A 4,700-Year Record of Lake Level and Isostasy for Lake Michigan

Abstract Four relative lake-level curves (RLLCs), produced from five sites bordering Lake Michigan, show similar timings of high and low lake-levels during the late Holocene. However, glacial isostatic-adjustments and possibly tectonism experienced at each site are superimposed on these records of relative lake-level change. This effect causes the RLLCs to diverge from each other with time. The absolute magnitudes of lake-level fluctuations for the late Holocene can only be determined by quantifying and subtracting the component of vertical ground-movement from each RLLC. Both an exponential rate and a constant-rate equation for a shoreline undergoing isostatic adjustment were used to model vertical movement for each site. Results show that for at least the last 4,000 calendar years (cal BP) of record, vertical movement in Lake Michigan has obeyed both types of equations. The two models yield similar results because rates of vertical movement of the shorelines around Lake Michigan are small and the time frame for which lake-level data are available is so short that the exponential nature of isostatic change is not expressed. Except for the southern shore of Lake Michigan, all the study sites have experienced uniform isostatic uplift consistent with trends reported by the Coordinating Committee on Great Lakes Basic Hydraulic and Hydrologic Data (1977) and Tushingham (1992). The southern shore of Lake Michigan, however, experienced a change in uplift rate relative to the Port Huron outlet about 1,400 cal BP. The residuals between the calculated rates of vertical movement at each site and its corresponding RLLC are a record of water-level change experienced at each site. Within the resolution of the technique used to construct the RLLCs, all the residual curves should be, and are, similar. A Fourier smoothing of the combined residual curves yields a “eustatic” lake-level curve for Lake Michigan over the past 4,700 cal BP. The results of the Fourier smoothing identify major lake-level fluctuations such as the Nipissing II and Algoma phases of ancestral Lake Michigan. The technique also resolves lower magnitude and shorter duration quasi-periodic lake-level fluctuations of about 160 years (120 to 200 years).

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