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TEACH Shoreline Geology

4 | Marshes, bogs, and swamps

The extensive freshwater wetlands of the Great Lakes' coasts are unique in ecological character, size and variety. They range from small wetlands nestled in scattered bays to extensive shoreline wetlands such as those of southwestern Lake Erie, freshwater estuaries such as the Kakagon Sloughs of northern Wisconsin and the enormous freshwater delta marshes of the St. Clair River.

A Minnesota Wetland. Click for a larger image

What is a wetland?
Wetlands are areas that are not dry land and not open water - they're a little bit of both. Found between the areas of dry land and open water, they include marshes, swamps, bogs, fens and barrier beach wetlands. Differing from inland lakes, wetlands are shaped and influenced by waves, seiches, and lake level fluctuations. Wetlands are one of the most productive ecosystems in the world, being home to many rare species. The U.S. Fish and Wildlife Service estimates that 43 percent of all threatened and endangered species rely on wetlands for their survival. Wetlands are an important part of the migratory cycle, providing food, resting stops and habitats for many migratory birds, such as ducks and geese. Wetlands also improve water quality by slowing surface runoff, and processing organic waste and sediments before they reach open water, thus protecting aquatic life and sources of drinking water. Flooding is also controlled with the help of wetlands. Wetlands will store water during times of flood or high lake levels, and then will slowly release the water during times of drought or low lake levels.

How were the Great Lakes' wetlands created?
Lake Ontario wetland. Click for larger image. Glaciers helped to create wetlands in the Great Lakes region around 12,000 years ago. Large wetlands formed when glaciers dammed rivers and created valleys. Large blocks of ice left behind by receding glaciers also created smaller wetlands by forming pits and depressions in the land.

Erosion and deposition of sediment on adjacent lands during floods can also create wetlands along rivers and streams. Periodic flooding provides the moisture needed to sustain the wetland, and they undergo constant change and transformation.

What are some threats to wetlands?
Purple loosestrife. Click for larger image. Wetlands were once considered wasted land that would be of better use if they were drained, filled, and used for agriculture or development. Although we have recently begun to recognize the significance and importance of wetlands, the United States and Canada have been destroying wetlands for the past 200 years. Consequently, more than two-thirds of the natural Great Lakes wetlands have already been filled in or drained for agriculture, urban uses, shoreline development and recreation. For example, 83 percent of western Lake Ontario's marshes have been filled in and lost to urbanization. Shoreline modification to prevent erosion and flooding, such as dikes and revetments (seawalls), have also threatened wetlands by affecting lake level fluctuations and sediment nourishment. These issues will be discussed in more detail in December 2000 in TEACH's Erosion segment.

Non-native invasive plant and animal species can also hurt wetlands. These species can choke out native plant species, thus reducing habitat diversity and the health of the wetland ecosystem. Purple loosestrife, Eurasian watermilfoil, and zebra mussels are all examples of non-native species that affect wetlands. See TEACH's module on non-native invasive species for more information.

Check out GLIN's Wetlands page for more resources on wetlands in your state, and find out what you can do to help!

Graphics: Minnesota wetland; Lake Ontario wetland; and, Purple Loosestrife in a wetland area.

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