LAKE-MICHIGAN BEACH-RIDGE AND DUNE DEVELOPMENT, LAKE LEVEL, AND VARIABILITY IN REGIONAL WATER-BALANCE

A sequence of northern Lake Michigan beach ridges records lake-level fluctuations that are probably related to changes in late Holocene climate, Historically, episodes of falling and low lake level associated with regional drought led to the formation of dune-capped beach ridges, The timing of prehistoric ridge formation, estimated by radiocarbon dating of plant macrofossils from early-successional dune species, shows that return periods of inferred drought, averaged for time intervals of 100 to 480 yr, ranged between 17 and 135 yr per drought during the last 2400 yr, In five of ten of these time intervals, the average return period ranged between 17 and 22 yr per drought. These intervals of frequent ridge formation and drought were associated with the development of parabolic dunes, which is indicative of high lake level and moist climate, This seeming paradox suggests that unusually moist decades alternated with unusually dry decades during these time intervals, Regional water balance probably varied less during the time intervals when ridges formed less often and the lake produced no evidence of high level. (C) 1995 University of Washington.