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Recovery Potential Screening

Ecological Indicators

Ecological IndicatorsThis web page describes several example indicators for the RPS Ecological Indicators category. A brief summary of each indicator provides the general name, example metrics used, relevance to watershed condition and basic information about data sources and measurement. Many of these indicator summaries are further hyperlinked to indicator-specific reference sheets with more detailed information, including literature excerpts. Most but not all of these example indicators have been compiled for the conterminous US as values per HUC12 watershed units and are either already embedded in the data tables of state-specific RPS Tools or publicly available through the Watershed Index Online (WSIO) Indicator Data Library. Some Ecological Indicators that weren't compiled nationally are available at state or local scales, and these can and should be added to the RPS Tool by the user as appropriate to support their screening objectives.

On this page:

% Natural Cover (N-Index) 

  • Description: Indicators of the extent, distribution, and preservation of natural land cover within the watershed. Natural cover types typically include forest, wetlands, shrubland, and grassland. Barren land may be optionally counted if it occurs naturally within the study area, such as along beaches, in mountainous areas with exposed bedrock, or in arid regions with sparse vegetative cover. Indicators can characterize the present-day extent of natural cover throughout the entire watershed area, within the riparian zone or hydrologically connected zone, or in patches that are contiguous to surface waters. Indicators can also measure changes in the extent of natural cover over time.
  • Example metrics: % N-Index in Watershed; % N-Index in Riparian Zone; % N-Index in Hydrologically Connected Zone; % N-Index Change in Watershed; % N-Index Contiguous to Water
  • Why relevant: Large-scale land use change often provides nonpoint pollution (e.g. from urban areas, agriculture, transportation, mining) as well as altering runoff and infiltration patterns in ways that can destabilize stream channels and flow regimes. The percent of watershed area that is not transformed to non-natural cover types is generally associated with runoff and flow dynamics within normal range of variability, as well as reduced opportunity for pollutant runoff.
  • Data sources and measurement: Measured as the percentage of total watershed area that is classified as a natural cover type within watershed, riparian zone, hydrologically connected zone, or contiguous to surface water. Indicators of natural cover change over time are calculated as the present-day percentage minus the historical percentage. The National Land Cover Database (NLCD) Exitfor 2011, 2006, 2001 and 1992 are available for determining the area of natural cover within study watersheds and changes over time; alternative land cover datasets are also often available from state-specific sources. One or more forms of this indicator has been measured at the HUC12 scale for all lower 48 states and data are available from Watershed Index Online (WSIO).

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% Forest 

  • Description: Indicators of the extent, distribution, and preservation of forest cover within the watershed. Indicators can characterize the present-day extent of forest cover throughout the entire watershed area, within the riparian zone or hydrologically connected zone, or in patches that are contiguous to surface waters. Indicators can also measure changes in the extent of forest cover over time. See Reference Sheet.
  • Example metrics: % Forest in Watershed; % Forest in Riparian Zone; % Forest in Hydrologically Connected Zone; % Forest Change in Watershed; % Deciduous Forest in Watershed; % Evergreen Forest in Riparian Zone
  • Why relevant: Greater watershed forest cover reduces the risk of numerous impairment types, thus lessening the relative complexity of restoration of impaired waters in forested watersheds. Maintenance of forest cover can mollify anthropogenic influences on runoff and recharge, water temperature and overland pollutant transport and to help ensure that natural watershed processes are or can become functional once stresses are removed.
  • Data sources and measurement: Measured as the percentage of total watershed area that is classified as forest cover within the watershed, riparian zone, hydrologically connected zone, or contiguous to surface water. Indicators of forest cover change over time are calculated as the present-day percentage minus the historical percentage. The National Land Cover Database (NLCD) Exitfor 2011, 2006, 2001 and 1992 are available for determining the area of forest cover within study watersheds and changes over time; alternative land cover datasets are also often available from state-specific sources. One or more forms of this indicator has been measured at the HUC12 scale for all lower 48 states and data are available from Watershed Index Online (WSIO).

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% Wetlands 

  • Description: Indicators of the extent, distribution, and preservation of wetland cover within the watershed. Indicators can characterize the present-day extent of wetland cover throughout the entire watershed area, within the riparian zone or hydrologically connected zone, or in patches that are contiguous to surface waters. Indicators can also measure changes in the extent of wetland cover over time. See Reference Sheet.
  • Example metrics: % Wetlands in Watershed; % Wetlands in Riparian Zone; % Wetlands in Hydrologically Connected Zone; % Wetlands Change in Watershed; % Wetlands Remaining in Watershed
  • Why relevant: Wetlands are key features in watershed processing of nutrients in runoff, retention of excessive runoff during extreme weather events and acting as sinks for sediment and pollutants. In addition, wetlands provide vital recharge, detention and release in their role within groundwater/surface water interactions. Absence of wetlands degrades natural processing of pollutants and results in greater direct transport to a receiving water body, increasing or perpetuating impairment. A greater proportion of wetland area in the watershed positively influences recovery potential because watersheds with more wetlands have greater resilience to nutrient and sediment impairments.
  • Data sources and measurement: Measured as the percentage of total watershed area that is classified as wetland cover within the watershed, riparian zone, hydrologically connected zone, or contiguous to surface water. Indicators of wetland cover change over time are calculated as the present-day percentage minus the historical percentage. The National Land Cover Database (NLCD)Exitfor 2011, 2006, 2001 and 1992 are available for determining the area of wetland cover within study watersheds and changes over time; alternative land cover datasets are also often available from state-specific sources. If possible, current wetlands abundance can be contrasted with estimated pre-settlement wetlands abundance to develop an indicator more sensitive to wetland loss rather than current wetlands total. One or more forms of this indicator has been measured at the HUC12 scale for all lower 48 states and data are available from Watershed Index Online (WSIO).

