National Wetlands Inventory (NWI) Explained

CT National Wetland Inventory Wetland Functions 2010 is a map of wetland classifications and includes several descriptive attributes. This page attempts to make sense of the codes used in the attributes.  

Wetland Codes | NWI+ for CT | LLWW Codes | Wetland Functions | More info and links


Wetland Codes

The wetland classification codes are a series of letter and number codes that have been developed to adapt the national wetland classification system to map form. These codes correspond to the classification nomenclature that best describes the habitat. Full explanation is provided in this FGDC's Wetlands Subcommittee documents titled Classification of Wetlands and Deepwater Habitats for the United States.

To find the description of a wetland code, use the US Fish and Wildlife Wetland Code Interpreter.


NWI+ for Connecticut

The following information can be found in the Connecticut Wetlands: Characterization and Landscape-level Functional Assessment document.

The standard NWI classification is a hierarchical organization that categorizes wetlands into systems ans subsystems and was developed by L.M. Cowardin, et al. (1979) for the USFWS.The layers in the viewer contain more attributes then the original NWI.

The additional attributes are a result of recent techniques to add hydrogeomorphic-type descriptors to the standard NWI data, called the NWI+ database.

Systems and Subsystems
  Code Description
marine M Associated with the ocean. Sub tidal (M1), intertidal (M2).
estuarine E Associated with tidal brackish waters.
riverine R Associated with freshwater rivers and streams. Tidal (R1), lower perennial (R2), upper perennial (R3), intermittent (R4).
lacustrine L Associated with lakes or other bodies of fresh water. Limnetic (L1), littoral (L2).
palustrine P Freshwater wetlands not associated with rivers, streams or lakes.

To find a specific code, use the Wetland Code Interpreter. For a list of all codes with definitions see Appendix A of the Data Collection Requirements and Procedures for Mapping Wetland, Deepwater, and Related Habitats in the United States.


LLWW (Landscape position, Landform, Water flow path and Waterbody type) Codes

In order to extend the wetlands inventory so that it contains wetland function, the data have an attribute called LLWW Descriptor. LLWW uses four elements:

  • landscape position,
  • landform,
  • water flow path and
  • waterbody type.

For a complete list of LLWW codes and descriptors, see Section 4 (page 39) of the US Fish and Wildlife Dichotomous Keys and Mapping Codes for Wetland Landscape Position, Landform, Water Flow Path, and Waterbody Type Descriptors. Commonly used codes in Connecticut are listed below with descriptions.

Landscape Position describes the location of a wetland relative to a waterbody if present.
  Code Description
estuarine ES along tidal brackish waters
lentic LE in basins of lakes and reservoirs
lotic LR, LS along rivers (LR) and streams (LS) and subject to overflow
marine MA along the ocean. Because Long Island Sound is an estuary, there are no marine wetlands in Connecticut.
terrene TE sources of streams or isolated – completely surrounded by upland, or not affected by the aforementioned waters. TE1 is terrene headwater, TE2 is terrene riparian, TE3 is terrene non-riparian.
Landform describes the physical shape of the wetland. Several types are recognized.
  Code Description
basin BA depressional wetland
flat FL wetland on a nearly level plain
floodplain FP overflow land along rivers subject to periodic inundation
fringe FR wetland in water, within the banks of a river, or on an estuarine intertidal plain, island (wetland completely surrounded by water)
island IS wetland completely surrounded by water or a river, stream, lake or pond
slope SL wetland on a hillside
Water Flow path defines the direction of flow of water associated with the wetlands.
  Code Description
outflow wetland OA, OI, OU If the wetland is a source of a stream or a seep. Outflow-artificial (OA) when water flows out of the system through a ditch or manmade channel; no direct surface water inflow. Outflow-intermittent (OI) when water flows out of the system periodically usually during the wet season or during and shortly after heavy rains; no direct surface water inflow; typically associated with intermittent streams and groundwater discharge; may be the source of a stream. Outflow-perennial (OU) when water flows out of the system year-round; no direct surface water inflow; typically associated with perennial streams, rivers and groundwater discharge; often the source of a stream.
throughflow wetlands TA, TI, TH River and streamside wetlands with water running through them (both into and out of) during high water periods. Throughflow-artificial (TA) when water enters from a water source above and flows out of the system via a ditch or manmade channel or canal. Throughflow-intermittent (TI) when water enters from a water source above and flows out of the system via an intermittent stream; flow usually occurs during the wet season or during and shortly after heavy rains. Throughflow-perennial (TH) when water flows through the system more or less year-round via a perennial stream; wetlands subject to seasonal overflow.
inflow wetlands IN Wetlands that only receive water from channelized flow without any outflow.
isolated IS Some wetlands have no channelized inflow or outflow – essentially with no water flow path, although water undoubtedly
can enter via runoff from the land and exit via groundwater.
bidirectional BO, TB, BT Water levels rise and fall. Bidirectional-nontidal outflow (BO) when water levels rise and fall with water in an outflow lake. Bidirectional-nontidal throughflow (TB) when water levels rise and fall due to influences or by river flood stages, but water does not flow through the wetland. Bidirectional-tidal (BT) when water ebbs and flows largely in response to tides.
Waterbody Type
  Code Description
lake LK natural (LK1), dammed river valley lake (LK2), other dammed lake (LK3), deep excavated lake (LK4), shallow excavated lake (LK5), other artificial lake (LK6)
river RV perennial (RV1), unknown (RV2), ephemeral (RV3), intermittent (RV4), tidal (RV5), dammed (RV6)
stream ST perennial (ST1), unknown (ST2), ephemeral (ST3), intermittent (ST4), tidal (ST5), dammed (ST6)
pond-associated PD natural (PD1), dammed/impounded (PD2), excavated (PD3), beaver (PD4), other artificial (PD5)
estuary EY drowned river valley estuary (EY1), bar-built estuary (EY2), river-dominated estuary (EY3), rocky headland bay estuary (EY4), island protected estuary (EY5), shoreline bay estuary (EY6)

Wetland Functions

Appendix A of the Connecticut Wetlands: Characterization and Landscape-level Functional Assessment document is a correlation table showing wetlands of significance for each function.

Code Title Possible Values
BSS Bank and Shoreline Stabilization High, Moderate
CAR Carbon Sequestration High, Moderate
CSS Coastal Storm Surge detention High, Moderate
FAIH Fish and Aquatic Invertebrate Habitat High, Moderate
NT Nutrient Transformation High, Moderate
OWH Other Wildlife Habitat High, Moderate
SM Streamflow Maintenance High, Moderate
SR Sediment and other particulate Retention High, Moderate
SWD Surface Water Detention High, Moderate
WBIRD Waterbird Habitat High, Moderate, Wood Duck
UWPC Unique, Uncommon, or highly diverse Plant Communities Regionally significant

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