DELINEATION OF WETLANDS

6.3 DELINEATION OF WETLANDS

A variety of de finitions are used around the world to delineate wetlands. In the United States, the U.S. Army Corps of Engineers’ de finition of a wetland carries particular weight, since the Corps is responsible for delineating wetlands for regulatory purposes and issuing dredge-and- fill permits required by the Clean Water Act. The Corps of Engineers defines wetlands as “those areas that are inundated or saturated by surface or ground water at a

WETLANDS

frequency and duration su fficient to support, and that under normal circumstances do sup- port, a prevalence of vegetation typically adapted for life in saturated soil conditions” (US ACE, 1987). Wetlands delineated for regulatory purposes are called jurisdictional wet- lands . According to the U.S. Army Corps of Engineers Wetlands Delineation Manual (US ACE, 1987), three factors must generally be considered in delineating a wetland: vegeta- tion, soils, and hydrology.

6.3.1 Vegetation

Wetland vegetation is de fined as macrophytes that are adapted to inundated or saturated conditions. For purposes of delineation, plants can be grouped into five categories: obli- gate wetland plants (OBLs), facultative wetland plants (FACWs), facultative plants (FACs), facultative upland plants (FACUs), and obligate upland plants (UPLs). The char- acteristics of the five plant categories are given in Table 6.1. To meet the wetland vegeta- tion requirement, more than 50% of the dominant species in a delineated wetland area must be OBLs, FACWs, or FACs. Species lists of plants in these categories can be found in USACE (1987). Marsh grass (Spartina alterniflora) and bald cypress (Taxodium distichum ), both shown in Figure 6.6, are plant species that are almost always found in wetlands.

TABLE 6.1 Plant Categories Used in Wetland Delineation

Category

De finition Obligate wetland plant

Symbol

OBL

Plants that occur almost always (estimated probability ⬎99%) in wetlands under natural conditions but which may also occur rarely (estimated probability ⬍1%) in nonwetlands. Examples: Spartina alterniflora, Taxodium distichum.

Facultative wetland plant

FACW

Plants that usually occur in wetlands (estimated probability ⬎67 to 99%) but also occur in nonwetlands (estimated probability ⬎1 to 33%). Examples: Fraxinus pennsylvanica, Cornus stolonifera .

Facultative plant

FAC

Plants with a similar likelihood (estimated probability 33 to 67%) of occurring in both wetlands and nonwetlands. Examples: Gleditsia triaconthos, Smilax rotundifolia .

Facultative upland plant

FACU

Plants that sometimes occur in wetlands (estimated probability 1 to 33%) but occur more often in nonwetlands (estimated probability ⬎67 to 99%). Examples: Quercus rubra, Potentilla arguta .

Obligate upland plant

UPL

Plants that rarely occur in wetlands (estimated probability ⬍1%) but almost always occur in nonwetlands (estimated probability ⬎99%) under natural conditions. Examples: Pinus echinata, Bromus mollis.

DELINEATION OF WETLANDS

FIGURE 6.6 Wetland plants: (a) marsh grass (Spartina alterniflora); (b) bald cypress (Taxodium distichum ). (From NOAA, 2005b.)

6.3.2 Soils

Wetland soils, called hydric soils, are de fined as soils that are saturated, flooded, or ponded long enough during the growing season to develop anaerobic conditions in the upper soil layers that favor the growth and regeneration of hydrophytic vegetation. Most soils, includ- ing hydric soils, are composed predominantly of minerals such as quartz, feldspars, and clay minerals. However, hydric soils commonly have a buildup of organic matter at the soil surface, which can make the surface horizon dark colored. If the organic matter content (measured as organic carbon) is greater than 20 to 30% of the soil’s weight and this organic-rich layer is more than 40 cm thick, it is considered an organic soil. Peat is com- posed mostly of recognizable plant fragments that are only partly decomposed; muck con- tains highly decomposed organic matter. When drained of excess water and carefully managed, muck soils are among the most important vegetable-producing soils in the east- ern United States.

A property unique to hydric soils is their color or color patterns. Besides the dark shad- ing from the presence of organic matter, iron compounds are the most important coloring agents in soils. Hydric soils tend to exhibit gray or blue-gray colors (known as gleying or gleyed colors ), especially just beneath the topsoil or surface horizon, as shown in Figure 6.7. This results from the chemically reduced oxidation state of iron compounds as opposed to the rusty red (oxidized) and brown colors of drier, nonhydric soils. Where shallow water tables fluctuate, gray, yellow, and red colors can also occur as small splotches, or as thread- like or network patterns, created by accumulations or depletions of iron and manganese. Because they result from processes of reduction and oxidation, these color indicators of wetness are collectively termed redoximorphic features.

When a hydric soil is drained, it may no longer be referred to as a hydric soil, unless the supported vegetation is hydrophytic, and indicators of hydrology support the designa- tion as a hydric soil.

6.3.3 Hydrology

Hydrologic factors that determine whether an area is a wetland are the frequency, timing, and duration of inundation or soil saturation, as shown in Table 6.2 for nontidal areas. Zone I is

WETLANDS

FIGURE 6.7 Hydric soil. (From Suffolk County Soil and Water Conservation District, 2005.)

TABLE 6.2 Hydrologic Zones for Nontidal Areas

Percent of Time

Zone Name

Comments I Permanently inundated

Inundated

Inundation ⬎2 m mean water depth. Aquatic, not wetlands. II Semipermanently to nearly

Inundation de fined inundated or saturated

as ⬍2 m mean water depth. III

Regularly inundated or

saturated IV Seasonally inundated or

saturated V Irregularly inundated or

Many areas having these saturated

hydrologic characteristics are not wetlands.

VI Intermittently or never

Areas with these hydrologic inundated or saturated

characteristics are not wetlands. Source: USACE (1987).

WETLAND HYDROLOGY

aquatic, zones II, III, and IV are wetlands, and zone VI is upland. Zone V may or may not

be considered a wetland, depending on the other indicators. Recorded water-level data from adjacent streams and lakes, as well as model predictions, can be used to establish the hydrologic condition of an area.

If an area meets any of the wetland criteria regarding vegetation, soils, and hydrology, the area is classi fied by the Corps of Engineers as a jurisdictional wetland.