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% Woody Vegetation 

  • Description: Indicators of the extent, distribution, and preservation of woody vegetation cover within the watershed. Woody vegetation includes forest, woody wetlands, and shrubland. Indicators can characterize the present-day extent of woody vegetation cover throughout the entire watershed area, within the riparian zone or hydrologically connected zone, or in patches that are contiguous to surface waters. Indicators can also measure changes in the extent of woody cover over time. See Reference Sheet.
  • Example metrics: % Woody Vegetation in Watershed; % Woody Vegetation in Riparian Zone; % Woody Vegetation in Hydrologically Connected Zone; % Woody Vegetation Change in Watershed; % Woody Vegetation Contiguous to Water
  • Why relevant: This metric is relevant for reasons similar to watershed forest and watershed natural cover, and provides a more appropriate indicator choice in regions that are not naturally forested. Greater watershed forest and shrub cover reduces the risk of numerous impairment types, thus reducing the relative complexity of restoration. Woody vegetation has a broad array of influence on the capacity to recover, including intercepting and moderating the timing of runoff, buffering temperature extremes (which can also reduce certain toxicities), filtering pollutants in surface or subsurface runoff, providing woody debris to stream channels that enhances aquatic food webs and stabilizing excessive erosion.
  • Data sources and measurement: Measured as the percentage of total watershed area that is classified as woody vegetation cover within the watershed, riparian zone, hydrologically connected zone, or contiguous to surface water. Indicators of woody vegetation cover change over time are calculated as the present-day percentage minus the historical percentage. The National Land Cover Database (NLCD) ExitLand Cover datasets for 2011, 2006, 2001 and 1992 are available for determining the area of woody vegetation cover within study watersheds and changes over time; alternative land cover datasets are also often available from state-specific sources. One or more forms of this indicator has been measured at the HUC12 scale for all lower 48 states and data are available from Watershed Index Online (WSIO).

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% Other Natural Cover Types 

  • Description: Indicators of the extent, distribution, and preservation of other natural land cover types within the watershed not mentioned in the above metric descriptions. Other natural cover types can include shrubland/scrubland, grasslands, barren land, or areas of year-round snow/ice. Indicators can characterize the present-day extent of the natural cover type throughout the entire watershed area, within the riparian zone or hydrologically connected zone, or in patches that are contiguous to surface waters. Indicators can also measure changes in the extent of natural cover types over time.
  • Example metrics: % Shrub/Scrub in Watershed; % Grassland in Riparian Zone, % Open Water in Hydrologically Connected Zone; % Barren Land in Watershed; % Perennial Ice/Snow in Watershed
  • Why relevant: Large-scale land use change often drives nonpoint pollution (e.g. from urban areas, agriculture, transportation, mining) and alters runoff and infiltration patterns in ways that can destabilize stream channels and flow regimes. The percent of watershed area that is maintained as natural cover types is generally associated with runoff and flow dynamics within the normal range of variability, as well as reduced potential for pollutant runoff.
  • Data sources and measurement: Measured as the percentage of total watershed area that is classified as the natural cover type of interest within the watershed, riparian zone, hydrologically connected zone, or contiguous to surface water. Indicators of natural cover change over time are calculated as the present-day percentage minus the historical percentage. The National Land Cover Database (NLCD) Exitfor 2011, 2006, 2001 and 1992 are available for determining the area of natural cover types within study watersheds and changes over time; alternative land cover datasets are also often available from state-specific sources. One or more forms of this indicator has been measured at the HUC12 scale for all lower 48 states and data are available from Watershed Index Online (WSIO).

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Topographic Complexity 

  • Description: Indicators of variation in slope gradient or elevation within the watershed. See Reference Sheet.
  • Example metrics: Slope, Standard Deviation in Watershed; Elevation, Range in Watershed; Channel Relief Ratio
  • Why relevant: Although likely not a strong causal influence on ecological condition, topographic complexity is associated with higher biodiversity, better water quality and reduced nutrient pollution in some studies. The metric may be indirectly related to limiting the extent of some forms of land use that may degrade aquatic condition, also associating it with greater recovery potential in general.
  • Data sources and measurement: The USGS National Elevation Dataset (NED) Exitis available for calculating watershed elevation and slopes indicators. Higher resolution elevation data may be available for state-specific projects. The NHDPlus Value-Added Attributes dataset Exitcontains information on maximum and minimum elevation for each flowline in the dataset and can be used to calculate channel relief ratio as maximum elevation minus minimum elevation divided by length. One or more forms of this indicator has been measured at the HUC12 scale for all lower 48 states and data are available from Watershed Index Online (WSIO).

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Forest Patch Mean Area 

  • Description: The mean area of contiguous forested patches within the watershed.
  • Example metric: Mean Area of Forest Patches in Watershed
  • Why relevant: Patch size is a direct indicator of how fragmented a watershed's natural cover is. Larger average patch size is likely to be associated with less fragmentation in a watershed. Forest fragmentation and its causes are associated with runoff changes and potentially greater pollutant loading. Larger forest patches surrounding stream segments are also more likely to harbor functionally intact waters.
  • Data sources and measurement: Calculated as the total forested land area in the watershed divided by forest patch count. For land cover data, the National Land Cover Database (NLCD) ExitLand Cover dataset for 2011, 2006, 2001 and 1992 is accessible; alternative land cover mapping datasets may also be available from state-specific sources.

