The paper suggests that a landscape approach might be useful in evaluating the effects of cumulative impacts on freshwater wetlands. The reason for using the approach is that most watersheds contain more than one wetland, and that their effects on water quality depend on the type of wetland and its position in the landscape. Riparian areas that border uplands appear to be important sites for nitrogen processing and retention of large sediment particles. Fine particles that are associated with high concentrations of phosphorus are retained in downstream wetlands where flow rates are slowed and where the surface water passes through plant litter. Riverine systems also may play an important role in processing nutrients primarily during flooding events. Lacustrine wetlands appear to have the least impact on water quality, due to a small ratio of vegetated surface to open water. Examples are given of changes that occurred when the hydrology of a Maryland floodplain was altered. (Copyright (c) 1988 Springer-Verlag.)