Wetlands: A Hydrological Perspective
In Canada and the United States, it is generally agreed that wetlands are lands saturated for most of the growing season which allow the development of hydric soil, or the support of hydrophytes, or prolonged flooding to a depth of 2 m [Cowardin et al., 1979; National Wetland Working Group (NWWG) 1987]. The areas of wetlands are estimated to be 1.27 × 106 km2 in Canada (Zoltai, 1988) and 0.28 × 106 km2 in the United States (Hofstetter, 1983). These estimates should be qualified by the shrinking trend of wetland areas. The loss of wetlands in the United States, for example, may have amounted to about 2.2 × 103 km2a−1 during the nineteenth and the twentieth centuries (Mitsch and Gosselink, 1986).
Mapping the distribution of wetlands in North America is not an easy exercise. Canadian wetland maps show the percentage of land occupied by wetlands (NWWG, 1988), whereas United States maps often indicate the distribution of various wetland types (e.g., marshes, swamps, bogs) or areas designated as peatlands, without indicating the percentage of wetland cover (see maps in Hofstetter, 1983). It is extremely difficult to compile different maps into a single wetland distribution sheet, particularly across the border between the two countries. Satellite mapping on a global scale cannot be harmonized easily with groundbased maps unless extensive ground-truthing is performed. In view of these difficulties, the wetland distribution map given in figure 8.1 is a subjective reconciliation from several sources, a sketch of wetland distribution on a continental scale subject to considerable error, including all inaccuracies inherent in the source maps. The overall pattern reveals a concentration of wetlands in the subarctic and boreal zones of Canada and Alaska, in glacial depositional terrain from Minnesota to southern Ontario, and on the coastal plain that lies to the south and southeast of the Fall Line in the United States (fig. 8.1).
Wetland literature has expanded greatly over the past few decades. Wetlands in North America have been studied from different viewpoints. Traditionally, much work has been done on the engineering aspects, for trafficability, peat production, or drainage and, nowadays, restoration of wetlands (e.g., MacFarlane, 1969; Galatowitsch and van der Valk, 1994). Wetlands have received considerable attention by biologists who primarily maintain an ecological focus (e.g., Moore and Bellamy, 1974). More recently, wetlands have been examined from multiple perspectives, concerning different wetland functions (e.g., Greeson et al., 1979) or the management of wetland resources (Williams, 1990). The hydrological factor has always been given consideration because the distribution, storage, and movement of water into, out of, and within the wetlands are of fundamental importance to their initiation, preservation, or degradation. In addition, the role of wetlands in climatic change, through their emission of “greenhouse gases,” has interested soil scientists and climatologists (e.g., Matthews