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% Canopy Cover 

  • Description: Indicators of the extent of tree canopy cover in the watershed. Indicators can characterize the extent of canopy cover throughout the entire watershed, within the riparian zone, or within the hydrologically connected zone.
  • Example metrics: % Canopy Cover, Mean Value in Watershed; % Canopy Cover, Mean Value in Riparian Zone; % Canopy Cover, Mean Value in Hydrologically Connected Zone
  • Why relevant: This metric is relevant for reasons similar to watershed forest and watershed woody vegetation cover, and may be a more appropriate indicator choice depending on data availability and accuracy.
  • Data sources and measurement: Measured as the percentage of total watershed area with tree canopy cover within the watershed, riparian zone, or hydrologically connected zone. The National Land Cover Database (NLCD) Exittree canopy data for 2011 and 2001 are available for determining the area of canopy cover within study watersheds; alternative tree canopy cover datasets may also often available from state-specific sources. One or more forms of this indicator has been measured at the HUC12 scale for all lower 48 states and data are available from Watershed Index Online (WSIO).

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Soil Resilience 

  • Description: Soil characteristics within the watershed that are associated with resilience to erosion, nutrient washoff, or other nonpoint source pollution. See Reference Sheet.
  • Example metric: Soil Stability, Mean in Watershed; Soil Stability, Mean in Hydrologically Connected Zone; Soil Stability, Mean in Riparian Zone
  • Why relevant: Soil texture, organic matter content, permeability, and other characteristics affect the degree of nitrogen and phosphorus retention, bank stability, overland flow, and erosion potential. Sediment and nutrient levels in water bodies can therefore be associated with soil properties. Watersheds with resilient soils may show more positive responses to restoration relative to soils that are susceptible to erosion and nutrient leaching. Corridor soils have potentially greater influence than erosive watershed soils in general due to proximity.
  • Data sources and measurement: Mapped and measured from soil survey data throughout the entire watershed, within the riparian zone, or within the hydrologically connected zone. Indicators can measure the extent of specific soil types documented as better for nitrogen processing, stability/erosion resistance and other factors as appropriate to the study area. Numeric soil attributes (e.g., erodibility factor) can also be used to calculate average values per watershed. Soil survey data are available for most areas from the Natural Resources Conservation Service (NRCS) Soil Data Access Exitwebsite. The availability and coverage of digital soil survey data varies by state. States with fully digitized county soil survey-level information can use this metric most effectively. One or more forms of this indicator has been measured at the HUC12 scale for all lower 48 states and data are available from Watershed Index Online (WSIO).

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% Streamlength Unimpaired 

  • Description: The length of streams that are not listed as impaired under Section 303(d) of the Clean Water Act.
  • Example metric: Streamlength Unimpaired in Watershed; % Streamlength Unimpaired in Watershed
  • Why relevant: From the standpoint of watershed-scale condition and functionality, the proportion of waterbodies reported as impaired is likely associated with the difficulty and complexity of a watershed-wide restoration. Functionally healthy stream reaches upstream of impairments may aid the recovery of impaired segments via dilution and through recruitment of aquatic biota.
  • Data sources and measurement: Measured as the proportion of total stream length per watershed that is unimpaired. Caution should be taken that streams not reported as impaired aren't simply waters that have not been assessed for impairment status. A national geospatial impaired waters dataset is available through EPA's Assessment TMDL Tracking and Implementation System (ATTAINS). This source contains information on 303(d)-listed waters by state and by semi-annual reporting cycle. One or more forms of this indicator has been measured at the HUC12 scale for all lower 48 states and data are available from Watershed Index Online (WSIO).

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Watershed Shape 

  • Description: Indicators that describe the geometric shape of the watershed. See Reference Sheet.
  • Example metric: Watershed Shape Factor; Watershed Elongation Ratio
  • Why relevant: A more circular watershed shape has been associated with degraded water quality primarily due to greater risk of a more frequently destabilized channel. Runoff from rounder watersheds tends to concentrate and reach the mouth more quickly and with greater erosive power and velocity. Further, the shortened channel length associated with rounder watersheds enables less travel time to naturally process excess nutrients. Elongate watersheds tend to lessen the effects described above, which would lower the risk of repeated destabilization during recovery efforts.
  • Data sources and measurement: Measured from a geospatial dataset of watershed boundaries. An example watershed shape factor can be calculated by locating the watershed centroid; measuring the length of axis (A) through the centroid most nearly parallel to the main channel; measuring the length of three additional axes (B, C, D) in 45 degree increments; then calculating A divided by the mean of the four axes. Nearly round watersheds approach a value of 1, elongate watersheds have higher values.

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Watershed Size 

  • Description: The total area of the watershed, including all land and water encompassed by the watershed boundary. See Reference Sheet.
  • Example metric: Area of HUC12 Watershed
  • Why relevant: Related more to the rate of recovery than absolute capacity to recover. As a general principle, smaller ecological systems are known to recover faster than larger ones if all else is equal. Also, size is correlated with many additional, directly and indirectly contributing recovery factors: for example, increasing complexity of larger systems delaying full recovery, larger systems' restoration often being more complex and expensive, larger watersheds usually having more complex ownership and multiple jurisdictions, larger lakes' far longer residence time, and larger river systems affected by more upstream factors that are less easy to isolate and address as part of a smaller system's restoration can often do.
  • Data sources and measurement: Measured from a geospatial dataset of watershed boundaries in polygon or gridded (raster) format. One or more forms of this indicator has been measured at the HUC12 scale for all lower 48 states and data are available from Watershed Index Online (WSIO).

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Bank Stability/Soils 

  • Description: Indicators of stream bank and lake shore soil characteristics that affect bank and shore stability. See Reference Sheet.
  • Example metric: % Stream Bank with Highly Erodible Soils; % Lake Shore with Highly Erodible Soils
  • Why relevant: Specifically at the banks of rivers and shores of lakes, soils that are erosion prone are unstable and have a greater likelihood of excess sediment load. Continual erosion and excess sediment are often linked to instream habitat degradation and diminished spawning success of lithophilic spawners, and may also add to other impairments involving nutrients or water temperature.
  • Data sources and measurement: This metric reflects bank and shore, not corridor, soil characteristics. Soil survey data may identify specific soil types rated as 'highly erosive'. This metric could be based on the percentage of total stream length and/or lake shore length passing through highly erosive soil types. A small buffer (e.g., 1 meter) can be applied to streams and lakes to represent the land/water interface, then the measurement can be based on the percentage of buffer area that contains highly erosive soil types. Physical properties of soils are available for most areas as part of the Natural Resources Conservation Service (NRCS) Soil Data Access Exitwebsite. One or more forms of this indicator has been measured at the HUC12 scale for all lower 48 states and data are available from Watershed Index Online (WSIO).

