Stream 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.

  • Temporary means impacts will occur for a period of one year or less and restoration of the aquatic system will occur following termination of the permitted activity.

  • Permanent means project impacts will occur for a period of greater than one year.

Dominant Impact

Dominant Impact categories are defined as follows.

  • Armor refers to riprap, bulkhead, or other rigid methods to contain stream channels.

  • Clear refers to activities, such as clearing streambank vegetation without disturbing the existing topography or soil stratigraphy. Mitigation for impacts associated with clearing may be required if the impact occurs as a result of, or in association with, an activity requiring a permit.

  • Culvert refers to routing a stream through pipes, box culverts, or other enclosed structures for a distance of 100 feet or less. Culverts should be designed to allow unimpeded natural stream processes such as sediment transport and fish migration. The width, height, and gradient of a proposed opening should be such as to pass the average historical low flow and spring flow without adversely altering flow velocity. The culvert bottom including headwalls and toe-walls should be embedded to a depth of no less than 12 inches below ground line. If rock runs throughout the culvert area, a bottomless culvert should be used. Culvert extensions where the cumulative length of the existing culvert and the additional extension exceeds 100 feet will be considered a pipe impact.

  • Detention/Weir refers to placing a weir in a stream to slow or to divert water when bankfull is reached. The structure should be designed to allow ingress and egress of aquatic organisms and to pass flows below bankfull stage.

  • Fill refers to the permanent placement of fill material in a stream channel.

  • Impound/Flood means to convert a flowing system to a still water system such as a reservoir, pond, or lake. Installation of a dam that modifies the stream to facilitate sediment control and/or stormwater management is considered impoundment. For creation of still water systems, the footprint of the impoundment structure is considered a Fill impact and the footprint of the pool is considered an Impound/Flood impact.

  • Morphologic change means to channelize, dredge, or otherwise alter the established or natural dimension, pattern, or profile of a stream.

  • Pipe refers to routing or diverting a stream through a pipe, culvert, or other enclosed structure for a distance greater than 100 feet.

  • Shade refers to intercepting or blocking sunlight. Examples of projects causing shading impacts include bridges, piers, and buildings constructed on pilings at such elevation that vegetation will be impacted.

  • Utility crossings refer to open cut construction or other pipeline/utility line installation methods that require disturbance of the streambed and reestablishment of pre-project contours after installation.

Seasonal or Perennial Stream Type

Seasonal or Perennial Stream Type categories are based on the suite of functions that they perform and are defined as follows.

  • Non-RPWs are non-navigable tributaries that are not relatively permanent. (i.e. the tributaries typically do not have continuous flow for at least 3 months of the year).

  • 1st and 2nd Order RPWs are non-navigable tributaries of traditional navigable waters that are relatively permanent where the tributaries typically have continuous flow at least seasonally (e.g., typically three months)

  • All other streams include greater than 2nd order tributaries and traditional navigable waters.

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 Appendix 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. Projects that result in impacts to 6000 linear feet or less of stream channel will be assigned a cumulative impact factor between 0.1 and 1.5. Projects that result in impacts to greater than 6000 linear feet of stream channel will be assigned a cumulative factor of 3.0.