stream temperature, controlling light quantity and quality, enhancing habitat diversity, modifying stream morphology, and enhancing food webs and species richness, may equal their value for pollutant reduction (EPA 1995; USDA 1997). Nitrate reduction by riparian wetlands in subsurface waters moving toward the stream has been studied more than most other water-quality functions. This function is widely recognized, but the amount of nitrate reduction depends on stream morphology, sediment chemistry, hydraulic conductivity of sediments and soils, carbon content, relative wetness or depth to shallow groundwater, and so forth (Hill 1978; Peterjohn and Correll 1984; Lowrance et al. 1984; Johnston 1993; Bohlke and Denver 1995; Hill and Devito 1997; O'Connell 1999; Prestegaard 2000). The ability of riparian wetlands to remove sediment and phosphorus from surface runoff water may also be diminished by channelized flow (Daniels and Gilliam 1996). Although there are variations in the effectiveness of riparian wetlands for various water-quality functions, in general, they are extremely effective. Because of this, Gilliam et al. (1996) have considered headwater riparian wetlands as the most important factor controlling nonpoint source pollution in humid areas, and it is a national policy of the U.S. Department of Agriculture's Natural Resources Conservation Service to promote use of the general model of riparian buffers presented by Welsch (1991). Wetlands in other watershed locations are important for water quality, but they cannot substitute for the effect of riparian wetlands present on low-order streams.
The value of wetlands for water quality is highly recognized for the Mississippi River drainage basin where a scientific panel from the National Oceanic and Atmospheric Administration (NOAA) recommended significant increases in riparian zones and wetlands to help with hypoxia problems in the Gulf of Mexico (Mitsch et al. 1999). The NOAA committee recommended restoring and/or creating 24 million acres of wetlands and predicted that this increase would reduce the nitrogen input into the Gulf by 40%. This NOAA-appointed committee also recognized that placement of the wetlands in the watershed is of vital importance.
Tremendous water-quality improvement has been documented for constructed storm-water wetlands (Schueler 1992; Brix 1993; Bingham 1994; Brown and Schueler 1997; Malcom 1989) and waste-water treatment wetlands (Reddy and Smith 1987; Hammer 1989; Cooper and Findlater 1990; Moshiri 1993; Corbitt and Bowen 1994; DuBowy and Reaves 1994; Hammer 1997). Constructed wetlands have much potential for assimilating nutrients and improving water quality in a watershed, but treatment wetlands and, to a lesser extent storm-water wetlands, often evolve to dense monoculture stands of Typha, Scirpus, or Phragmites, which will “effectively remove target contaminants from influent waters while providing habitat for a few muskrats, blackbirds and some songbirds but