EASI Walkthrough
A guided tour of running a screening assessment in EASI (Ecosystem Assessment Screening Index).
Where EASI works
EASI scores streams in the National Hydrography Dataset. The map draws one solid blue stream network. To inspect data coverage, enable StreamCat coverage in the Layers menu; this distinguishes StreamCat reaches from other streams and shows the data-source reach and downstream connector. The option starts off and only changes the display. The assessment point, reach, and watershed remain distinct.
After choosing a stream, wait for the StreamCat source-reach note above Delineate. The selected point stays visible during lookup, with a small progress circle beside the status text. EASI retries temporary routing failures up to three times, after pauses of 5, 10, and 15 seconds. The circle continues during retry pauses and stays stationary if reduced motion is preferred. If lookup remains unresolved, choose Retry StreamCat lookup to retry the same point, or select another stream. Delineation and screening remain disabled until the source reach is resolved; individual metric values are retrieved during screening.
Within 150 ft of an NHDPlus V2 reach, the StreamCat lookup engine supplies watershed metrics from EPA StreamCat in seconds. Elsewhere, the STAF site engine calculates the HR reach watershed, usually in well under a minute (up to about five minutes on a large basin), with progress shown while it runs. Some reach-keyed metrics, including low flow and biological integrity, can use the nearest StreamCat reach downstream. Affected desktop evidence is identified beside the metric and with a dagger explained below the report table. Detailed sources retain the reach, routed distance, and drainage-area ratio. The two engines are defined on the Computation Engines page.
Regional reference criteria
EASI uses Alternative 2, adopted after the September 2026 alternatives study, with 34 frozen reference curves and unchanged scoring weights. It rates twenty functions as Good, Fair or Poor. Woody corridor cover for Habitat provision and Light and thermal regime, natural corridor cover for Carbon processing, and EROM monthly flow variability for Low flow use EPA NARS-9 reference curves. When a region has no usable curve, EASI uses the national curve and labels that fallback in the scoring panel.
Each of those three curve families has nine regional references and one national fallback. The regions are Coastal Plains, Northern Appalachians, Northern Plains, Southern Appalachians, Southern Plains, Temperate Plains, Upper Midwest, Western Mountains and Xeric. The three slope-specific entrenchment curves and their national fallback bring the total to 34. Regions are not further split by slope or stream size. Regional nutrient thresholds remain fixed thresholds, separate from the fitted reference curves.
Alternative 1 and the completed study remain archived. Adopting Alternative 2 is the owner’s decision; it does not revise the study’s conservative recommendation to retain Alternative 1. No scoring-method selector is exposed.
The reference panels represent reaches passing the fixed least-disturbed desktop screen. Its road-density cap is 2.0 km/km², with complete and exploratory panel floors of 100 and 30. The shipped artifact records the historical dataset, screen and curve-engine provenance. These curves express regional expectations within a screening assessment.
Entrenchment ratio for Floodplain connectivity and Channel evolution uses national curves for slopes below 0.5%, 0.5% to below 2%, and 2% or greater. Missing or invalid slope uses the pooled national entrenchment curve. Bank-height ratio and impervious-cover components keep their fixed bands. The scoring panel identifies the resolved reference and its sample size.
| Rating | EASI index anchor | Function score |
|---|---|---|
| Good | 0.85 | 13 |
| Fair | 0.545 | 8 |
| Poor | 0.195 | 3 |
The reference curve’s interpolated index determines a Good / Fair / Poor band at 0.69 and 0.39. That band maps to the index anchor above, then to a rounded score out of 15. These anchors are the midpoints of the Good (0.70 to 1.00), Fair (0.40 to 0.69) and Poor (0.00 to 0.39) bands. The outcome weights, rollup and condition boundaries remain the same.
Interpreting the updated proxies
- Low flow: the population standard deviation of twelve monthly EROM QE estimates divided by their mean, rounded to six decimals. Lower variability rates better. Missing flow evidence or a nonpositive mean is unknown. This proxy remains unvalidated for field low-flow condition.
- Bed composition: watershed agriculture share, with the existing 30 / 50 bands. This is a disclosed correlated pair with Catchment hydrology because both use agriculture evidence. It does not measure bed material directly.
- Population support: the EPA StreamCat benthic probability
prg_bmmi0809, using theotherarea of interest. Good is at least 0.50, Fair is 0.25 to below 0.50, and Poor is below 0.25. Where the model has no value, the catchment/watershed integrity products remain a labeled fallback.
