In 1974, the U.S. Fish and Wildlife Service directed its Office of Biological Services to design and conduct a new National inventory of wetlands. Whereas the single purpose of the 1954 inventory was to assess the amount and types of valuable waterfowl habitat, the scope of the new project is considerably broader (Montanari and Townsend 1977). It will provide basic data on the characteristics and extent of the Nation's wetlands and deepwater habitats and should facilitate the management of these areas on a sound, multiple-use basis.

Before the 1954 inventory was begun, Martin et al. (1953) had devised a wetland classification system to serve as a framework for the National inventory. The results of the inventory and an illustrated description of the 20 wetland types were published as U.S. Fish and Wildlife Service Circular 39 (Shaw and Fredine 1956). This circular has been one of the most common and most influential documents used in the continuous battle to preserve a critically valuable but rapidly diminishing National resource (Stegman 1976). However, the shortcomings of this work are well known (e.g., see Leitch 1966; Stewart and Kantrud 1971).

In attempting to simplify their classification, Martin et al. (1953) not only ignored ecologically critical differences, such as the distinction between fresh and mixosaline inland wetlands but also placed dissimilar habitats, such as forests of boreal black spruce (Picea mariana) and of southern cypress-gum (Taxodium distichum-Nyssa aquatica) in the same category, with no provisions in the system for distinguishing between them. Because of the central emphasis on waterfowl habitat, far greater attention was paid to vegetated areas than to nonvegetated areas. Probably the greatest single disadvantage of the Martin et al. system was the inadequate definition of types, which led to inconsistencies in application.

Numerous other classifications of wetlands and deepwater habitats have been developed (Stewart and Kantrud 1971; Golet and Larson 1974; Jeglum et al. 1974; Odum et al. 1974; Zoltai et al. 1975; Millar 1976), but most of these are regional systems and none would fully satisfy National needs. Because of the weaknesses inherent in Circular 39, and because wetland ecology has become significantly better understood since 1954, the U.S. Fish and Wildlife Service elected to construct a new National classification system as the first step toward a new National inventory. The new classification, presented here, has been designed to meet four long-range objectives: (1) to describe ecological units that have certain homogeneous natural attributes; (2) to arrange these units in a system that will aid decisions about resource management; (3) to furnish units for inventory and mapping; and (4) to provide uniformity in concepts and terminology throughout the United States.

Scientific and common names of plants (Appendix A) and animals (Appendix B) were taken from various sources cited in the text. No attempt has been made to resolve nomenclatorial problems where there is a taxonomic dispute. Many of the terms used in this classification have various meanings even in the scientific literature and in some instances our use of terms is new. We have provided a glossary (Appendix C) to guide the reader in our usage of terms.