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Bank Stability/Woody Vegetation 

  • Description: Indicators of stream bank and lake shore vegetative characteristics that affect bank and shore stability. See Reference Sheet.
  • Example metric: % Stream Bank with Woody Vegetation; % Lake Shore with Woody Vegetation
  • Why relevant: Stream banks and lake shores without woody vegetative cover may be particularly prone to erosional damage during extreme high flow/flood events and slower to recover in the aftermath. The prevalence of streambank stabilization projects involving woody plantings in restoration practice reflects the widespread opinion that the relative proportion of stable banks and woody vegetation needs to be high for the system to recover.
  • Data sources and measurement: Land cover datasets coarsely identify woody vegetation (e.g. forest, shrub, forested wetland, shrub wetland) and can be used to determine the percentage of stream bank or lake shore length with woody cover. Making this a linear metric (i.e. length of woody cover actually in contact with stream/river banks, as mapped) discerns this metric from the "% riparian woody vegetation" metric, which considers a larger corridor and relates to additional recovery factors. A small buffer (e.g., 1 meter) can be applied to streams and lakes to represent the land/water interface, then the measurement can be based on the percentage of buffer area that contains woody vegetation. Land cover datasets are available through the National Land Cover DatabaseExit. Land cover for coastal areas is available through the National Oceanic and Atmospheric Administration (NOAA) Digital Coast Exitwebsite. Orthophoto maps or remote imagery can be a good source for detailed local information.

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Corridor Slope 

  • Description: The slope gradient in the stream corridor of the watershed. See Reference Sheet.
  • Example metric: Mean Slope in Riparian Zone; % Riparian Zone with >3% Slopes; % Riparian Zone with >10% Slopes
  • Why relevant: Waters near low-gradient land surfaces tend to discourage geomorphic characteristics (gullying, destabilized floodplain features, etc.) that perpetuate some impairments or make restoration more difficult, complex or expensive. Low-slope areas may also have superior water retention and favor more stabilizing vegetative growth. Note that corridor slope and channel slope are different metrics that do not have identical implications for recovery potential.
  • Data sources and measurement: Digital elevation model (DEM) data from the National Elevation Dataset (NED) Exitor other sources can be mapped into slope classes, which can be merged with a selected corridor width to yield a percentage of corridor area in selected slope classes or a mean percent slope for the corridor overall. For finer resolution, use local DEM data.

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Natural Channel Form 

  • Description: Indicators of the presence and extent of stream channels in the watershed that have maintained natural geomorphology and not undergone anthropogenic straightening, widening, deepening, leveeing, or other alterations. See Reference Sheet.
  • Example metric: % Unaltered Streamlength in Watershed
  • Why relevant: Natural channel form helps to maintain flow regimes and sediment transport dynamics within a natural range of variability and supports the establishment of native and healthy biotic communities. Although a wide variety of natural channel forms exist and some may be unstable or impaired for other reasons, the absence of any natural channel form (i.e. channelization) provides no generally preferred habitat as a starting point for biotic or natural fluvial process recovery. See also Channelization under stressor indicators.
  • Data sources and measurement: Because channelization tends to occur in straight-line segments that join at right angles, detection is best done manually by visual identification on mapped or remote data with high resolution. Once detected, the linear percentage of total reach length with natural channel form can be measured with common GIS software. Some monitoring programs note channel form among other field-gathered data and this is occasionally adaptable to a metric. Data sources include the high resolution National Hydrography DatasetExit, state/locally compiled channelization metrics from previous studies, or other digital source.

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Corridor Groundwater Level 

  • Description: The depth from the land surface to the groundwater table in the stream corridor of the watershed. See Reference Sheet.
  • Example metric: Mean Groundwater Depth in Riparian Zone
  • Why relevant: Potentially related in multiple ways to waterbody recovery. Shallower groundwater depth is likely to be related to the retention of alternating influent and effluent reaches along stream corridors, implying greater likelihood that groundwater/surface water interactions and exchanges are functional rather than isolated and disconnected. Also related to the likelihood of successful reestablishment of riparian vegetation and subsequent bank stabilization.
  • Data sources and measurement: State-specific data sources on groundwater depths may be available; could be based on the depth of the water table as an average value in a specific buffer size around water bodies.

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Channel Slope 

  • Description: The slope gradient of stream channels in the watershed.
  • Example metric: Mean Channel Slope in Watershed
  • Why relevant: Stream restoration practices often involve restoring natural channel slopes, implying that specific channel gradients are more dynamically stable than others, and thus less prone to channel restoration failure. Often, moderately sloped (e.g. 2 to 3%) channels are more stable than either lower or higher gradient channels. Note that corridor slope and channel slope are different metrics that do not have identical implications for recovery potential.
  • Data sources and measurement: Measured as the change in elevation over channel length for a specified segment or interval; can be averaged for longer segments or watersheds. The NHDPlus Version 2Exit includes channel slope estimates for each flowline in the dataset. For finer resolution, use local Digital Elevation Model (DEM) data or LIDAR if available.

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Sinuosity 

  • Description: The degree of stream channel sinuosity in the watershed. Sinuosity is a measure of the oscillation or curvature of a channel across the landscape.
  • Example metric: Mean Channel Sinuosity in Watershed
  • Why relevant: Highly sinuous channels generally are more prone to longer-term sediment problems if they are impaired by excessive sediment loads. On the other hand, sinuous channels appear to allow for more nutrient processing. Relevance to recovery can vary with the impairment type and whether the sinuosity is human-altered.
  • Data sources and measurement: Measured as channel segment length divided by the straight line length between the segment start and end. A high resolution hydrography dataset such as the high resolution National Hydrography Dataset Exitis preferable for measuring sinuosity.