The 16-state development dataset that was used to evaluate and refine these criteria is archived as a GitHub prerelease with a provenance record; it is a development dataset, not an assessment product. The in-app Nationwide screening map that reads it is a developer preview and is not part of the public application (it mounts only when an operator sets EASI_NATIONAL_VIEWER=1).
Observed evidence. Two functions take an observation in place of the terrain estimate. On the Channel evolution card you can record a documented channel class (stable or recovered, moderately adjusting, severely adjusting) with the indicators you saw, and on the Channel and floodplain dynamics card the eroding and armored percentages of the bank. An observation applies only when complete, and it then outranks both the automatic rating and a rating set by hand. The Excel calculator has the same entries.
Excel calculator. The EASI calculator applies the same methodology in a spreadsheet: enter the desktop quantities, the NARS-9 region and the channel slope, and the workbook rates the 20 metrics with the same criteria and reference curves, then applies the STAF rollup. The last row of each function is an optional Override Score (Good, Fair or Poor) that replaces the computed rating, as a rating changed on the Assessment page does. The workbook is generated from the app’s own scoring definitions and checked against the app’s engine on a retained case set, so the same inputs give the same results.
Get Forms. On the Assessment page, Get Forms lists the 20 desktop metrics with the values each was rated from. From there you can download that list as a PDF, the completed workbook (the calculator with the site’s values, your ratings, your observations, your notes and the twelve monthly flows entered), or the blank workbook. The report and the batch dialog offer the completed workbook too, and the latest blank calculator is on the Apply STAF page.
Operators can select the former criteria with EASI_CRITERIA_SET=legacy. Both sets use the current anchors above. The regional set is the default. The app’s extended verification and validation report is a historical record with earlier criteria and the same 13/8/3 scores. Its cached field comparisons have not been rerun to validate these regional changes.
The displayed physical-value crossings are rounded approximations. Ratings use interpolation on the stored curve at the exact 0.39 / 0.69 index edges. The static curve examples below use the national fallback so this page has a single reproducible example for each quantity. An assessment’s scoring panel shows the resolved criteria for its own Level II region or slope class. Nutrient cycling retains its NARS nine-region benchmark table.
Screening metric reference
The Screening tier rates twenty stream-function metrics automatically from national desktop data, as described on the Scoring and Condition page. The screening app shows the inputs and the Good / Fair / Poor criteria for each metric while you work. The reference below is the fuller detail: what each metric measures, where its scoring boundaries come from, why each input is used, and the limits to keep in mind when reading a result.
Hydrology
Watershed Land-Cover Pressure
Stream function Runoff and infiltration sustain natural flow regime, carry appropriate sediment and nutrients from uplands, and reliably cue spawning/migration of aquatic life.
Watershed land-cover pressure on catchment hydrology. Scored automatically on the more limiting of two indicators: impervious cover (speeds runoff) or agricultural cover (where farming, not pavement, is the dominant hydrologic alteration).
| Boundary | What it marks |
|---|---|
| 10% impervious | Boundary between the sensitive and impacted classes of the Impervious Cover Model (Schueler 1994). |
| 25% impervious | Boundary of the non-supporting class of the Impervious Cover Model (Schueler 1994). |
| 30% agriculture | Lower agricultural transition. Fish-index decline and subsidy-stress responses are reported above about 30 percent watershed agriculture (Allan 2004). |
| 50% agriculture | Upper agricultural screen. Habitat and biotic integrity decline become apparent above 50 percent watershed agriculture (Wang et al. 1997), aligned with NRSA reference-site screening practice. |
- Watershed impervious cover: Impervious cover represents altered infiltration, runoff routing, and hydrologic flashiness.
- Watershed agricultural cover: Agricultural cover represents a separate soil-disturbance, drainage, and runoff pathway that impervious cover does not capture.
- The worse input governs because hydrologic alteration through one pathway is not offset by low pressure in the other.
- Agricultural bands are national screening tiers, not ecological criteria.
- A result computed from one input is labeled partial.
- Sensitive macroinvertebrate taxa decline well below the 10 percent impervious Good boundary (community thresholds near 0.5 to 2 percent), so a Good rating is not a no-effect finding (King et al. 2011).
