Chapter 2 



Wetland Types 



CHAPTER SUMMARY 



Wetlands, including marshes, swamps, bogs, 

 bottom lands, and tundra, occur along sloping areas 

 between upland and deepwater environments, such 

 as rivers, or form in basins that are isolated from 

 larger water bodies. Wetlands are either periodically 

 or continually inundated by water and genersdly 



covered by vegetation adapted to saturated soil con- 

 ditions that emerges through any standing water. 

 Most wetlands have formed as a result of past gla- 

 ciation, erosion and sedimentation, beaver activi- 

 ty, freezing and thawing in arctic areas, activities 

 of man, and other processes. 



ORIGINS OF WETLANDS 



The U.S. Fish and WUdlife Service (FWS) used 

 the term "wetland" in 1952 to describe a number 

 of diverse environments, typically of high produc- 

 tivity, that share characteristics of both aquatic and 

 terrestrial habitats — i.e. , they are at least temporari- 

 ly inundated and have "emergent" vegetation 

 adapted to saturated soil conditions. While a wide 

 range of environmental conditions exist within this 

 categorization — from salt marshes flooded and ex- 

 posed daily to bottom land forests inundated only 

 during spring flooding — wedands also share similar 

 hydrologic and habitat characteristics. These char- 

 acteristics primarily stem from three interrelated 

 factors: the wetland's origin, hydrology, and vege- 

 tation. 



Six basic processes are responsible for wetland 

 formation: glaciation, erosion and sedimentation, 

 beaver dams, freezing and thawing, activities of 

 man, and miscellaneous processes (6). 



Glaciation 



A principal band of wetiands (fig. 1) — lying along 

 the northern tier of the United States, including 

 Alaska, Maine, New York, Michigan, Wisconsin, 

 Minnesota, North Dakota, and Washington — was 

 formed in three ways as glaciers melted 9,000 to 

 12,000 years ago. First, the melting of large blocks 

 of ice left by receding glaciers created pits and de- 

 pressions in glacial moraines, till, and outwash. 



Lakes and wetlands formed where the depressions 

 intersected the ground water table or where fine 

 clay and organics sealed their bottoms and per- 

 mitted the coUection of runoff waters. The majority 

 of wetlands in the Northern United States were 

 formed in this manner. Second, glaciers dammed 

 rivers, often creating glacial lakes, sometimes 

 thousands of square, miles in area. Once the ice 

 retreated, the lakes were drained partially, resulting 

 in extensive low-lying areas with peat deposits. 

 These areas form some of the large wetlands in the 

 once glaciated Northern States. Third, glaciers 

 scooped out and scoured river valleys and soft bed- 

 rock deposits, creating large and deep lakes such 

 as the Great Lakes, and shallow depressions and 

 wetland areas, such as the prairie potholes. 



Erosion and Sedimentation 



Another principal band of wedands is found (fig. 

 1) along the gulf and Atlantic coasts, where sedi- 

 ment has been deposited in the still waters be- 

 hind barrier islands or reefs and in bays and 

 estuaries. Wetland formation is favored by low- 

 elevation topography along the Atlantic and gulf 

 coasts. The sediment deposited behind Georgia 

 coastal marshes, for instance, may be up to 10 

 meters in thickness and has formed extensive flat 

 or gently sloping topography conducive to growth 

 of wetland plants. 



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