28 • Wetlands: Their Use and Regulation 



water. Geologic movements have shaped still other 

 wetlands. Reelfoot Lake in Tennessee, for exam- 

 ple, was formed by the sudden sinking of the earth 



from earthquakes. Similarly, San Francisco Bay 

 was formed by movement along the San Andreas 

 Fault. 



HYDROLOGIC CHARACTERISTICS OF WETLANDS 



Wedands may be located on the transitional slop- 

 ing areas between upland and deepwater environ- 

 ments where the water is shallow and calm enough 

 for emergent vegetation to grow. Wetlands also 

 may form in basins that generally are isolated from 

 larger water bodies. These basins: 1) are either at 

 or below the ground water table, or 2) because of 

 poor drainage, retain much of the water that flows 

 into them. The interaction among the hydrologic 

 regime, the wetland topography, and its underly- 

 ing substrata (e.g. , soU) largely controls the general 

 characteristics of a wetland and most, if not all, of 

 the ecological services that it performs. 



The two hydrologic characteristics that have the 

 greatest influence in ultimately determining the 

 habitat values of a wetland are the depth of the 

 water and the paf fern of fluctuation of water depth 

 (8). The average depth of water varies greatly 



among wetlands. Bogs, for instance, typically are 

 saturated to their surfaces, but rarely have stand- 

 ing water. In contrast, a wooded swamp or deep 

 marsh may have standing water several feet deep. 

 Annual fluctuations in water level also vary wide- 

 ly, ranging from those that are wet year-round, to 

 those inundated irregularly for only a fraction of 

 the year, to those flooded and exposed daily by tidal 

 action. One of the most important factors influenc- 

 ing average water depth and patterns of fluctua- 

 tion is the source of water, whether from direct sur- 

 face runoff of snowmelt, from a river during spring 

 flooding, or from tidal action in coastal areas. 

 Climate, in addition to influencing the source of 

 water — precipitation, snowmelt, and flooding — 

 also determines seasonal patterns of drying. In the 

 prairie-pothole region of the United States, for in- 

 stance, shallow wetlands may dry out completely 

 in some years. 



WETLAND VEGETATION 



A diversity of plant forms is found in wetlands, 

 ranging from deciduous trees to rooted floating 

 plants, such as water lilies. Depending on the soil 

 type, water availability, water quality, and temper- 

 ature patterns, the dominant plants in wedand areas 

 may be mosses, grasses, sedges, bulrushes, cattails, 

 shrubs, trees, or any combination of these. A com- 

 mon distinction among wetland types is the vege- 

 tation type: trees or shrubs dominate swamps; 

 grasses, sedges, cattails, and bulrushes dominate 

 marshes; and mosses and lichens dominate bogs. 



With the exception of the severe, limiting effect 

 of high salinity on plant type, water depth and fluc- 

 tuation are perhaps the dominant physical factors 



influencing the type and distribution of plants. 

 Plants often have a narrowly defined tolerance for 

 hydrologic conditions. In a typical New England 

 salt marsh, for instance, Spartina alterniflora (salt 

 marsh cordgrass) dominates the water's edge; as 

 the marsh gains elevation, Spartina patens (salt- 

 meadow cordgrass), and then Juncus (rushes) dom- 

 inate the marsh (see fig. 2). In a freshwater marsh, 

 a typical progression from deep to shallow water 

 would include hard-stemmed bulrush, narrowleaf 

 cattail, and broadleaf cattail. Bald cypress, black 

 willow, willow oak, and swamp chestnut oak are 

 representative species found in a bottom land hard- 

 wood forest, from the areas most regularly flooded 

 to those irregularly inundated. 



