In order to communicate effectively about the extravagant diversity of wetlands, people tend to group similar types together, and this is the core of wetland classification. Various wetland classifications have been proposed or used in Oregon over the years, but changing concepts and technology require periodic revisions or new approaches to the problem.
Three primary wetland classifications are currently in use for Oregon's wetlands.
Cowardin Classification (1979). Developed for use with the National Wetland Inventory (NWI). It classifies both wetland and deepwater habitats, often treated separately in other classifications. Primary ranks ("Systems") are: Marine, Estuarine, Riverine, Lacustrine, and Palustrine. Units are defined by hydrology, substrate, and non-specific structure of dominant vegetation (aquatic bed, emergent herbaceous, moss-lichen, scrub-shrub, or forested). Additional attributes define water regimes and man-made alterations. Classification by this method is fairly straightforward because it is based on vegetation structure that is easily identified, often from aerial photography, and water regimes can often be inferred by knowledge of local or regional conditions.
Cowardin, L.M., V. Carter, F.C. Golet & E.T. LaRoe. 1979. Classification of wetlands and deepwater habitats of the United States. USDI Fish & Wildlife Service, Biological Services Program. FWS/OBS-79/31. 103 pp.
Hydrogeomorphic Classification (HGM; 1993, with revisions). Developed as an alternative to vegetation-based classifications. It classifies vegetated wetlands according to their hydrology and geomorphology, and does not include deepwater or marine habitats. Wetlands are characterized by their landscape position, how water moves through them, and by extension how wetlands influence water quality, groundwater, habitat, and biodiversity. Primary ranks ("Classes") are: Riverine, Depressional, Mineral Soil Flats, Organic Soil Flats, Slope, Lacustrine Fringe, and Estuarine Fringe. Units are defined by the source and direction of moving water (unidirectional, bidirectional, vertical, horizontal). Classification is done by onsite inspection or use of digital elevation models and other data combined with knowledge of local or regional conditions.
Adamus, P.R. 2001. Guidebook for Hydrogeomorphic (HGM)-based Assessment of Oregon Wetland and Riparian Sites: Statewide Classification and Profiles. Oregon Division of State Lands, Salem. 162 pp. Further information available at Oregon Department of State Lands Wetlands Program HGM Guidebooks Website.
Adamus, P.R. 2006. Hydrogeomorphic (HGM) Assessment Guidebook for Tidal Wetlands of the Oregon Coast, Part 1: Rapid Assessment Method. Report to Coos Watershed Association, US Environmental Protection Agency, and Oregon Department of State Lands, Salem. 85 pp.
U.S. National Vegetation Classification (NVCS; 1998, with revisions). Developed by NatureServe to facilitate vegetation mapping and habitat conservation at a variety of scales. Used by federal agencies and state and provincial natural heritage programs throughout the United States and Canada for mapping vegetation and tracking wetlands of conservation concern. It includes both wetland and deepwater habitats, with an emphasis on existing or seral vegetation instead of potential or climax vegetation. Eight hierarchical ranks classify vegetation progressively finer scales of resolution for use with different types of mapping data. Coarse-scale upper ranks are classified by physiognomy, middle ranks by physiognomy and floristics, and finer-scale lower ranks by floristics only. Plant association, the finest scale of resolution, is the most useful rank for tracking individual wetlands of conservation concern. The NVCS is under continuous revision nationally by NatureServe and locally by ORBIC (Oregon Biodiversity Information Center). The latest version can be accessed via the Major Wetland Types page. Classification at coarse scales is fairly straightforward because it is based on vegetation structure that is easily identified, often from aerial photography. Classification at fine scale, particularly at the rank of plant association, can only be done by onsite inspection. Interpretation of plant communities at this scale is inherently subjective in nature and sometimes difficult to replicate among a range of different observers. For a full description, see NatureServe's publication Seeing the Forest and the Trees
Faber-Langendoen, D., D. L. Tart, & R. H. Crawford. 2009. Contours of the revised U.S. National Vegetation Standard. Bulletin of the Ecological Society of America 90:87-93.