Wetland Mitigation

Compensatory Mitigation Guidelines Working Draft, Subject to Change Last Revised October 7, 2010.


Duration means the length of time the adverse impacts are expected to last. For example, if a forested wetland is cleared to construct a temporary access road it will take more than 10 years for a similar forested canopy to develop.

Dominant Impact

Dominant Impact categories are defined as follows:

  • Clear means to remove vegetation without disturbing the existing topography of the soils.

  • Draining means ditching, channelization, or excavation that results in the removal of water from an aquatic area causing the area, or a portion of the aquatic area, to change over time to a non-aquatic area or a different type of aquatic area.

  • Dredge means to dig, gather, pull out, or excavate from waters of the United States.

  • Fill means depositing material used for the primary purpose of replacing an aquatic resource with dry land or changing the bottom elevation of a water body or wetland.

  • Impound means to collect or confine the flow of a riverine system by means of a dike, embankment, or other man made barrier.

  • Impoundments may result in the formation of ponds, lakes, reservoirs, detention basins, etc, or they may limit the reach of high waters, such as levees or flood dikes.

  • Shading means to shelter or screen by intercepting radiated light or heat. Examples of projects causing shading impacts include bridges, piers, and buildings on pilings.

Wetland Type/Lost Type

These categories are based on the suite of functions that they perform and are defined as follows.

  • Type A means:
    • Tidal vegetated systems
    • Shallow subtidal bottoms
    • Riverine systems including headwaters
    • Bottomland hardwoods and riparian zones
    • Intertidal flats
  • Type B means:
    • Seeps and bogs
    • Depressions
    • Savannahs and flatwoods
    • Pocosins and bays
  • Type C means:
    • Man-made lakes and ponds
    • Impoundments
    • Vegetated lake littoral
    • Shallow cove areas

Habitat types that are not categorized will be evaluated on a case-by-case basis with consideration of any comments provided by the resource agencies.

The following is a brief description of the major systems of wetlands under the Cowardin system, Classification of Wetlands and Deepwater Habitats of the United States.

  • Marine -- Open ocean overlying the continental shelf and coastline exposed to waves and currents of the open ocean shoreward to (1) extreme high water of spring tides; (2) seaward limit of wetland emergents, trees, or shrubs; or (3) the seaward limit of the Estuarine System, other than vegetation. Salinities exceed 30 parts per thousand (ppt).

  • Estuarine - Deepwater tidal habitats and adjacent tidal wetlands that are usually semienclosed by land but have open, partly obstructed, or sporadic access to the ocean, with ocean-derived water at least occasionally diluted by freshwater runoff from the land. The upstream and landward limit is where ocean-derived salts measure less than .5 ppt during the period of average annual low flow. The seaward limit is (1) an imaginary line closing the mouth of a river, bay, or sound; and (2) the seaward limit of wetland emergents, shrubs, or trees when not included in (1).

  • Riverine - All wetlands and deepwater habitats contained within a channel except those wetlands (1) dominated by trees, shrubs, persistent emergents, emergent mosses, or lichens, and (2) which have habitats with ocean-derived salinities in excess of .5 ppt.

  • Lacustrine - Wetlands and deepwater habitats (1) situated in a topographic depression or dammed river channel; (2) lacking trees, shrubs, persistent emergents, emergent mosses, or lichens with greater than 30% areal coverage; and (3) whose total area exceeds 8 hectares (20 acres); or area less than 8 hectares if the boundary is active wave-formed or bedrock or if water depth in the deepest part of the basin exceeds 2 m (6.6 ft) at low water. Ocean-derived salinities are always less than .5 ppt.

  • Palustrine - All nontidal wetlands dominated by trees, shrubs, persistent emergents, emergent mosses, or lichens, and all such tidal wetlands where ocean-derived salinities are below .5 ppt. This category also includes wetlands lacking such vegetation but with all of the following characteristics: (1) area less than 8 ha; (2) lacking an active waveformed or bedrock boundary; (3) water depth in the deepest part of the basin less than 2 m (6.6 ft) at low water; and (4) ocean-derived salinities less than .5 ppt. The majority of the adverse impacts authorized by Department of the Army permits occur within the following three classes of the Palustrine system:

    • Forested Wetland- Forested Wetlands are characterized by woody vegetation that is 6 m tall or taller. They normally possess an overstory of trees, an understory of young trees or shrubs, and a herbaceous layer.

    • Scrub-Shrub Wetland- Scrub-Shrub Wetlands include areas dominated by woody vegetation less than 6 m (20 feet) tall. The species include true shrubs, young trees, and trees or shrubs that are small or stunted because of environmental conditions. Scrub-Shrub Wetlands may represent a successional stage leading to Forested Wetland, or they may be relatively stable communities. They are one of the most widespread classes in the United States and are known by many names, such as shrub swamp, bog,and pocosin.

