EVIDENCE OF AN ~ 30-YEAR CYCLE OF SAND SUPPLY CHANGE IN MODERN DUNE-BUILDING AT SILVER LAKE ON THE EASTERN SHORE OF LAKE MICHIGAN

Six sets of aerial photographs were compared in a geographic information system to determine rates of sand supply over a 55 year period in a transgressing dune system at Silver Lake State Park on Lake Michigan's eastern shore. Each photo set (and one additional set) was also interpreted to determine percent of dune system vegetative cover at the various photo set dates. Recent research suggests dune building is associated with many ~150-yr Great Lakes highstands previously inferred from beach ridge studies. The variations in sand supply and vegetative cover at the Silver Lake dunes were plotted against the lake levels for Lake Michigan. Between 1938 and 1992, sand supply and Lake Michigan exhibited two peaks and three nadirs of ~30 years frequency. Sand supply and Lake Michigan levels were positively correlated. Sand supply and vegetative cover exhibited negative correlation. A shorter-frequency sand supply cycle appears to occur at Silver Lake repeating the signals of ~30-yr lake-level fluctuations found by beach ridge research in protected bays of Lake Michigan.