BEACH-RIDGE DEVELOPMENT IN LAKE-MICHIGAN - SHORELINE BEHAVIOR IN RESPONSE TO QUASI-PERIODIC LAKE-LEVEL EVENTS

Strandplains of arcuate beach ridges are common in coastal embayments in parts of the Great Lakes. Similarities in beach-ridge development a nd geomorphology are recognizable in many of the embayments in the Lak e Michigan basin despite differences in size and shape, available sedi ment type and supply, predepositional slope and topography, and hydrog raphic regime between the embayments. These similarities are primarily a product of three scales of quasiperiodic lake-level variation rangi ng in time from 30 to 600 years and in water level change from 0.5 to 3.7 m. The interaction of these three lake-level variations can be rep resented on a Curray (1964) diagram (rate of water level change versus rate of sediment supply). The position of any shoreline on the diagra m and the type of behavior the shoreline is experiencing is a product of the interaction of the three variations. Two large strandplains of late Holocene beach ridges occur at opposite ends of Lake Michigan (To leston Beach and Thompson embayment). The two areas exhibit similar pa tterns of beach-ridge development for the past 2600 calendar years. Th at is, both areas form beach ridges about every 30 years. Groups of 4 to 6 beach ridges reflect a longer-term lake-level variation of about 150 years. Only during the largest variation of about 600 years in dur ation do the two areas differ. The rise to the 1700 cal yr B.P. high c aused the erosion of beach ridges back to 2800 cal yrs B.P. in norther n Lake Michigan. In southern Lake Michigan, no erosion occurred during this lake level high. Differences in shoreline development between th e two areas are related to the rate of sediment supply to the shorelin es. As the sediment sink for the southern half of Lake Michigan, the s outhern strandplain received a greater sediment flux than the northern strandplain during the latter part of the late Holocene and produced a continuous record of beach-ridge development.