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Wetlands geology
12.20.02
Science and Health:
Troubled Waters
More on This Story:
Understanding the Wetlands

Wetlands are areas where water covers the soil, or is present either at or near the surface of the soil all year or for varying periods of time during the year, including during the growing season. Wetlands vary widely because of regional and local differences in soils, topography, climate, hydrology, water chemistry, vegetation, and other factors, including human disturbance. Wetlands exist on every continent except Antarctica, from the tundra to the tropics.

Wetlands include marshes, swamps, bogs, and similar areas found in generally flat vegetated areas, in depressions in the landscape, and between dry land and water along the edges of streams, rivers, lakes, and coastlines. Because they are so varied, wetlands can be difficult to recognize. Some are wet all of the time; some may look completely dry most of the time.

Over the last 200 years, the continental U.S. has lost more than half of its wetlands. Often regarded as wastelands, wetlands have been destroyed in a number of ways - drained and converted to farmland, filled for housing developments and industrial facilities, or used to dispose of household and industrial waste.

Only recently have we begun to understand the importance of wetlands. They improve natural water quality; reduce flood and storm damages; provide important fish and wildlife habitat; control shoreline erosion; and support hunting, fishing, and other recreational activities. According to the EPA, wetlands have been degraded in many ways, some not as obvious as others. These include chemical contamination, excess nutrients, and sediment from air and water.

Major Causes of Wetland Loss and Degradation Human Actions

  • Drainage
  • Dredging and stream channelization
  • Deposition of fill material
  • Diking and damming
  • Tilling for crop production
  • Levees
  • Logging
  • Mining
  • Construction
  • Runoff
  • Air and water pollutants
  • Changing nutrient levels
  • Releasing toxic chemicals
  • Introducing nonnative species
  • Grazing by domestic animals
Natural Threats
  • Erosion
  • Subsidence
  • Sea level rise
  • Droughts
  • Hurricanes and other storms


National Wetlands Inventory
The National Wetlands Inventory (NWI) of the U.S. Fish & Wildlife Service produces information on the characteristics, extent, and status of the Nationís wetlands and deepwater habitats.



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