What Is A Wetland?

What Is a Wetland (Legally Speaking)?

Wetland Hydrology

Hydritic Soils

Hydrophytic Vegetation

Types of Wetlands

Values of Wetlands

Some Local Wetlands

What Is A Wetland?

Wetland ecosystems are transitional between terrestrial (upland) and aquatic ecosystems. Wetlands can be defined as areas that are either covered by shallow water or where the soil is saturated with water. Because of this wetlands have characteristic soils and contain specialized vegetation adapted those soils.

The controlling factor in wetlands is the saturation of the soil. When water fills up the spaces within the soil oxygen from the atmosphere cannot circulate into the soil and the soil becomes anoxic or anaerobic (without oxygen). With little or no oxygen present a chemical characteristic of the soil called the redox potential is changed, and when that happens it causes many of the chemical processes in the soil to change drastically.

If the soil is anoxic for a significant portion of the growing season a hydritic soil develops. Only specialized plants, known as hydrophytic plants, have developed the ability to grow and reproduce in these anoxic hydritic soils. These hydrophytic plants are characteristic of wetlands.

What Is A Wetland (Legally Speaking)?

When legislation was enacted to protect wetlands it became necessary to define wetlands more rigidly. The lawyers and politicians required exact definitions and delineations of the terms the ecologists used.

In the original legislation three "diagnostic environmental characteristics" were defined - wetland hydrology, hydritic soils, and hydrophytic vegetation. Any area possessing any one on these characteristics was legally considered a wetland. Subsequently the Bush Administration, by executive order, changed the rule so that an area had to possess all three of these characteristics to be considered a wetland. This change substantially reduced the number of wetlands under legal protection.

Wetland Hydrology:

To be considered to possess "wetland hydrology" an area must be either innundated or the soil must be saturated at some time during the growing season. Under these conditions hydritic soils are likely to develop and support wetland vegetation.

The water source can be from direct precipitation, from ground water, from flooding, or from some combination of these sources. The shape of the land (topography), underlying bedrock, and the permeability of the soil all affect how rapidly incoming water moves through the area. Dense plant cover may retard the drainage of water through the area but the increased transpiration of the vegetation may speed up the removal of water from the soil.

Whether or not an area possesses "wetland hydrology" can be determined in a variety of ways. There may exist recorded gauging data available from the U.S. Army Corps of Engineers, the U.S. Geological Survey, or from other agencies. There are also field indicators such as visual observation of innundation or soil saturation, watermarks on fixed objects, drift lines, and sediment deposits.

Hydritic Soils:

Hydritic soils develop where the soil is saturated during enough of the growing season to develop the anaerobic soil conditions that favor hydrophytic vegetation.

As the oxygen in the soil declines below the 0.5% level significant changes in the chemical environment in the soil begin to occur. One of the most significant of these changes is in the potential for ions & molecules to gain or lose electrons during chemical reactions. This tendency is quantified as the redox potential and indicates whether a given chemical reaction will tend to proceed from oxidation to reduction or the reverse.

In a reducing environment (with a low redox potential), such as occurs in anoxic saturated soil, ions & molecules such as ammonia, ferrous iron, hydrogen sulfide, and methane accumulate. In addition to the lack of oxygen, these toxic substances make it difficult for plants to metabolize.

These chemical changes occur within a few days of the soil becoming waterlogged. While many plants can survive under these conditions for a short time only the plant species found in wetlands can persist under these conditions. If the anoxic conditions persist long enough the soil develops hydritic characteristics that are indicated by soil colors and other physical characteristics.

A rule of thumb is that if the upper 12 inches of the soil is saturated more than 1/8th (12.5%) of the growing season (approximated by the freeze-free season) then the soil will be hydritic and the area will be a wetland. If it is saturated between 5% and 12.5% of this time the area may or may not be a wetland. Areas saturated less than 5% of the time are non-wetlands.

Hydritic soils can be classified into two broad categories: organic and mineral. Organic hydritic soils are commonly known as peats or mucks. All other hydritic soils are mineral soils. These have a range of textures and colors. There are a variety of indicators that signal the presence of hydritic soil.

