26 • Wetlands: Their Use and Regulation 



Figure 1.— General Distribution of Wetlands of tfie United States 



Note: Shaded portions incorporate generai wetiand areas. Each dot represents about 10,000 acres. 



SOURCE: Adapted from Samuel P. Stiaw and C. Gordon Fredine, "Wetlands of the United States: Their Extent and Their Value to Waterfowl and Other Wildlife.' 

 Fish and Wildlife Service, U.S. Department of the Interior, Circular 39, 1956. 



Major wetlands also are located along the flood 

 plains of low-gradient rivers such as the Mississip- 

 pi. River flood plains are created by the deposition 

 of river alluvium on adjacent lands during floods. 

 Rivers may cut new channels, abandoning old 

 water courses, which may then become lakes or wet- 

 lands. Extensive wetland areas, such as the Mis- 

 sissippi Delta, are found where sediment is de- 

 posited at the mouths of rivers and streams. The 

 deposition of sand, gravel, or silt also can create 

 wetlands along the shores of, or adjacent to, lakes. 

 Vast marshes of this type form along the Great 

 Lakes. 



Beaver Dams 



At one time, beaver dams played a major role 

 in forming smaller inland wetlands in the forested 



areas of the Nation. While beaver populations fluc- 

 tuate due to variability in trapping pressure, their 

 presence can be a major factor in increasing wedand 

 acreage in some regions of the country. For exam- 

 ple, in an analysis of wetland trends in 15 Massa- 

 chusetts towns between 1951 and the 1970's, beaver 

 activity was the third most important cause of in- 

 creases in wetland acreage out of 1 1 identified fac- 

 tors (9). 



Freezing and Thawing 



In the Arctic, wedands are created when the Sun 

 melts the surface of frozen organic soils while the 

 underlying soil remains permanently frozen. In ad- 

 dition, frost action segregates rock and soil particles 

 of various sizes and shifts them in such a way that 

 shallow, water-filled basins are formed. 