- National Rivers and Streams Assessment 2018-19 Technical Support Document
- Schueler 1994, The importance of imperviousness
- Allan 2004, Landscapes and riverscapes: the influence of land use on stream ecosystems
- Wang et al. 1997, Influences of watershed land use on habitat quality and biotic integrity in Wisconsin streams
- King et al. 2011, Stream community thresholds at exceptionally low levels of catchment urbanization
Percent Wetlands in Watershed
Stream function Wetlands and storage features store floodwater, recharge groundwater, sustain baseflow, and provide low-velocity habitat.
Share of the watershed that is wetland, which stores water, buffers peak flows, and sustains baseflow.
| Boundary | What it marks |
|---|---|
| 1% | Lower national screening transition. Watershed wetland extents of 1 to 5 percent are associated with water-quality benefit and about 7 percent with flood attenuation (Mitsch and Gosselink, as reviewed by the Chesapeake Bay Program 2016). |
| 5% | Upper national screening transition, the top of the same reviewed 1 to 5 percent benefit range. |
- Woody wetland cover: Woody wetlands contribute surface storage and hydrologic attenuation.
- Herbaceous wetland cover: Herbaceous wetlands contribute storage not represented by woody-wetland cover.
- Natural wetland abundance varies substantially among regions.
Concentrated Runoff / Stormwater Inputs
Stream function Quantity + quality of inflow (tributaries, ditches, and pipes) does not provide harmful peaks or pollution, and supports diverse habitat conditions.
Road density in the watershed, a stand-in for concentrated runoff pressure. It does not count actual outfalls or crossings.
| Boundary | What it marks |
|---|---|
| 1 km/km² | Lower STAF screening transition. It is not a demonstrated ecological threshold. |
| 3 km/km² | Upper transition grounded in NRSA least-disturbed-site road-density screening. |
- Watershed road density: Road density is a directional proxy for concentrated runoff sources and hillslope-to-channel connectivity.
- Road density does not count actual outfalls or establish whether runoff is treated.
- The 1 km/km² breakpoint is a screening judgment.
- USFS watershed-condition guidance classes road density more strictly (good under 1, impaired over 2.4 miles per square mile, about 0.62 and 1.49 km/km²), derived from salmonid watersheds. EASI keeps the NRSA-anchored national screen and discloses the stricter alternative.
Flow Alteration (Regulation / Water Use)
Stream function Integrates the range of "typical" flows experienced by other processes. Examines the degree to which upstream infrastructure or land uses have fundamentally altered flow regimes (e.g., hydropeaking, dams, withdrawals).
Degree of regulation: upstream reservoir storage relative to the river's annual runoff volume. It does not account for how dams are operated or for diversions.
| Boundary | What it marks |
|---|---|
| 2% DOR | Onset-of-regulation boundary. Comparative global assessment treats storage above 2 percent of annual flow as regulated (Lehner et al. 2011). |
| 15% DOR | Severe-regulation boundary. The free-flowing rivers analysis uses 15 percent as its operational pressure limit, the high end of a published 2 to 15 percent range (Grill et al. 2019). It is not a regulatory standard. |
- Upstream normalized storage: Upstream storage is the regulated volume available to alter the natural flow regime.
- Annual runoff: Annual runoff converts storage to a dimensionless degree of regulation relative to the river's annual water volume.
- Degree of regulation does not describe operating rules, diversions, seasonality, or dam location on the network.
- Missing or non-positive runoff is not converted to zero and leaves the metric unscored.
Hydraulics
Low-flow Wetted Connectivity
Stream function Ensures habitat availability and water quality during low water levels, and indicates non-storm conditions experienced the majority of the time.
Unvalidated screening proxy for low-flow condition based on modeled monthly flow variability. It does not measure daily low flow or channel wetted connectivity.
Reference: national curve, 53,197 reference reaches; crossings are approximate
| Boundary | What it marks |
|---|---|
| Reference index 0.39 | Fair begins at an interpolated reference index of 0.39. |
| Reference index 0.69 | Good begins at an interpolated reference index of 0.69. Physical crossings depend on the reference stratum. |
- Monthly flow variability: Population standard deviation of all twelve mean monthly flows divided by their mean. All months must be finite and the mean must be positive.
- EROM mean annual flow: context only Mean annual modeled flow provides scale context; it does not determine the rating.
- Expected NHD flow regime: context only Flow classification provides context for natural intermittency and does not determine the rating.