    • Emergent Wetland- Emergent Wetlands are characterized by erect, rooted, herbaceous hydrophytes, excluding mosses and lichens. This vegetation is present for most of the growing season in most years. These wetlands are usually dominated by perennial plants. Emergent Wetlands are known by many names, including marsh, meadow, fen, prairie pothole, and slough. Areas that are dominated by pioneer plants which become established during periods of low water are not Emergent Wetlands and should be classified as Vegetated Unconsolidated Shores or Vegetated Streambeds.

Priority Category

Priority Category is a factor that recognizes the importance of aquatic resources that provide valuable functions and services on a watershed scale, that occupy important positions in the landscape, or that are considered important because of their rarity. Adverse impacts to primary priority areas should be avoided and minimized to the maximum extent practicable.

Primary priority

Primary priority areas include:

  • National Estuarine Sanctuaries
  • Anadromous fish spawning waters
  • Wild and Scenic Rivers.
  • State Heritage Trust Preserves
  • Designated Shellfish Grounds
  • National Wildlife Refuges
  • Outstanding Resource Waters
  • Waters officially designated by State or Federal agencies as high priority areas
  • Essential Fish Habitat
  • Trout waters
  • Old growth climax communities that have unique habitat structural complexity likely to support rare communities of plants or animals
  • All tidal waters

And the following categories of rare aquatic systems:

  • Hillside Herb Bog
  • Piedmont Seepage Forest
  • Upland Bog
  • Limestone Sink
  • Atlantic White Cedar Bog
  • Pine Savannah
  • Depression Meadow
  • Interdune Pond

Secondary priority

Secondary priority areas include the following categories of vulnerable or uncommon aquatic systems that do not fall into the designated primary priority category:

  • Carolina Bay
  • Swale Pocosin
  • High Elevation Seep
  • Pond Cypress Pond
  • Bay Forest
  • Seepage Pocosin
  • Salt Shrub Thicket
  • Upland Depression Swamp Forest
  • Waters on the 303(d) list

Tertiary priority

Tertiary priority areas include the following categories of aquatic systems that do not fall into the designated primary priority category:

  • Bald Cypress-Tupelo Gum Swamp
  • Non-alluvial Swamp Forest
  • Swamp Tupelo Pond
  • Pond Pine Woodland
  • Pocosin (other than seepage or swale)
  • Pine flatwoods
  • Bottomland hardwood

Note: descriptions of these community types may be found in Appendic C and The Natural Communities of South Carolina, Initial Classification and Description (Nelson, John B).

Existing Condition

Existing Condition means the degree of disturbance relative to the ability of a site to perform its physical, chemical, and biological functions. This factor evaluates site disturbances relative to the existing functional state of the system.

  • Fully functional means that the typical suite of functions attributed to the aquatic resource type are functioning naturally. Existing disturbances do not substantially alter important functions. Examples include: pristine (undisturbed) wetlands, aquatic resources with nonfunctional ditches or old logging ruts with no effective drainage, or minor selective cutting.

  • Partially impaired means that site disturbances have resulted in partial or full loss of one or more functions typically attributed to the aquatic resource type but functional recovery is expected to occur through natural processes. Examples include: clear-cut wetlands, aquatic areas with ditches that impair but do not eliminate wetland hydrology, or temporarily cleared utility corridors.

  • Impaired means that site disturbances have resulted in the loss of one or more functions typically attributed to the aquatic resource type and functional recovery is unlikely to occur through natural processes. Restoration activities are required to facilitate recovery. Examples include: areas that have been impacted by surface drainage and converted to pine monoculture or agriculture, areas that are severely fragmented, or wetlands within maintained utility corridors.

  • Very impaired means that site disturbances have resulted in the loss of most functions typically attributed to the aquatic resource type and functional recovery would require a significant restoration effort. Examples include: filled areas, excavated areas, or effectively drained wetlands (hydrology removed or significantly altered).

Cumulative Impact

Cumulative Impact is defined by the National Environmental Policy Act as the impact on the environment which results from the incremental impact of an action when added to other past, present, and reasonably foreseeable future actions regardless of what agency or person undertakes such other actions. Cumulative impacts can result from individually minor but collectively significant actions taking place over a period of time. The total acreage of permanent and temporary wetland impacts are added together to determine the value (0.1 - 2.0) of the cumulative impact factor for a proposed project. The same value is used to calculate the RMC for each adverse impact associated with the proposed project.