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Confinement Ratio 

  • Description: The confinement ratio measures the width of a stream valley/floodplain relative to stream channel width.
  • Example metric: Mean Confinement Ratio in Watershed
  • Why relevant: Highly confined channels tend to be more sensitive and prone to high-energy bank erosion and channel destabilization. This sensitivity can make the banks of confined channels with sediment impairments more difficult to restore through establishment and management of vegetated buffers. Relevance to other impairment types is unknown.
  • Data sources and measurement: To calculate the confinement ratio, divide the valley floor width by the stream channel width. Very confined segments may have values around 1 to 2; very broad unconfined valley types with abandoned terraces may have a ratio of 10 or more. Measurement can require field work or be performed using aerial photography.

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Channel Evolution Status 

  • Description: Stream channels typically evolve in form over time after a significant disturbance. These indicators describe the stage of stream channel evolution in the watershed. Stages can include stable, recently disturbed, aggregating, or degrading.
  • Example metric: Mean Channel Evolution Stage in Watershed
  • Why relevant: The tendency of rivers is to seek their own flow-related and sediment-related stability. Following disturbance, streams will try to reestablish the dimension, pattern and profile of a pre-disturbance morphology. Channels in a highly destabilized state undergoing channel evolution may be more difficult to restore temporarily until approaching a more stable condition.
  • Data sources and measurement: The existing stream form must be compared to the potential stable form it is likely to take on in time. Widespread spatial data on this factor are not likely to be found, but helpful guidance on evaluating successional status is available in the literature.

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Fine Sediment Transport Capacity 

  • Description: Indicators describing the capacity of streams in the watershed to transport fine sediment to downstream waters. See Reference Sheet.
  • Example metric: Fine Sediment Transport Capacity in Watershed
  • Why relevant: Moderate- to high-gradient streams and rivers are normally coarse-bedded and have aquatic communities adapted to coarse sediments. Fine sediment inputs commonly impair those communities. A system's capacity to move fine sediment downstream and reestablish dynamic equilibrium affects how quickly it can recover from excess fine sediment loading.
  • Data sources and measurement: The capacity of a stream to transport fine sediment is a function of stream discharge volume and velocity. Various equations have been developed to estimate sediment transport capacity based on discharge, channel dimensions, slope, and bed characteristics. These data are typically limited and site-specific measurements are likely needed.

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Natural Flow Regime 

  • Description: Indicators that describe the maintenance of the natural flow regime in the watershed. A stream’s flow regime can be characterized by the magnitude, timing, duration, frequency, and rate of change of average flows, high flow events, or low flow events.
  • Example metric: Baseflow Index; Frequency of High Flow Events; Annual Streamflow Magnitude
  • Why relevant: Stream condition is largely dependent not only on 'enough' water but on naturally dynamic changes in flow. Streamflow is strongly correlated with water temperature, channel form and habitat, thus acting as a primary factor that determines aquatic communities instream and affects numerous human uses near streams. Five components of the flow regime include magnitude, frequency, duration, timing and rate of change; significant alteration of any of the five can affect stream ecosystem condition and strongly influence the potential for recovery from impairments of many kinds.
  • Data sources and measurement: Indicators should characterize flow regime characteristics that are ecologically relevant to the study watersheds. Daily streamflow records may be limited but are extremely valuable to recovery potential screening where available. Can be measured as present-day values of flow metrics in the watershed or as a departure from reference (pre-development) flow conditions.

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Median Flow Maintenance 

  • Description: Maintenance of reference annual or monthly median streamflow in the watershed.
  • Example metric: August Median Monthly Flow; Annual Median Flow
  • Why relevant: See also natural flow regime. Depending on geographic region, median flow during specific times of the year influences stream and biological community condition. Surplus flows or deficits during crucial times of year may influence salmonid egg development, spawning, growth rates and survival, among many other effects. Although stream flow naturally varies, degree of departure from median flow on a monthly basis has been associated with significant biological impacts to current condition, and higher departure from expected flow range would likely work against recovery.
  • Data sources and measurement: Typically measured as the median monthly flow for a selected month throughout a period of record. Departure from reference median monthly flow can be estimated with reference gauging stations in the area. A departure threshold (such as within 10% of reference values) can be used to score maintenance of natural conditions.

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Low Flow Maintenance 

  • Description: Maintenance of reference low flow magnitude, frequency, and duration in the watershed.
  • Example metrics: Annual 7-Day Minimum Flow Magnitude; Mean Duration of Low Flow Events; Number of Low Flow Events
  • Why relevant: See also natural flow regime. Seasonal low flow is a vulnerable time for stream communities due to greater risk of elevated water temperature, lower dissolved oxygen, predation and more concentrated pollutants. As such, metrics that address the maintenance of low flows above harmful levels are useful in addressing whether flow volume might sustain or repeatedly interfere with stream recovery.
  • Data sources and measurement: A number of different metrics can address low flow. Annual 7-day minimum flow is a commonly used statistic in water monitoring. Also, the number of times and duration that flow drops below a given low flow threshold make useful measures from which to examine how well flow is maintained under low-flow scenarios.

