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Wetland Types and Classification

The term ‘wetland’ is often used interchangeably with other terms such as ‘marsh,’ ‘swamp,’ ‘slough,’ ‘pond,’ ‘pothole,’ ‘fen,’ or ‘bog.’ It is important to understand that there are several different types of wetlands, and often these wetland terms can be confusing. For the purposes of using a consistent naming convention, the term ‘wetlands’ is used to represent the collective group of all wetland types found in Iowa.

In general terms, wetlands are lands where saturation with water is the dominant factor determining the nature of soil development and the types of plant and animal communities living in the soil and on its surface. The water creates severe physiological problems for all plants and animals except those that are adapted for life in water or in saturated soil. Drained hydric soils that are incapable of supporting hydrophytes are not considered wetlands. However, they do provide a valuable record of historic wetlands and indicate areas that may be suitable for restoration. This does not mean that wetlands and farmlands are necessarily exclusive- many wetlands can be farmed during dry periods. If these wetlands were not actively farmed, they would likely develop natural communities of hydrophytes.

The Cowardin system for wetland classification was officially adopted by the U.S. Fish and Wildlife Service (FWS) in 1979 to define and classify wetlands and deepwater habitats across the U.S. (Cowardin, et al. 1979). It was subsequently used to define and classify wetlands for the National Wetlands Inventory (Tiner, 1984). Cowardin et al. (1979) noted that there was “no single correct, indisputable, ecologically sound definition for wetlands, primarily because of the diversity of wetlands and because the demarcation between dry and wet environments lies along a continuum.” Nevertheless, FWS needed to define wetlands to accurately identify and delineate the Nation’s wetland resources. FWS defined wetlands as follows:

"Wetlands are lands transitional between terrestrial and aquatic systems where the water table is usually at or near the surface or the land is covered by shallow water. For purposes of this classification, wetlands must have one or more of the following three attributes: 1) at least periodically, the land supports predominantly hydrophytes; 2) the substrate is predominantly undrained hydric soil; and 3) the substrate is nonsoil and is saturated with water or covered by shallow water at some time during the growing season of each year."

Fig 1- From Cowardin et al. (1979) illustrating the relationship of wetlands, deepwater habitats and uplands, as well as the water table, on the landscape

The U.S. Army Corps of Engineers (COE) and the U.S. Environmental Protection Agency define wetlands as follows:

“Those areas that are inundated or saturated by surface or ground water at a frequency and duration sufficient to support, and that under normal circumstances do support, a prevalence of vegetation typically adapted for life in saturated soil conditions. Wetlands generally include swamps, marshes, bogs, and similar areas.”

COE uses three characteristics of wetlands when making wetland determinations: vegetation, soil, and hydrology. Unless an area has been altered or is a rare natural situation, wetland indicators of all three characteristics must be present during some portion of the growing season for an area to be a wetland.

In defining wetlands from an ecological standpoint, the FWS emphasized 3 key attributes of wetlands: 1) hydrology – the degree of flooding or soil saturation, 2) wetland vegetation (hydrophytes) and 3) hydric soils. Most wetlands have hydrophytes growing on hydric soils but all wetlands must have enough water at some time during the growing season to stress plants and animals not adapted to life in water or saturated soils. It is important to note that the FWS did not include permanently flooded deepwater areas as wetlands. Instead, these water bodies (generally deeper than 3 meters or 6.6 feet) were defined as deepwater habitats because water is the principle medium in which dominant organisms live. In summary, the FWS’s definition of a wetland is based on the degree of flooding or soil saturation and the presence of wetlands plants and/or hydric soils.

The structure of the Cowardin classification is hierarchical, progressing from systems and subsystems to classes, subclasses and dominance type. The classification structure to the class level is illustrated in the diagram below. Modifiers for water regime, water chemistry, and soils are applied to classes, subclasses and dominance types. Special modifiers are used to describe wetlands and deepwater habitats that have been created or highly modified by man or beavers.

