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The Great Lakes’ water levels have fluctuated dramatically since record keeping started in the early 1900's due to variation in precipitation and evaporation rates. In high water years, stands of emergent plants die off or become uprooted by wave and ice action. Historically, because of the flat landscape the marsh usually was able to migrate inland in shallow water areas that were once wet meadow.
As the cycle continues, water levels eventually fall, allowing the rhizomes of emergent plants destroyed above ground to produce stems and recolonize shallower open water over time. This natural dynamic system of "lateral displacement" (where vegetative zones expand and contract) sets back succession, accelerates nutrient cycling, increases habitat diversity and enhances coastal wetland values for wildlife.
For more information, see: Coastal Habitat Assessment, Section V (PDF)