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Strahler Stream Order 

  • Description: Indicators of stream order at the watershed outlet, the range of stream order in the watershed, or the extent of one or more stream orders of interest in the watershed. Strahler stream order is a hierarchical system that categories streams by their size: where the lowest value stream order is the smallest headwater stream, the headwater stream is a tributary to the second order stream, the second order stream is a tributary to the third order stream, and so on. See Reference Sheet.
  • Example metrics: Stream Order at Watershed Outlet; Headwater Flag; % Draining to 1st, 2nd, or 3rd Order Streams in Watershed
  • Why relevant: Stream size is strongly related to many condition-relevant attributes but the recovery potential of different stream sizes varies with the attribute. The smallest headwater streams appear to be most sensitive to riparian stresses, suggesting lower recovery potential, yet their small size and high disturbance regime may imply greater resiliency and more rapid recovery than larger orders, as well as less complex and expensive restoration needs. Generally higher biodiversity associated with small to moderate orders (2nd to 4th order) may imply a more complex and resilient biotic community structure that may respond well to restoration efforts. Another recovery factor favoring a focus on the recovery of smaller orders is their favorable downstream influence on the condition of larger order streams.
  • Data sources and measurement: Strahler stream order is manually calculable from hydrographic maps or is reported in the NHDPlus Value-Added Attributes datasetExit. The NHDPlus is a medium resolution hydrography dataset (1:100,000 scale), which misses many finer order streams; thus orders may be lower than field-measured data, but may show relative rather than absolute differences in order adequately for general comparisons. If high resolution hydrography is available it is possible to derive stream order with geoprocessing tools in GIS software.

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Biotic Community Integrity 

  • Description: Indicators of the health of aquatic biological communities in the watershed. See Reference Sheet.
  • Example metrics: Fish Index of Biotic Integrity in Watershed; Macroinvertebrate Index of Biotic Integrity in Watershed
  • Why relevant: The very complex concept of natural processes integrity is difficult to impossible to represent well using generalized geographic or impaired waters assessment data in a screening process. Nevertheless, several primary natural processes are exceedingly important influences on the prospects of recovery. As a substitute for measuring them all, biotic integrity integrates all other processes reasonably well. This recovery potential metric orients toward Karr's five major factors determining the condition of the water resource: flow regime, chemistry, habitat structure, biotic factors and energy. Severe degradation in any of the five likely represents severely reduced recovery potential. Many other more narrowly defined recovery metrics relate to one or more of these factors; this concept as a metric presents an opportunity to capture any other severe limiting factors that may be known but are unaddressed by the other recovery metrics in use. The increasing use of biotic integrity indices in state biomonitoring provides an important source of at least the biotic component of natural structure and process.
  • Data sources and measurement: Index of Biological Integrity (IBI) data on fish or benthic invertebrates are available in some state monitoring program datasets (for example, the Benthic IBI for the Puget Sound Lowlands)Exit. Also, NatureServe Exitprovides ecological integrity assessments for wetland mitigation in some regions of the country.

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Rare Taxa Presence 

  • Description: Presence/absence of rare aquatic species in the watershed or the extent of rare aquatic habitat in the watershed. See Reference Sheet.
  • Example metrics: Count of At-Risk Aquatic Species; Count of At-Risk Aquatic Animal Species; Count of At-Risk Wetland Species; Count of At-Risk Terrestrial Plant Species; % Rare Ecosystem in Watershed
  • Why relevant: Rare taxa have repeatedly been associated with more diverse and functionally intact ecosystems, including aquatic ecosystems. Rare taxa are also often more sensitive to stressors, and their presence in an impaired water may imply that the impairment is not severe. Increased eligibility and options for protection or restoration, elevated public and scientific concern and motivation to act, and other social factors may also be associated with rare taxa. These reasons support a probable association of the presence of rare aquatic taxa with generally higher recovery potential.
  • Data sources and measurement: Species rarity has been organized and categorized for most major taxonomic groups as part of Natural Heritage Programs in most states and through NatureServe Exitconservation status assessment methodologies. National datasets can be found through the NatureServe Explorer or the USDA Plants DatabaseExit. In addition, the Critical Habitat ExitPortal has GIS data for threatened and endangered species. More detailed datasets can be found through Natural Heritage Programs available in most states. It is possible to score the presence/absence of rare taxa with corresponding values of 1 and 0. One or more forms of this indicator has been measured at the HUC12 scale for all lower 48 states and data are available from Watershed Index Online (WSIO).

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Trophic State 

  • Description: The trophic state of water bodies in the watershed. Trophic state is a measure of the total biomass in a water body, and thus the associated nutrients. A highly eutrophic waterbody is rich in biomass and nutrients whereas a highly oligotrophic waterbody has relatively low biomass and nutrients
  • Example metrics: Mean Lake Trophic State in Watershed
  • Why relevant: Trophic state is often associated with water body condition relative to nutrients, with highly eutrophic systems frequently considered nutrient-impaired. Beyond nutrients, trophic state also has implications for biological impairment, oxygen depletion, sediment and other impairment types, the recovery from which can be hindered.
  • Data sources and measurement: Sources of trophic state data usually do not exist unless compiled through state monitoring programs or special studies. Measurement can be categorical with weights assigned between eutrophic and oligotrophic extremes.

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NFHP Fish Habitat Condition Index 

  • Description: The National Fish Habitat Partnerships (NFHP)Exit Habitat Condition Index (HCI) score in the watershed. The NFHP HCI Score is based on a national assessment of fish habitat condition using 17 metrics related to fish habitat condition.
  • Example metrics: Mean Habitat Condition Index in Watershed; HCI in buffer zones; cumulative and local versions available
  • Why relevant: Better-scoring catchments support fish habitats that are more likely to represent fully functional systems, generally implying greater likelihood of restorability.
  • Data sources and measurement: Data are publicly available for browsing and download through the NFHP data systemExit. Scores have been calculated at the scale of NHDPlus catchments, HUC12s and HUC8s.