- EROM monthly flow variability: Use all twelve modeled monthly flows; missing or nonpositive-mean flow remains unscored.
- Unvalidated screening proxy for low-flow condition. Agreement with the field wetted-channel measure was weak (0.18).
- Monthly climatological variability is not a measurement of daily low flow, baseflow contribution, or wetted connectivity.
- NARS-9 curves compare variability with regional reference expectations; lower variability rates better. A national curve is the fallback.
- All twelve months are required. Missing months are unknown, not zero. Natural intermittent and ephemeral streams require interpretation.
Floodplain Engagement Frequency (bankfull recurrence)
Stream function Peak flows reshape channel without chronic instability, redistribute wood and sediment, reset habitats, and supply nutrients to floodplains.
How readily high flows reach the floodplain, from the bank-height ratio rather than a modeled recurrence interval.
| Boundary | What it marks |
|---|---|
| BHR 1.3 | Upper boundary of the functioning floodplain-connectivity class. |
| BHR 1.5 | Boundary above which published stream-quantification guidance identifies non-functioning connectivity. |
- Bank-height ratio: BHR directly measures the vertical separation between bankfull flow and the low floodplain surface.
- A BHR below 1.0 remains Good but requires geometry verification.
- The DEM-derived cross section is a screening estimate and may require surveyed geometry.
Floodplain Access / Entrenchment
Stream function Frequent overbank flows reduce flood peaks, support riparian vegetation, create off-channel refugia, and extend nutrient processing time.
Whether the channel has lateral access to a floodplain, via the entrenchment ratio (floodprone width ÷ bankfull width).
Reference: national curve, 28,287 reference reaches; crossings are approximate
| Boundary | What it marks |
|---|---|
| Reference index 0.39 | Fair begins at an interpolated reference index of 0.39. |
| Reference index 0.69 | Good begins at an interpolated reference index of 0.69. Physical crossings depend on the reference stratum. |
- Entrenchment ratio: ER directly represents lateral floodplain width available relative to the bankfull channel.
- National reference curves group channel slopes below 0.5 percent, from 0.5 to below 2 percent, and at least 2 percent.
- An unsplit national reference curve is used when slope is unknown. Naturally confined valleys still require field interpretation.
- Entrenchment ratio measures floodplain access; it does not establish channel stability or flood frequency.
Hyporheic Exchange Indicators
Stream function Surface water-groundwater exchange moderates temperature, transforms nutrients, supplies oxygen, and supports aquatic life.
Potential for surface water to exchange with the shallow subsurface (hyporheic zone), scored on the better of two pathways: channel slope (vertical bedform-driven exchange) or reach sinuosity (lateral meander-driven exchange).
| Boundary | What it marks |
|---|---|
| 0.003 m/m | Lower screening transition, carried over from the prior exchange-potential index. A screening judgment, not a published threshold. |
| 0.006 m/m | Upper screening transition, carried over from the prior exchange-potential index. A screening judgment, not a published threshold. |
| 1.05 | Boundary between straight and sinuous planform in common geomorphic classifications. A screening judgment, not a published threshold. |
| 1.2 | Boundary above which meandering is well expressed in generalized reach geometry and lateral exchange across bends is expected (Boano et al. 2006, Cardenas 2009). A screening judgment, not a published threshold. |
- Channel slope: Slope sets the vertical hydraulic gradient that drives bedform-driven bed exchange, the dominant hyporheic pathway in steeper channels.
- Reach sinuosity: Sinuosity indicates lateral exchange through meander necks and point bars, the dominant hyporheic pathway in low-gradient meandering channels. Because the better pathway governs, sinuosity understated by generalized reach geometry can only fail to credit lateral exchange, never lower the rating.
- Slope screens vertical bedform-driven exchange and sinuosity screens lateral meander-driven exchange. Bed hydraulic conductivity is a co-dominant control that is unavailable, so steep reaches over bedrock or fine beds can overpredict exchange.
- The better pathway governs because vertical and lateral exchange are alternative mechanisms. Either alone indicates exchange potential.
- Sinuosity measured over generalized reach geometry understates planform sinuosity. Under the better-pathway rule that bias can only fail to lift a rating, never lower one. The earlier weighted slope and sinuosity composite was retired for exactly that downward bias.
- A result computed from one pathway is labeled partial.
