LAKE LEVEL

Analyses of the stratigraphy of the late Quaternary sediments 
deposited in Lake Michigan provide a good record of the last major 
glacial advance and retreat, of at least two incursions of flood 
waters from glacial Lake Agassiz, and of drastically lowered lake 
levels when ice retreat opened the North Bay outlet draining the 
Lake Michigan to the Chippewa low, at least 80 m below its present 
level. The subsequent rise in water level from the Chippewa low to 
the Nipissing high is well documented by the distinctive, planar, 
trangressive Chippewa unconformity that left little evidence of 
intermediate lake-level positions.  A dramatic change in conditions 
in the lake occurred after about 5 ka;  ostracodes and mollusks are 
poorly preserved, eliminating many tools for recontructing 
paleolimnologic conditions.  However, short-term variations in grain 
size and magnetic properties of the sediments, over time periods of 
200 to 500 years, may be due to lake level changes.  

Before 5 ka, the complex response of lake level to isostatic changes 
in the elevation of the lake outlets and to climatic changes has been 
deciphered by analysis of various species of ostracodes, whose 
abundance and occurrence are related to lake volume and chemistry. 
Stable-isotope compositions of ostracodes and mollusks also reflect 
changes in lake volume, temperature, and chemistry.  Five periods 
have been identified. Data for the oldest suggest a cold, dilute lake;  
then they indicate falling lake level probably coincident with the 
opening of North Bay but also a drier climate which may have been 
driving the ice retreat;  the third period appears to be a cold, dilute, 
and trangressing lake probably in response to isostatic rebound;  the 
fourth event appears to be a time of lake regression controlled 
mainly by drier climate; and the fifth event indicates an increase in 
solute concentration by climatic control and possible closure of the 
outlet as water inflow dropped below outflow.  As already noted, the 
most  recent changes in lake level cannot be determined with this 
technique because of the poor preservation of ostracodes after 5 ka. 

Variations in paleomagnetic declination and inclination are largely 
coherent among core sites in southern Lake Michigan and with  
secular-variation curves from other North American sites.    
Tentative age assignments and correlations of strata can be 
achieved.  Long-term (1,000-2,000 yr) variations in the character 
and concentration of magnetic minerals correspond with long-term 
lake-level changes that have been reconstructed from the effects of 
isotatic rebound, from the changes in drainage, and from shoreline 
features.  Short-term variations (200-500 yr) in the concentrations 
and character of magnetic minerals may record short-term 
fluctuations in lake levels at a high degree of resolution not easily 
obtained by other methods.  

Beach ridges have been mapped to provide a record of former water 
level positions through time.  Where shorelines prograde extensive 
beachridge complexes may develop.  The highest altitude of upper 
shoreface deposits in each beach ridge marks the position of 
maximum wave runup on the beach.  Although differential isostatic 
uplift complicates the interpretation of beach ridge complexes in 
Lake Michigan and Lake Huron, this technique shows great promise of 
providing a reliable lake level curve when dating is complete and 
records for different complexes are correlated.