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According to Cowardin et al. (1979), wetlands are lands transitional between terrestrial and aquatic systems where the water table is usually at or near the surface or the land is covered by shallow water. Additionally, a site must have one or more of the three following attributes to be defined as a wetland:

(1) The site must, at least periodically, support hydrophytes.

(2) The land at the site must be predominantly undrained hydric soil. Hydric soils are defined by the U.S. Soil Conservation Service (1985) as those that in their undrained condition are saturated, flooded, or ponded long enough during the growing season to develop anaerobic conditions favoring the growth and regeneration of hydrophytic vegetation.

(3) The site substrate is nonsoil and is saturated with water or covered by shallow water at some time during the growing season each year. Nonsoils are defined by the U.S. Soil Conservation Service (1975) as barren areas such as ice, rock, or substrates underlying deep water.

Nearly all natural basins in the Prairie Pothole Region of the Dakotas have both the first and second of these attributes, and thus are wetlands according to the Cowardin et al. (1979) classification. The basins are underlain by hydric soils, and when surface water is present, can support hydrophytes. Many of the basins are intensively cultivated; the soil may be bare or commercial crops may be present during dry conditions, but when water is replenished during the growing season, communities of hydrophytes quickly develop.

Few basins in the region contain deep-water habitat. This habitat will not be dealt with in this report. Deep-water habitats are permanently flooded lands lying below the deep-water boundary of wetlands. For the wetlands described in this report, the boundary between them and deep-water habitats lies at a depth of 2 m below low water; however, if emergents, shrubs, or trees grow beyond this depth at any time, their deep-water edge is the boundary. Substrates of deep-water habitat are nonsoil whose water depths are in excess of that required to support emergent vegetation (Cowardin et al. 1979; U.S. Soil Conservation Service, Soil Survey Staff 1975). In addition, a small proportion of the wetlands in the region are of the water regimes "intermittently exposed" and "saturated." These will also be omitted from this report.

Prairie wetlands occur in glacially or postglacially derived basins in the northern grassland biome (see Section 2.1). The basins are roughly round or oval in shape, although it is fairly common to encounter some with relatively convoluted shorelines composed of several bays or peninsulas, or, more rarely, with one or several islands.