What is a wetland, and why do we have so many types?

Figure 1. Schematic cross-section with names of scarps (shorelines) and sedimentary formations between them. (From Fred Beyer, North Carolina: The Years Before Man (Durham, N.C.: Carolina Academic Press, 1991). More about the illustration)

The legal definition of a wetland has become controversial as wetlands have gained a measure of protection from uncontrolled ditching and draining. This protection has been accorded them as their role in sustaining high water quality and wildlife habitat has been documented and recognized. The current definition used by the U.S. Fish and Wildlife Service is:

Wetlands are lands transitional between terrestrial and aquatic systems where the water table (the level at which the soil is saturated) is usually at or near the surface or the land is covered by shallow water.

The definition goes on to say that to be officially classified as a wetland, an area must have one or more of the following:

1. It must support the growth of specific types of water loving plants.
2. The soil must be of a moisture-filled (hydric) type.
3. The soil must be saturated or covered with water at some time during each growing season.

The diverse array of wetlands on North Carolina’s coastal plain results from its width and poor drainage, its warm temperate climate, and the mix of habitats created by its history of being covered and uncovered by the sea. The role of sea level change in establishing our states wetland heritage is the less well understood than its geography and climate, so I will describe this change and its role in wetland development in a little detail.

As mentioned in the introduction, sea level history includes a long-term downward trend during the last 90 million years, and a recent short term upward trend for the last 20,000 years. The downward trend left sandy ridges across the coastal plain. These ridges were once the edge of the sea. These old shorelines consist of a sandy ridge flanked on the westward side by an area of flat, sandy soils and on the eastward side by a downward sloping surface that grades into dark, muddy soils.

The three sections of the old shorelines represent the sand flats, dunes, and seaward-sloping beaches of ancient barrier islands. These islands were left behind during sea level’s 90 million year decline. Six major shorelines have been identified and dated by geologists, who refer to them as the “seven steps to the sea.” Figure 1 represents a diagrammatic cross-section of these features taken from Fred Beyer’s book entitled North Carolina: The Years before Man. Note that the old dune ridges are called scarps in the diagram, a scarp is defined by Webster as “a steep slope,” but in many cases, these eastern North Carolina examples make a mockery of the term. Their slopes are gradual at best, and almost imperceptible at worst.

Nonetheless, the scarps were found by people interested in building towns, roads, and railways. As a result, we can find them now by simply looking for the routes of state and federal highways built upon them. Examples include sections of U.S. routes 13 and 17, as well as N.C. routes 24, 32, 45, and 306. In the time before roads were paved, there were great advantages to have them on well-drained sandy ridges rather than on the frequently wet muddy areas between them. Those advantages are less important now, so new highways are built where land is cheap and available, sometimes even on top of perfectly good wetlands.

The recent short term rise in sea level has also played a role in forming our coastal wetlands. The rising sea has not only flooded coastal regions that once were dry land, it has converted rivers that were fast-flowing when sea level was lower into the meandering, brown and blackwater rivers we find today. The recent sea level rise also raised the water table under the flatlands between the scarps thereby creating wetlands in areas that were once dry.

In fact, were it not for humans digging ditches to drain land between the scarps, much of that found east of I-95 would meet today’s legal definition of a wetland. That fact alone explains why wetland definition is a contentious issue. When politicians pledge programs that will result in “no net loss of wetlands,” many people may feel that their property rights are being unfairly affected. Thus wetland preservation remains a hot-button topic in much of eastern North Carolina.

Some of the controversy about wetland preservation and drainage is based on the wide array of wetlands that exist in our outer coastal plain. Few observers of the open woodlands dominated by longleaf pines would argue strongly that these seasonal wetlands should be ditched and drained for use in agriculture, but many owners of pocosin wetlands might think something more useful might be developed in their place. The type of wetland that people are thinking about while discussing wetland development underlies much of the difference of opinion about their use. This virtual fieldtrip will describe and illustrate some of the major types of wetlands that occur on our outer coastal plain.

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