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The term "wetland" is very all-inclusive. Anything ranging from a small area of ground that is relatively spongy for only a portion of the year, to a large area that is covered year-round by shallow water may be a wetland. As a result, there is not one all-encompassing wetland type. Swamps, marshes, bogs, fens, backwater lakes and sloughs, small streams, shallow ponds, lake and river shores, wet meadows and prairies, and bottomland hardwood areas are all considered wetlands. Each one has its own unique set of attributes and resulting functions. In order to identify and categorize the various different wetland types a technical classification system has been developed by the USF&WS.

USF&WS Wetland Classification System
In the USF&WS wetland classification hierarchy, wetland types are categorized by special modifiers which describe: hydrophytic plant populations, hydrologic regimes, substrate types, and human activities that can artificially effect wetlands. The highest level of the classification hierarchy is called the system. In Illinois, three of the five nationally recognized wetland systems occur: Palustrine, Lacustrine, and Riverine.

The Palustrine system includes the wetland types most people think of when hearing the term "wetland". These are the soggy, transitional areas (i.e., marshes, bogs, swamps, bottomland forests, and small ponds). Wetlands classified in the Lacustrine system are more closely associated with open water areas (i.e., lakes, reservoirs, and impounded rivers). Wetlands of the Riverine system are associated with free-flowing bodies of water (i.e., un-impounded rivers and streams).

The next classification division is the subsystem. The distinction between subsystems is made on the basis of large differences in hydrology within the system. The Palustrine system has no such large differences and therefore has no subsystems.

The Lacustrine system has two subsystems: the Littoral and Limnetic. Littoral subsystems consist of everything from the shoreline to a depth of 2 meters below low-water, or to the extent that non-persistent emergents are present (if they grow at greater depths). Limnetic subsystems consist of all deepwater habitats at depths greater than two meters below the average low water level (Cowardin et al. 1979).

The Riverine system, because it has the least constant hydrology of the three systems, has four subsystems. Only three of these, the Lower Perennial, Upper Perennial, and Intermittent subsystems, occur in Illinois. Lower Perennial subsystems consist of areas with a low gradient, slow water velocity, no tidal influence, and a constant flow of at least some water throughout the year. Upper Perennial subsystems exist in areas with a high gradient, fast water velocity, no tidal influence, and a constant flow of at least some water throughout the year. Intermittent subsystems occur in areas in which the channel contains flowing non-tidal water for only part of the year. When water stops flowing through this subsystem it may remain in isolated pools or be entirely absent above the surface.

After the subsystem level, wetlands are further grouped into smaller organizational units called classes. Each subsystem may contain numerous classes. Classes are based on the condition of the wetland's substrate and the hydrophytic vegetation present, both of which are a function of hydrology.

Subclasses and Modifiers
Classes are further divided into subclasses based upon the relative dominance of the specific plant species present. A wetland may be described in detail beyond the subclass level. In order to accomplish this, a series of modifiers may be applied. These modifiers represent specific details about the water regime, substrate type, or water chemistry of the wetland. There are also a series of special modifiers that describe how human activities (i.e., draining and impounding) have affected the wetland.

By their very nature, however,wetlands are dynamic resources. The characteristics of any given wetland are constantly in flux. The biotic communities and physical qualities that classify a wetland are always changing. For example, through succession an area previously classified as Lacustrine may fill up from the gradual accretion of sediment and deposition of decaying organic material. After years of this activity, the physical changes to the ecosystem and the biotic community may cause the wetland to be more accurately classified as a Palustrine marsh. There are also many other ways wetlands undergo changes, and these natural and artificially induced changes make the monitoring and management of wetlands very difficult.

[Introduction | Delineation | Hydric Soils|Wetland Hydrology| Hydrophytic Vegetation| Wetland Classification |Summary]

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