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Confluence Density 

  • Description: The density of stream channel confluences (intersections) in the watershed.
  • Example metrics: Stream Confluence Density in Watershed
  • Why relevant: For impairments affecting biological communities in streams, recruitment from tributaries, particularly those large enough to support similar species assemblages, is one factor influencing speed of recovery. Tributaries per linear mile of impaired stream represent possible recolonization sources. On a watershed basis, confluence density is a measurement of this property.
  • Data sources and measurement: Measured as the count of confluences per mile of watershed total stream length (optionally within 1 Strahler stream order).  Strahler stream order (if used) is available from NHDPlus Value-Added Attributes datasetExit. Confluence count can be manual or automated. Where available, dam locations can be used to further assess and verify accessibility. Note that the NHDPlus dataset is a medium resolution (1:100,000 scale), and likely missing many finer order streams.  When possible, high resolution data on Strahler stream order should be used.

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Recolonization Access 

  • Description: Along an impaired water body, the density of confluences (intersections) with water bodies that are not listed as impaired under Section 303(d) of the Clean Water Act. See Reference Sheet.
  • Example metrics: Unimpaired Confluences Density in Watershed
  • Why relevant: Loss or degradation of aquatic life, usually affecting a more sensitive subset of the resident fish or stream invertebrates, is an impairment whose recovery can be highly influenced by access and proximity to the nearest appropriate source for recolonization after conditions improve. Same or similar-sized streams within the same drainage are more likely to support similar aquatic life and act as biotic refugia and recruitment sources for recolonizing the impaired segment. Most relevant where aquatic life use support is impaired (many, perhaps most listed waters). Impaired waters with unimpaired tributary confluences or bracketed by unimpaired upstream and downstream segments may be good prospects, as are listed waters where species of concern are reduced in number but not totally lost. In contrast, impaired waters isolated from similar systems may have poor prospects for recruitment or even be dependent on manmade reintroductions to recover fully, even if physical conditions have become suitable.
  • Data sources and measurement: Measured as the count of confluences with unimpaired tributary channels per mile of impaired segment on a watershed basis (optionally within 1 Strahler stream order). Can also include connections with upstream and downstream segments of the same water body. Impaired segment shapefiles are available from EPA through the Assessment and TMDL Tracking and Information System (ATTAINS) and can be measured for length. Strahler stream order is available from the NHDPlus Value-Added Attributes dataset Exitfor both impaired segments and their tributaries. Where available, dam locations should be used to further assess and verify accessibility. Note that both the ATTAINS dataset and NHDPlus are medium resolution (1:100,000 scale) datasets, and likely missing many finer order streams.  When possible, high resolution data on Strahler stream order should be used.

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Stream Density 

  • Description: The density of streams in the watershed, measured as total stream length per watershed area.
  • Example metrics: Stream Density in Watershed
  • Why relevant: Stream density is a surrogate measurement related to opportunities for biotic recruitment from tributaries. For impairments affecting biological communities in streams, recruitment from tributaries, particularly those large enough to support similar species assemblages, is one factor influencing speed of recovery.
  • Data sources and measurement: Measured as the total stream length in the watershed divided by total watershed area. The NHDPlus Exitdataset contains stream features for the conterminous United States as well as many territories. Note that the NHDPlus is a medium resolution (1:100,000 scale) dataset and are missing many finer order streams.

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Contiguity with Green Infrastructure Corridor 

  • Description: Indicators that describe the degree of contiguity between surface waters and green infrastructure corridors within the watershed. See Reference Sheet.
  • Example metrics: % Streamlength Contiguous to Green Infrastructure
  • Why relevant: Extensive documentation supports the importance of connectivity among suitable habitats to promote more diverse and resilient ecological communities. This metric evaluates the direct connection between stream/lake segments and green infrastructure corridors. Corridors increase effective habitat size and access, afford migration and movement to avoid temporary stressors and aid recruitment and recolonization into impaired areas. Impaired water segments that are near, or hydrologically connected to, functionally intact corridors identified by a green infrastructure (GI) mapping effort have greater recovery potential than isolated impaired waters for the reasons above. Generally, GI corridors have relatively unimpaired aquatic systems and relatively uninterrupted, naturally vegetated riparian corridors.
  • Data sources and measurement: This factor can be measured as corridor length or area within the watershed, but that does not directly address connectivity. Measured on a stream segment basis, one example system for scoring would be:
    • 0 = no surface hydrologic connection to green infrastructure corridor;

    • 1 = no proximity to green infrastructure corridor (e.g., connected hydrologically but greater than 2 kilometers from corridor terminus);

    • 2 = proximate to green infrastructure corridor (e.g., connected hydrologically and less than 2 kilometers from corridor terminus);

    • 3 = connected to green infrastructure corridor;

    • 4 = connected to and bridging two or more green infrastructure corridors.

The National Ecological Framework (NEF) is a GIS-based model of the connectivity of natural landscapes in the conterminous United States. Other state-specific GI datasets may also be available.

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Proximity to Green Infrastructure Hub 

  • Description: Indicators of the proximity of surface waters to green infrastructure hubs or the area of green infrastructure hubs in the watershed. See Reference Sheet.
  • Example metrics: % National Ecological Framework in Watershed; % National Ecological Framework Hubs in Watershed; Mean Proximity to Green Infrastructure Hubs
  • Why relevant: Based on extensive documentation of island biogeographic principles and the importance of habitat size/extent supporting more diverse and resilient ecological communities. Green hubs and connected corridors increase effective habitat size and access, afford migration and movement to avoid temporary stressors, and aid recruitment and recolonization of impaired areas. Impaired water segments that are near, or hydrologically connected to, functionally intact hubs and important corridors identified by a green infrastructure (GI) mapping effort have greater recovery potential than isolated impaired waters for the reasons above. Generally GI hubs contain major or multiple unimpaired aquatic systems and constitute larger, relatively uninterrupted, naturally vegetated communities with connections to multiple, naturally vegetated riparian corridors.
  • Data sources and measurement: This factor can be measured as the percent area of GI hubs in the watershed or measured on a stream segment basis as the mean proximity to GI hubs. The National Ecological Framework (NEF) is a GIS-based model of the connectivity of natural landscapes in the conterminous United States. Other state-specific GI datasets may also be available.  One or more forms of this indicator has been measured at the HUC12 scale for all lower 48 states and data are available from Watershed Index Online (WSIO).

