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COMPENSATING FOR WETLAND LOSSES UNDER THE CLEAN WATER ACT
which is dependent on the wetland function(s) of interest. The purpose of this chapter is to demonstrate that these units are hydrologically connected, and thus wetland functions are integrated on a watershed basis. Consequently, wetland mitigation should be considered on a watershed basis. Examples of watershed assessment approaches are presented in Chapter 8.
Several approaches used to determine wetland function focus on the position of wetlands in the landscape. The hydrogeological approach (Winter 1986), hydrogeomorphic approach (Brinson 1993), and hydrological equivalency (Bedford 1996, 1999) all have elements of watershed-scale assessment. Hydrological equivalency and landscape position have been viewed as important components of wetland restoration in recent years (Bedford 1996; Bell et al. 1997).
Restored and created wetlands should be self-sustaining (Mitsch and Wilson 1996); to be self-sustaining, they must be properly sited in the watershed. One way to target mitigation sites to appropriate landscape positions is through the development of basinwide wetland restoration and mitigation plans. The evaluation of watershed position on the functions of existing wetlands and on restored, created, and enhanced wetlands has been aided by the development of new technology. Global positioning system (GPS) technology provides a simple means for locating wetland sites in the landscape. GPS information is easily used with geographical information system (GIS) databases and analysis tools to facilitate watershed analysis. The U.S. Geological Survey supports watershed analysis efforts by providing topographic databases and watershed boundaries that have been coded according to the relative size of the watershed.
How Does Position in the Watershed Affect Hydrology?
One of the most frequently cited functions of wetlands is their ability to reduce the effects of flooding by temporarily storing storm water and gradually releasing it to streams as modulated surface flow (Dennison and Barry 1993) and/or groundwater discharge that constitutes stream base flow. Novitzki (1985) showed that watersheds in the northeastern United States with 4% or greater wetland areas had 50% lower peak flows compared with watersheds without wetland areas. To provide this function, the receiving wetland must occur at a relatively lower topographic elevation within the watershed than the contributing uplands. Typical inland wetlands that provide floodwater storage include riparian or floodplain wetlands.
Riparian wetlands (wetlands immediately adjacent to streams) receive significant groundwater and/or surface-water runoff from a con-