?8.'iod of Distribution of Standard 



Observation Rates of Retreat Deviation (m/yr) 



20 10 



Advance 



30 tm/yr) 



2»r 



4yr 



6»r 5 



rZOyr 



I 



t^as 



Figure 14. Distributions of measured retreat rates. Note the spread in 

 the rates o£ shoreline change decrease as the period of ob- 

 servation lengthens. The histogram of 120-year rates is not 

 strictly comparable to the others as it is based on a larger 

 number of observations and includes effects of variations 

 encountered around the entire perimeter of the lake (Powers, 

 1958); however, it still illustrates a continued reduction 

 in the spread of retreat rates as the time interval lengthens. 



M0NTI1.Y MEAN LAKE LEVEL 



1967 1968 1969 1970 1971 1972 1973 1974 1975 1976 1977 



Figure 15, Lake Michigan hydrograph showing changes in lake 

 level between survey periods. 



31 



