refers to the protection of an existing and well-functioning wetland from prospective future threats. Preservation does not involve alteration of the site.
A compensatory mitigation project is the creation, restoration, enhancement, or preservation of a wetland designed to offset permitted losses of wetland functions in response to special conditions of a permit. The mitigation project provides a desired set of hydrological, water quality, and/ or habitat functions in the watershed. (A process for identifying the desired functions for a watershed is described in Chapter 7.) As noted above, wetland functions include water quality, water retention, and habitat contributions of wetlands to watersheds.
Functional assessment methods provide useful guidelines for measurement of wetland functions. Such methods that consider how wetland structure (see below), location in the watershed, and the resulting hydrological, geochemical, and biological processes related to that structure and location give rise to certain wetland functions. (Functional assessment is described in Chapter 7.)
The no-net-loss goal is focused on wetland functions; however, the area of a wetland type is often used as a proxy for wetland functions. Wetland type describes wetlands according to the U.S. Fish and Wildlife Service's (FWS) classification system (Cowardin et al. 1979; see also Box 1–1). If wetland type is used as a proxy to represent wetland function, compensatory mitigation projects might be expected to result in some number of acres that can be classified as a wetland.
Wetland types in the Cowardin system are differentiated by their
Wetland Classification System of the U.S. Fish and Wildlife Service
Cowardin et al. (1979) developed a hierarchical system to classify wetland types for purposes of mapping and inventory. It has at its highest level the “system,” of which five are defined (marine, estuarine, riverine, lacustrine, and palustrine). Subsystems further define hydroperiod attributes of the first four systems. Wetland classes are based on substrate type and flooding regime (six classes: rock bottom, unconsolidated bottom, rocky shore, unconsolidated shore, streambed, and reef) or on vegetation types (five classes: aquatic bed, moss-lichen wetland, emergent wetland, scrub-shrub wetland, and forested wetland). Finally, wetlands are classified by their dominance type, based on dominant plants or animals. Various modifying terms are added to describe water regimes, salinity, pH, soil type, or human modifications.