Forested wetlands occupying floodplains of major rivers in the Southeast are highly productive and diverse ecological systems. The wetlands are produced and maintained by fluvial processes and unique hydrologic regimes consisting of periodic flooding and subsequent drydown. Fluctuations in soil chemistry and biology resulting from this flooding and drydown provide a broad range of environmental conditions that are important in determining the role of forested wetlands in maintaining and improving water quality. The periodic shift between aerobic and anaerobic conditions in floodplain soils in response to flooding facilitates the assimilation of nutrients and organic matter, hastens the degradation of persistent pesticides, and decreases the bioavailability of heavy metals.