Wetland Primer

What Are Wetlands?

Wetlands are lands that are flooded or saturated at or near the ground surface for varying periods of time during the year. Water comes from rainfall, snowmelt, river overflow, ocean-driven tides, rising lake levels, or ground water coming from beneath the soil surface. There are many wetland definitions - some are technical definitions used by scientists to describe and inventory wetlands, while other are regulatory definitions that define lands covered by government regulations and zoning ordinances.

Figure 1. Many wetlands occur along water bodies like rivers and streams.

1890 wetland definition: One of the earliest wetland definitions used in the United States comes from an 1890 federal government report on wetlands "General Account of the Freshwater Morasses of the United States" (Nathaniel Shaler): "...all areas... in which the natural declivity is insufficient, when the forest cover is removed, to reduce the soil to the measure of dryness necessary for agriculture. Wherever any form of engineering is necessary to secure this desiccation, the area is classified as swamp." So, for a long time, wetlands have been recognized as areas that are too wet to farm or too wet to build upon without draining or filling.

Presently, there are two definitions in wide use in the United States. One is the U.S. Fish and Wildlife Service's (FWS) definition used for conducting the National Wetlands Inventory, a government program that is mapping wetlands across the country. The other is the federal regulatory wetland definition used to identify wetlands subject to federal regulations under the Clean Water Act. These definitions are given below. Note that the FWS definition includes both vegetated and nonvegetated habitats in its definition, while the federal regulatory definition emphasizes vegetation and therefore only includes vegetated wetlands.

FWS definition:
"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."
Note: hydrophytes are plants capable of growing in water or waterlogged soils/substrates; hydric soils are waterlogged soils that support plant growth; nonsoil is a nonvegetated substrate like a mudflat or rock outcrop.

Federal Regulatory definition:

Wetlands are "those areas that are inundated or saturated 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."
Note: "under normal circumstances" means that such areas support plant growth unless such growth is removed or prevented from growing by man or infrequent natural events (like mudslides or volcanic eruptions).
While the above two definitions are widely used, recognize that wetlands may be defined somewhat differently in your state or town for regulatory purposes, so consult these agencies for details.