A reconstruction of Lake Michigan-Huron water levels derived from tree ring chronologies for the period 1600-1961

A dendrochronolgy of annual precipitation and air temperatures from six Great Lakes locations was used to reconstruct Lake Michigan-Huron water levels front 1600-1961 representing the present St. Clair River channel conditions and basin land cover. The reconstructions are based upon a multi-linear regression model relating multi-year annual precipitation and air temperature to annual water levels. An increased frequency of low lake levels was found to occur prior to the twentieth century, accompanied by a major extreme in water levels, greater than that experienced in the historical record, in the early 1600s. The comparison of simulated and measured water levels also indicates that the impact of some of the channel changes in the St. Clair River may be underestimated and that the major drop in lake level in the 1880s may be due to erosion as well as to decreased precipitation. The occurrence of extreme levels around 1640, in 1838, and in 1986 suggests a return interval of 150-190 years for extreme lake levels. The analysis also suggests that the variability of lake levels has greatly decreased over the last century when comparing tree-ring-derived level variability. Thus climatic periods used for the development of the current regulation plans may not be representative of the longer-term climate and lake levels.