The Cowardin wetland classification system to the class level (Cowardin et al. 1979)

In Iowa, most wetlands fall under the Palustrine System. The Palustrine System includes all nontidal wetlands dominated by trees, shrubs, persistent emergent vegetation or emergent mosses or lichens. It also includes wetlands lacking such vegetation but with of the following 3 characteristics: 1) area less than 8 ha (20 acres), 2) active wave-formed or bedrock shoreline features lacking, and 3) water depth in the deepest part of the basin less than 2 meters at low water. This system includes vegetated wetlands traditionally referred to as marshes, swamps, bogs, fens, and prairie wetlands, including the small, shallow, intermittent water bodies often called potholes or ponds. Palustrine wetlands may be situated shoreward of lakes or river channels, on floodplains, in isolated catchments, or on slopes. The emergent vegetation adjacent to rivers and lakes is often referred to as “the shore zone” or the “zone of emergent vegetation” (Reid and Wood 1976) and is generally a different type of habitat from the river itself. There are often many similarities between wetlands lying adjacent to lake or rivers and isolated wetlands of the same class.

Examples of wetland habitats in the Palustrine System (Cowardin et al. (1979)

The second most common classification for wetlands in Iowa is under the Lacustrine System . The Lacustrine System includes wetlands and deepwater habitats with all of the following characteristics: 1) situated in a topographic depression or a dammed river channel; 2) lacking trees, shrubs, persistent emergents, emergent mosses or lichens with greater than 30% surface coverage and 3) the total area exceeds 8 ha (20 acres). Similar wetland or deepwater habitats totaling less than 8 ha are also included in the Lacustrine System if an active wave-formed or bedrock shoreline feature makes up all or part of the boundary, or if the water depth in the deepest part of the basin exceeds 2 m (6.6 feet) at low water. Typically, these are permanently flooded lakes or reservoirs with extensive areas of deep water and considerable wave action. Islands of palustrine wetlands may lie within the boundaries of a lacustrine wetland.

Examples of wetland habitats in the Lacustrine System (Cowardin et al. (1979)

Most linear wetland features in Iowa would fall under the Riverine System. The Riverine System includes all wetlands and deepwater habitats contained within a channel, with the two exceptions: 1) wetlands dominated by trees, shrubs, persistent emergents, emergent mosses, or lichens, and 2) habitats with waters containing ocean derived salts in excess of 0.5o/oo. A channel is “an open conduit either naturally or artificially created which periodically contains moving water, or which forms a connecting link between two bodies of standing water” (Langbein and Iseri 1960:5). Water is usually, but not always, moving within the Riverine System. Palustrine Forested Wetlands, Emergent Wetlands, Scrub-Shrub Wetlands, and Moss-Lichen Wetlands occur adjacent to the Riverine System, often on a floodplain. Some biologists suggest that all the wetlands occurring on the river floodplain should be part of the Riverine System because their presence is often the result of river flooding.

Examples of wetland habitats in the Riverine System (Cowardin et al. (1979))

However, Cowardin and others (1979) agreed with Reid and Wood (1976) that the floodplain is a flat expanse of land bordering an old river and that it often takes the form of a very level plain occupied by the present day stream channel. It may never, or only occasionally, be flooded. It is the subsurface water, i.e., the ground water that controls to a great extent the flow of streams, the water level in lakes, and the extent of swamps and marshes.

The other two wetland classification systems developed by Cowardin et al. (1979), Marine and Estuarine, have no application in Iowa.

The second most common wetland classification system used in the United States was developed by Martin and others (1953) and republished in U.S. Fish and Wildlife Circular 39 (Shaw and Fredine 1956). Because this wetland classification system has been in circulation for more than 50 years, it has been used to describe wetlands in thousands of published research papers and continues to be used to describe wetlands for many long-term waterfowl and wetland studies. It is a somewhat simpler system for wetland classification than Cowardin and others (1979) system and is based on criteria such as water depth and permanence, water chemistry, life form of vegetation, and dominant plant species. This system recognizes 20 different types of wetlands, but Types 9 through 20 describe saline wetlands and salt marshes that are not found in Iowa. Table 1 illustrates how wetlands would be classified under the Circular 39 system and the Cowardin et al. system

Comparison of wetland types described by the U. S. Fish and Wildlife Service. Source: Cowardin et al. 1979

