Glossary

Glossary of terms used in Wetland Indicators

  • Anadromous Fish - Fish that migrate up rivers, from marine environments, to reproduce in fresh water.
  • Catadromous Fish - Fish that migrate down rivers to reproduce in marine environments.
  • Down-cut Stream Channel - a stream channel that is eroding or has previously eroded vertically creating an incised channel. This process, called degradation, causes the stream to be removed from its floodplain, the confinement of flood flows within the channel and the stream banks to become unstable.
  • Flow Function - wetlands are often areas where ground water is discharged to streams, rivers and lakes where it then becomes surface water (note arrows for direction of flow in the diagram). At certain times during the year, the ground water table does not intersect the land surface and ground water is recharged by surface water (arrows for direction of flow in the diagram would be reversed).
  • Hydrogeomorphic Classification - In 1993, the U.S. Army Corps of Engineers (the Corps) developed a hydrogeomorphic (HGM) Approach or classification system to address abiotic wetland functions. The HGM Approach emphasizes the hydrologic and geomorphic controls that influence many wetland functions. The HGM Approach focuses on the location of a wetland in a watershed (its geomorphic setting), its sources of water, and its hydrodynamics. The HGM approach first classifies wetlands based on their differences in functioning, second it defines functions that each class of wetland performs, and third it uses reference to establish the range of functioning of the wetland. A series of geographically based models or "functional profiles" for various wetland types are being created for use in functional assessments. The classification is designed for on-site application and requires considerable field effort for model development. The HGM models could help broaden our understanding of the range in performance of selected functions by different wetland types.
  • Hydrograph - a graphical representation of stream discharge (volume/time) during a storm or flood event.
  • Inlet/Outlet Class - inlet and outlet class are determined by local topography, surface water and ground water flow, residence time (storage of water), and water inflow and outflow.
  • Interspersion of Open Water and Vegetation - the relative spatial distribution of open and vegetated areas within a wetland boundary.
  • Land Use - the presence of natural processes and productivity or human-made forms of productivity (agriculture, forestry, mining) or use (recreation, residential, commercial, industrial) on a parcel of land.
  • Microrelief- variations in the topography of the land surface within a wetland, specifically the difference between the highest and lowest elevations. Microrelief within a wetland is also called surface "roughness" such as hummocks (mounds, small hills or islands) and depressions. The rougher the wetland surface the more slowly water will flow through the wetland. Microrelief is an important feature which influences the storage capacity of wetlands and, therefore, the flood control value of wetlands and the provision of wildlife habitat.
  • Outlet Restriction - the degree of outlet restriction of a wetland basin is a function of the area of the outlet which controls the rate of discharge or the hydraulic outflow.
  • Overbank Flooding - the flow of water or stream discharge beyond the stream channel. This excess flow is often accommodated within the floodplain or wetland areas adjacent to the stream.
  • Overland Flow - the flow of water over a land surface due to direct precipitation. Overland flow generally occurs when the precipitation rate exceeds the infiltration capacity of the soil (Fetter, 1994).
  • Potentiometric Surface - the elevation of the ground water table for an unconfined aquifer. The regional potentiometric surface reflects the subsurface geology and topography.
  • Sediment Retention - the deposition and storage of sediment in a topographic depression, such as a lake or basin, or a river drainage system.
  • Sediment Source - the place of origin of particles composed of organic or inorganic materials. Organic particles are produced by the decay of plants and animals. Inorganic materials are composed primarily of rock fragments and minerals derived from weathered and eroded uplands, particularly mountains and other high topographic features nearby. Local inorganic sediment sources may include valley and floodplain deposits composed of gravel, sand and/or clay.
  • Springs and Seeps - Springs are localized points on the land surface where ground water discharges from the surficial or near surface geologic rock units as a point source and becomes surface water. Seeps are broad areas where ground water discharges to the land surface and becomes surface water.
  • Stem Density - the number of plant stems per unit area. Stem density offers resistance to water flow through a wetland, adding to the surface roughness. This decreases water flow velocity and results in sedimentation and reduced erosion.
  • Stream Sinuosity - a dimensionless measure of the longitudinal contour (a measure of curvature) of a stream channel. Sinuosity is calculated using the following ratios and measurements: stream length divided by valley length OR valley slope divided by channel slope.
  • Topographic Position within a Watershed - streams are assigned a stream order number (1-3) with respect to the topographic position within the watershed drainage system. Tributary or headwater streams are low-order streams (1 or2) located at higher elevations within the watershed.
  • Vegetation Cover Distribution - the spatial arrangment and density of vegetation in a given area.
  • Vegetation Interspersion - the relative spatial arrangement and density of different types of vegetation in a given area.
  • Water Chemistry - the type and concentration of ions present (dissolved) in water. (See periodic table for common ions (including metals) found in fresh water and ocean water.)

    The chemical composition of water influences the types of vegetation, fauna, and soils present in a wetland. Certain types of wetlands have unique water chemistry:

    • Estuarine wetlands contain ocean-derived water having low to high salinity
    • Wetlands in arid to subhumid climates contain water having extremely high salinity; these wetlands have no outflow and high rates of evaporation
    • Wetlands in northern climates form nutrient-poor bogs and nutrient-rich fens
    • Wetlands formed in limestone (calcareous rock type) contain alkaline, nutrient-rich water
  • Water Regime - the duration and timing of surface water inundation resulting from surface water (overland flow), precipitation and ground water inflow.
  • Wetland Edge - the shape or contour of the wetland boundary or edge. Terms used to describe a wetland edge are regular, irregular, linear, circular, eliptical, etc. These terms can also be combined (for example, an irregular, linear edge).

Fetter, C.W., 1994, Applied Hydrogeology: Macmillan, New York.

Fugro East, Inc., 1995, A Method for the Assessment of Wetland Function, for the Maryland Department of Natural Resources​