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Habitat Assessment

Water Level Fluctuations

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.

Due to Lake St. Clair's relatively small surface area, its water levels can respond rapidly and fluctuate significantly in response to climatic factors and short-term weather events across the region. The St. Clair River provides about 97 percent of the total water supply with drainage from the immediate watershed providing the other three percent. For this reason, the lake is particularly susceptible to even small changes in its connecting channels, the St. Clair and Detroit rivers.

Wetland scientists have determined that water level fluctuations are critical to water, nutrient and energy exchange in coastal marsh wetlands. For example, during the breakdown of detritus (dead plant material), nutrients are released which are used for new plant growth. This process however, requires oxygen. Periodic de-watering of the marsh during low water periods allows wetland bottom soils to aerate, which increases detritus breakdown and nutrient exchange.

For more information, see: Coastal Habitat Assessment, Section V (PDF)