The various elevations at which the lakes in the Michigan and Huron basins stood during the Holocene Epoch were previously thought to be the result of isostatic uplift of the northern outlets and discontinuous erosion of the southern outlets. Lake-level fluctuations of smaller magnitude, however, were caused by climatic changes. Correlation of climatic variables and lake-level variations over the past 100 years indicate that high lake levels occurred during cold periods when arctic air frequently converged over the Great Lakes with warm moist air from the Gulf of Mexico, causing increased rainfall. At the same time, increased cloud cover decreased evaporation. In addition, lower temperatures caused more winter precipitation to be held in storage in the basin as snow and ice, later to be made available for run-off during the spring. Major periods of climatically induced high lake levels can be recognized where sediments along the beach were eroded during the high levels, and in areas behind the beach where streams aggraded, active marshes formed, and soils formed on eolian sands. Radiocarbon dates of organic material that accumulated in these areas during high-water periods, coupled with analysis of the historic record of lake-level fluctuations, indicate that water level has varied cyclically, with a short 100- to 150-year cycle superimposed on a longer 400- to 500-year one.