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Historic Species Occurrence 

  • Description: Indicators of the historical occurrence of one or more aquatic species of interest in the watershed. See Reference Sheet.
  • Example metrics: Native Trout Historical Occurrence in Watershed
  • Why relevant: Although single-species oriented, this metric is appropriate where a restoration target or even a water quality criterion directly addresses a species of concern (e.g., naturally reproducing salmon or trout populations) or indirectly alludes to an aquatic condition exemplified by a keystone species (e.g., Eastern Brook Trout exemplifying a coldwater biotic community target). Verified historical occurrence does not necessarily ensure recovery potential due to the many additional factors that may interfere, but should provide a starting point for comparative evaluations of numerous potential restorations involving the species as a target. Verified historical absence is valuable for avoiding inappropriate restoration investments due to low recovery potential.
  • Data sources and measurement: Limited to individual species of concern that have been researched sufficiently to establish historical presence/absence data. Assuming the probability that historical data are incomplete or imperfect, this metric at a minimum allows for the following three ranking categories (lowest to highest):
    • 0 = historically not found;

    • 1 = unknown historical occurrence;

    • 2 = known historical occurrence.

Distributional information on historical presence/absence of the species of interest may not exist for many species. Threatened and Endangered Species Habitat can be found through the Critical Habitat PortalExit.  Historical information may be available through State Fish and Wildlife Service, as is the case in OregonExit. Biodiversity organizations and state natural heritage programs may have data on other major aquatic taxa of interest

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Species Range 

  • Description: Indicators describing whether the range of one or more aquatic species of interest extends into the watershed. See Reference Sheet.
  • Example metrics: Eastern Brook Trout Range Presence;  Mottled Sculpin Range Presence
  • Why relevant: Although single-species oriented, this metric is appropriate where a restoration target or even a water quality criterion directly addresses a species of concern (e.g., naturally reproducing salmon or trout populations) or indirectly alludes to an aquatic condition exemplified by a keystone species (e.g., Eastern Brook Trout exemplifying a coldwater biotic community target). The rationale regarding recovery is that the extremes of a species’ range generally have greater stressors and higher risks to restoration efforts than non-marginal range locations. Marginality concepts may be numerous (e.g., northern or southern extremes; elevation; waterbody traits such as size, channel gradient, substrate; precipitation regime) and need to be selected appropriately for the species of interest. Climate, land use, and other changes to watershed conditions may act to make marginal areas additionally unsuitable and difficult to restore.
  • Data sources and measurement: Dependent upon species range maps, which are often very generalized. Modification of range maps may be necessary by consulting with local experts on the species of concern. Threatened and Endangered Species Habitat can be found through the Critical Habitat PortalExit.  Historical information may be available through State Fish and Wildlife Service, as is the case in OregonExit. Biodiversity organizations and state natural heritage programs may have data on other major aquatic taxa of interest.

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Integrated Watershed Health Index and Sub-Indices 

  • Description: Watershed Health Index and Sub-Index scores from the EPA Preliminary Healthy Watersheds Assessments (PHWA). The PHWA calculated a Watershed Health Index score and six Sub-Index scores (Landscape; Habitat; Hydrology; Geomorphology; Water Quality; Biological Condition) for each HUC12 watershed in the contiguous US.
  • Example metrics: PHWA Watershed Health Index; PHWA Landscape Sub-Index; PHWA Hydrology Sub-Index; PHWA Geomorphology Sub-Index; PHWA Habitat Sub-Index; PHWA Biological Sub-Index; PHWA Water Quality Sub-Index
  • Why relevant: Under the PHWA assessment framework, watershed health is characterized by the presence of natural land cover that supports hydrologic and geomorphic processes within their natural range of variation, good water quality, and habitats of sufficient size and connectivity to support healthy, native aquatic and riparian biological communities. The PHWA aimed to develop screening-level information to evaluate relative watershed condition (i.e., comparisons of multiple watersheds across states and ecoregions) to help resource managers plan and target future watershed protection efforts. The PHWA framework is not designed to make a statement on the absolute condition of any watershed or water body, the assessment does not define a “healthy watershed” threshold and is instead for comparative purposes only.
  • Data sources and measurement: PHWA Watershed Health Index and Sub-Index scores for each   HUC12 in the contiguous US are available for download from the PHWA website. Scores were calculated by first identifying measurable indicators closely associated with each of six sub-index categories (Landscape; Habitat; Hydrology; Geomorphology; Water Quality; Biological Condition), compiling Sub-Index scores from indicator data, and then developing the integrated Watershed Health Index from the six sub-indices. Separate “statewide” and “ecoregional” index and sub-index scores are provided. Statewide scores reflect a HUC12’s condition relative to all other HUC12s in the state, while ecoregional scores reflect a HUC12’s condition relative to all other HUC12s in the ecoregion. All scores can range from 0 to 100, with higher scores representing better conditions relative to other watersheds in the state/ecoregion. One or more forms of this indicator has been measured at the HUC12 scale for all lower 48 states and data are available from Watershed Index Online (WSIO).

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Neutral Variable (Ecological) 

  • Description: The neutral variable indicator is equal to 0.5 for all watersheds in the RPS Tool. This is a “dummy” indicator used when you wish to omit the Ecological category from a screening.
  • Example metrics: Neutral Variable, Ecological Category
  • Why relevant: Users may want to run a RPS screening that does not include the Ecological indicator category. Selecting only the Neutral Variable for the Ecological category will allow the RPS Tool to run without errors and provide results that focus on just the Stressor and Social indicator categories.
  • Data sources and measurement: The neutral variable indicator is set to 0.5 for all watersheds.

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