2. Background . 



Alternating periods of sustained rise and fall are characteristic of 

 the annual mean surface elevations on the Great Lakes (Pig- 2). The 

 cumulative effect of these persistent changes in lake levels frequently 

 shifts monthly and annual mean surface elevations as much as a meter in 

 a few years (Table 1) . 



Table 1. Major 



increases 



in annua 



1 mean 



lake 1 



evel. 



Lake 



1925 

 (ra) 



-29 

 (ft) 



1949 

 (m) 



-52 

 (ft) 



1964 

 (m) 



-73 



Cft) 



Ontario 



0.70 



2,3 



0.63 



2.1 



0.91 



3.0 



Erie 



0.71 



2.3 



0.55 



1.8 



1.14 



3.8 



Michigan-Huron 



0.99 



3.3 



0.88 



2.9 



1.45 



4.8 



Superior 



0.47 



1.6 



0.19 



0.6 



0.26 



0.9 



Although the changes in water level on the Great Lakes (Table 1) may 

 not appear large relative to tidal ranges at many ocean beaches, the 

 long-term, gradual nature of the lake level fluctuations increases their 

 effect on shore erosion and property loss. During years of low water, 

 new property owners easily acquire a false sense of shore stability and, 

 as a result, often build structures too near the shore. Storm erosion 

 during years when the mean lake levels are high accelerates the rate of 

 shore retreat, and causes considerable destruction to shore property on 

 the Great Lakes. The long duration of high water periods also allows 

 time for a relatively broad area of the nearshore zone to adjust to the 

 elevated water surface (Hands, 1976). This adjustment involves offshore 

 transport of large volumes of beach material and, as a consequence, 

 greater shore retreat. After lake levels have declined sufficiently to 

 reverse conditions, the waves transport some material from offshore back 

 on the beach at most localities. 



Rates of shore erosion fluctuate in response to the long-term hydro- 

 logic cycle. The impact of high lake levels, while relatively strong 

 on all the lower lakes, is relatively weak on Lake Superior where the 

 variations in water level are small (Fig. 2) and rocky shorelines are 

 common . 



No comprehensive survey of shore damage has been made on the U.S. side 

 of the Great Lakes during the present episode of high water levels. The 

 recent lake level rise is, however, similar to the previous rise which 

 peaked in the early fifties (Fig. 2). A survey of economic loss sustained 

 over a 12-month period coincident with the high levels of 1951-52 attri- 

 buted $50 million worth of damage to wave erosion of U.S. property on the 

 Great Lakes (U.S. Army Engineer Division, North Central, 1965). Consid- 

 ering inflation and recent shoreline development, it has been estimated 

 that a recurrence of 1951-52 storms and high water levels would cause a 

 minimum of $120 million damage (Great Lakes Basin Commission, 1976) . 



