreat (landward displace- 

 ment of the Stillwater level) during the latter part of the recent period of 

 rising water was about six times greater than it had been during the preceding 

 120-year period, or about eight times greater than during the previous 50 

 years. This Increase reflects the effect of recent high lake levels. As the 

 lake levels rose the shore retreated roughly in proportion to the increase in 

 lake levels. Retreat rates remained high for several years after lake levels 

 stabilized; then as levels declined between 1975 and 1976 the beach began 

 prograding lakeward. During the last year of study, the average advance of 

 the shore was similarly proportioned to the drop in lake level during that 

 period. The horizontal change in shore positions averaged about 40 times the 

 vertical change in water level surface during those same periods. Simple 



13 