Hydrophytic Vegetation:

Hydrophytic vegetation is macrophytic vegetation (vegetation large enough to be seen with the naked eye) that is adapted to environments where the soil is either permanently saturated or at least saturated enough of the year to exert a controlling influence on the plant communities present.

How do plants
adapt to
wetland soils?

While many factors such as light, soil texture and permeability influence the development of communities composed of hydrophytic vegetation the most important factor is the hydritic (anaerobic, anoxic) soil that develops in areas with wetland hydrology. Without some kind of adaptation plants cannot persist in areas with these soils. There are many types of adaptations. both biochemical/physiological and structural.

Many of the plant species in the United States have been classified into categories that indicate how likely they are to be found in wetlands. (It should be realized that although the bureaucratic process has essentially frozen these species into their currently assigned categories the scientific information originally used in sorting the species was in some cases minimal.) The categories are described below. Each region of the United States has its own list.

Plant Wetland Indicator Status Categories
indicator category symbol definition examples
Obligate Wetland
OBL almost always (more than 99% of the time) found in wetlands wide leafed cattail
white water lily
marsh marigold
Facultative Wetland
FACW usually (between 67% and 99% of the time) found in wetlands silver maple
silky dogwood
FAC with equal liklihood (between 33% and 67% of the time) of being found in wetlands or uplands red maple
flat topped goldenrod
Facultative Upland
FACU may occur in wetlands (between 1% and 33% of the time) but are usually found in uplands sugar maple
russian olive
canada thistle
Obligate Upland
UPL rarely occur in wetlands (less than 1% of the time); almost always found in uplands canada plum
mapleleaf viburnum
oak fern
- the three facultative categories (FACW, FAC, FACU) can have + or - modifiers
        (+ is more likely in wetlands; - is less likely in wetlands)
- many introduced plants have not been categorized
- native plants that have not been categorized are by default considered upland plants

The basic rule is that for an area to be considered a wetland more than 50% of the dominant species must be in either the OBL, FACW+, FACW, FACW-, FAC+, or FAC categories.

Types of Wetlands:

Many different schemes for classifying wetlands have been devised. Each seems to have its own strengths and weaknesses and to be useful under certain circumstances.

Geographical Systems: These systems are based primarily on geographical location, with adjacent areas being delimited by differences in factors such as land surface form, hydrology, geology, soils, and potential natural vegetation.

In one such system, often referred to as the Ecological Units System, a given wetland would end up with a hierachical description using named regions for each of the following levels:


For example, most of the eastern United States is in the Humid Temperate domain.

Hydrogeomorphic Systems: This type of system is based on geomorphic setting, dominant water source, and dominant hydrodynamics. Seven general classes of wetlands are recognized:

Riverine: floodplains and riparian zones along rivers
Depressional: located in depressions where water can collect
Slope: on slopes where groundwater discharges directly to the surface but does not collect
Mineral Soil Flats: areas of low topographic relief (such as old lake bottoms) where direct precipitation is the main water source
Organic Soil Flats: areas of low topographic relief created by the accretion of organic matter (e.g. raised peatlands) where direct precipitation is the main water source.
Tidal Fringe: areas along oceans and estuaries where the ground water is influenced by adjacent ocean or estuary water.
Lacustrine Fringe: areas along lakes where the ground water is influenced by the water level of the lake.

Each of these classes can in turn be subdivided, several times if desired, to provide a more specific identification.

Habitat Based Systems: Wetlands are given names that are usually based on the vegetation associated with them. One of the more commonly used systems is the one developed by The Nature Conservancy.

This system is only partialy hierachical, dividing wetlands between open and forested categories and again between mineral soil and peatland categories. A few examples of wetland designations would be: deep emergent marsh, non-calcareous lakeshore & streambank, floodplain, red maple & hardwood swamp, vernal pond, and northern white cedar swamp.

Combined Systems: The above systems can be combined in an almost infinte variety of ways. The most important of these combined systems is the Cowardin System. It was first developed by the US F&W Service and has since become the official system of the federal government and of some state governments.

Values of Wetlands:

There are many possible values to wetlands. Some of these are listed below:

Some Local Wetlands

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