Many of the larger, altered wetland complexes consist of relatively flat, poorly drained lands. Examples include former lake bottoms formed during glacial periods (e.g., lands adjacent to the Great Lakes), wide floodplains or deltas (lower Mississippi), coastal areas of low relief (e.g., pocosins and pine flatwoods of the Southeast), the Florida Everglades, and alluvium in front mountain ranges. Many of these lands have been at least partially drained for agricultural purposes. However, the wetland soils and hydrology are otherwise largely intact.

These lands constitute the largest area of potential wetland restoration sites, not only because of their large acreage but also because wetland restoration may be less expensive and may be achieved through the filling of drainage ditches or installation of minor water-control structures. Land values may also be relatively low, particularly where such lands are no longer being used for agriculture and there are no alternative uses.

More than 80 percent of historic wetland loss in the 48 conterminous states has been due to agricultural conversions. Such conversions have involved clearing of the natural forested or herbaceous wetland vegetation, introduction of drainage, and flood control. Federally funded agricultural flood control and drainage projects have facilitated private conversions. In arid parts of the country, massive federally funded irrigation projects have promoted conversions of wetlands to agriculture.

Any national program designed to restore millions of acres of former wetlands will have to focus primarily on wetlands converted to agricultural use. In many cases, the technical problems of reconverting such agricultural lands to functioning wetland systems are not as challenging as those encountered in the urban context, where the physical components of natural wetlands have been severely altered by chemical pollution, fill, barriers to water movement, and vast changes in the watershed. Former wetlands now in agricultural use can typically be reestablished by cessation of planting crops or domestic animal foraging, breaking drainage tiles, filling in drainage ditches, and if feasible, removing flood control structures. In the prairie pothole region of the country, particularly in Minnesota, the U.S. Fish and Wildlife Service, the Soil Conservation Service, and the state of Minnesota have achieved partial restoration of thousands of acres of prairie