The third most common wetland classification system used in the upper Midwest was developed by Stewart and Kantrud (1971) to specifically serve the needs of researchers and wetland managers in the glaciated prairies region. Their system recognizes seven classes of wetlands that are distinguished by the vegetational zone occupying the central or deepest part and covering 5% or more of the basin. The classes reflect the wetland’s water regime, e.g., temporary ponds (Class II) are those where the wet-meadow zone occupies the deepest part of the wetland. The classification system includes six possible subclasses based on differences in plant species composition that are correlated with the average salinity of the surface water. The classification system also includes a cover type component that represents differences in the spatial relationship of emergent cover to open water or exposed bottom soil. The zones of the Stewart and Kantrud (1971) classification system are roughly equivalent to the water regime modifiers and water chemistry modifiers in the Cowardin et al. (1979) system (Table 2).

A comparison of wetland zones as described by the U.S Fish and Wildlife Service. Source: Cowardin et al. (1979)

Recommendations for defining wetlands in Iowa for the purposes of prioritizing wetlands as it relates to their protection and restoration

For the purpose of prioritizing wetlands for protection and restoration in Iowa, wetlands will be defined as those that are classified as Palustrine wetlands, as defined by Cowardin et al. (1979), with water regimes A-Z (temporarily flooded/saturated to permanently flooded/intermittently exposed) and including those with special modifiers b (beaver), d (partially drained/ditched), f (farmed), h (dikes/impounded), and x (excavated) (see Figure 6). Because the vast majority of wetlands in Iowa have been drained and converted to cropland, it is particularly important to include palustrine wetlands with the modifiers d and f as these wetlands have substantial restoration potential. Additionally, wetlands included in the Lacustrine System that are less than 8 ha (20 acres) in size but deeper than 2 meters (6.6 feet) should be included with palustrine wetlands for prioritization purposes as these smaller lacustrine wetlands are often developed to enhance water quality for streams, rivers, and lakes, often have emergent vegetation in their littoral zones, and substantially benefit wildlife due the high shoreline to surface acre ratio. Lacustrine wetlands larger than 8 ha (20 acres) and between 2 and 3 meters deep (6.6 – 9.9 feet) will be defined as shallow lakes.

The wetlands defined above are included in the following classes defined by Circular 39 ((Shaw and Fredine 1956) (also see Table 1):

  • Type 1 – Seasonally flooded basins, including wet meadows, incompletely drained pastures or crop fields that would support hydrophytes if not tilled, flooded bottomland hardwoods, and fresh water swamps
  • Type 2 – Inland fresh meadows, including fens and sedge meadows.
  • Type 3 – Inland shallow fresh marshes, usually semipermanently flooded.
  • Type 4 – Inland deep fresh marshes, usually semipermanently or permanently flooded.
  • Type 5 – Inland open fresh water, including open water wetlands with submergent aquatics, usually semipermanently or permanently flooded.
  • Type 6 – Shrub swamps.
  • Type 7 – Wooded swamps.
  • Type 8 – Bogs.

Literature Cited

  • Cowardin, L. M., V. Carter, F. C. Golet, and E. T. LaRoe.  1979.  Classification of Wetlands and Deepwater Habitats of the United States.  U.S. Fish and Wildlife Service.  FWS/OBS-79/31.  103  p.
  • Langbein, W. B., and K. T. Iseri.  1960.  General introduction and hydrologic definitions manual of hydrology.  Part 1.  General surface-water techniques.  U. S. Geol. Surv. Water-Supp. Pap. 1541-A.  29 p.
  • Martin, A. C., N. Hotchkiss, F. M. Uhler, and W. S. Bourn.  1953.  Classification of wetlands of the United States.  U.S. Fish Wildl. Serv., Spec. Sci. Rep. – Wildl. 20. 14 p.
  • Reid, G. K., and R. D. Wood.  1976.  Ecology of inland waters and estuaries.  D. Van Nostrand and Co., New York.  485 p.
  • Shaw, S. P., and C. G. Fredine.  1956.  Wetlands of the United States.  U.S. Fish Wildl. Serv., Circ. 39.  67 p.
  • Tiner, R. W. Jr.  1984.  Wetlands of the United States:  Current Status and Recent Trends.  59 p.

Wetland Classification Links