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Systematic sampling of wetland drainage in South Dakota by the U.S. Fish and Wildlife Service for the period 1974-1980 indicated that 4.4% of the area of seasonal and semipermanent wetland basins unprotected at the end of 1974 in the survey areas were destroyed during the period (Memorandum to the Area Manager, Fish and Wildlife Service, Pierre, South Dakota, 17 December 1980). Northeastern portions of the state had relatively low drainage rates (1.5% of area of wetlands destroyed), whereas rates were as high as 7.5% in southeastern South Dakota. Wittmier (1982) estimated that 34,505 ha of temporary, seasonal, and semipermanent basins have been drained in South Dakota since 1964. The latest private drainage survey data for North Dakota (Memorandum to the Area Manager, Fish and Wildlife Service, Bismarck, North Dakota, 29 May 1980) stated that 10.6% of the area of all privately owned natural basin wetlands in the Prairie Pothole Region of the state were drained from 1966 to 1980. The survey did not include temporary basins, so the loss rate is a conservative estimate.

Wetland drainage has been most extensive in the Central Lowlands portion of the Dakotas (Figure 1) (Nelson et al. 1984). Drainage rates vary widely among localities, in part depending on the proportion of the wetland base that is protected by perpetual easements acquired by the U.S. Fish and Wildlife Service under the Small Wetlands Acquisition Program.

The primary cause of wetland degradation in the Prairie Pothole Region, including the part occurring in the Dakotas, is agriculture (finer 1984). Intentional filling of basins with soil from adjacent uplands occurs, but is not common. More insidious is the gradual siltation of basins caused by soil erosion from adjacent cropland. This cause of wetland degradation has become more severe in recent years with the advent of larger, more powerful farm equipment that allows cultivation of entire basins and their steeply rolling watersheds. Increased siltation to basins can also be attributed partly to expanded planting of row crops, particularly corn, sunflower, and soybean. Row crops generally result in more soil erosion than small grains, because of the additional cultivation required during the growing season.

Agricultural chemicals exert a subtle but pervasive influence on prairie wetlands. Agricultural chemicals are widely used in association with growing intensification of land use in the Dakota. Most cropland in the two states is regularly treated with herbicides, but insecticide use is restricted primarily to sunflowers, which are now a widely grown crop: 65% of the sunflower hectarage in North Dakota was treated with insecticides in 1984 (Grue et al. 1988). Widespread aerial application of agricultural chemicals in the Dakotas exposes wetland animal life and plants to overspray and drift. Although more research is needed, preliminary studies in North Dakota indicate that the potential for agricultural chemicals to enter prairie wetlands and influence survival and reproduction of wildlife is high, particularly for the most toxic and widely used insecticides (Grue et al. 1986). The impact of herbicides on prairie wildlife is indirect and comes primarily from elimination of food and cover (Hudson et al. 1984; Hill and Camardese 1986).

If degradation and destruction of waterfowl breeding habitat continues at present rates into the next century, stocks of ducks can be expected to decline accordingly. Therefore, a wide array of measures will be needed to partially circumvent these losses, including increased production of waterfowl on publicly owned lands, restoration of lands that formerly were prime breeding habitat, and slowing further habitat loss.