Lake Michigan Dune Development. 3. Lake-Level, Beach, and Dune Oscillations

During periods of several successive years when lake levels are several feet below the mean level, sand is blown from the wide beach into a new foredune ridge. But much of it may be eroded by waves during ensuing years of unusually high mean lake level, aggravated by storms. When the level again falls and the beach widens, the beginning of a new ridge may cut off the sand supply from the ridge behind it. This "shore-dune cycle" may be repeated several times in a few decades. Protracted lowering of the lake may also help underwater sand bars emerge as barriers; these may be permanently colonized by vegetation if net longshore deposition of sand is sufficient to prevent their destruction during later episodes of high lake level. The connections between beach and dune forms and lake level and between lake level and precipitation and temperature account for many details of shore-line history around Lake Michigan for the past century and possibly for earlier episodes of the late postglacial climatic history.