- The slope and sinuosity bands are screening judgments, not published thresholds.
- EPA StreamCat Metrics and Definitions
- EPA Function-Based Framework for Stream Assessment and Restoration Projects
- Harvey et al., How hydrologic connectivity regulates water quality in river corridors (NEXSS synthesis)
- Boano et al. 2006, Sinuosity-driven hyporheic exchange in meandering rivers
- Cardenas 2009, A model for lateral hyporheic flow based on valley slope and channel sinuosity
Geomorphology
Channel Evolution Stage & Trends
Stream function Long-term changes in channel size and slope maintain dynamic equilibrium and diverse habitats while interacting with the floodplain.
Whether the channel is stable or actively incising/widening, its stage in the channel-evolution sequence.
Reference: national curve, 28,287 reference reaches; crossings are approximate
| Boundary | What it marks |
|---|---|
| BHR 1.3 | Upper boundary for functioning floodplain connection. |
| BHR 1.5 | Boundary above which incision/disconnection is Poor. |
| Reference index 0.39 | Fair begins at an interpolated reference index of 0.39. |
| Reference index 0.69 | Good begins at an interpolated reference index of 0.69. Physical crossings depend on the reference stratum. |
- Bank-height ratio: BHR is a documented incision and floodplain-disconnection clue used in channel-evolution interpretation.
- Entrenchment ratio: ER indicates lateral confinement and access to flood-prone width, another documented clue to adjustment state.
- NHD feature classification: context only A canal/ditch classification is decisive. Other FCODEs are neutral and cannot imply stable condition.
- Documented channel-stage assessment: A user-documented stable/recovered, moderately adjusting, or severely adjusting condition supersedes the proxy.
- Canal/ditch classification: A canal/ditch FCODE is directly classified Poor.
- BHR and ER susceptibility proxy: For other reaches, the worse BHR or ER index governs.
- This is a susceptibility proxy, not a formal channel-evolution stage assessment. Both BHR and ER are required.
- BHR retains its 1.3/1.5 bands; ER uses national reference curves by slope class, with an unsplit national fallback when slope is unknown.
- Canal/ditch classification and documented field-stage overrides remain decisive. Naturally confined valleys require field interpretation.
Bank Erosion & Armoring Condition
Stream function Examines the role of bank processes and erosion in channel change. Observes patterns in sinuosity and curvature affecting habitat complexity.
Susceptibility to bank erosion, from the bank-height ratio unless bank observations are entered. The proxy does not detect existing armoring.
| Boundary | What it marks |
|---|---|
| BHR 1.3 | Upper published functioning boundary, used here as the Good/Fair susceptibility transition. |
| BHR 1.5 | Published non-functioning floodplain-connectivity boundary, used here as the Fair/Poor susceptibility transition. |
- Bank-height ratio: BHR is an established incision and floodplain-connectivity measure used here only as a susceptibility proxy, not as measured erosion or armoring.
- Observed erosion and armoring: Complete user observations of both components supersede the proxy.
- BHR susceptibility fallback: Used automatically when complete bank observations are unavailable.
- BHR does not detect existing armoring and does not directly measure erosion rate or eroding-bank extent.
- The DEM-derived cross section is a susceptibility screen and may require surveyed geometry.
- BHR is reused by floodplain engagement and channel adjustment and is therefore correlated evidence.
Sediment Supply Potential (watershed/banks)
Stream function Balanced sediment supply and transport preserves bed elevations, substrate sizes, spawning/benthic habitats, and supports riparian succession.
Potential for excess sediment from watershed sources, scored on the most limiting of agricultural cover, soil erodibility, and road density. Bank-derived supply is not represented.
| Boundary | What it marks |
|---|---|
| 30% agriculture | Lower agricultural transition (Allan 2004), shared with the land-cover pressure metric. |
| 50% agriculture | Upper agricultural transition (Wang et al. 1997). |
| K 0.25 | Lower boundary of the moderate RUSLE erodibility class (silt loams). |
| K 0.40 | Upper boundary of the moderate RUSLE erodibility class. Values above 0.40 mark the most erodible high-silt soils. |
| 1.24 km/km² | 2 miles per square mile, the properly-functioning limit in NMFS salmonid watershed criteria (as compiled in KRIS). |
| 1.86 km/km² | 3 miles per square mile, the not-properly-functioning boundary in the same criteria. Fine sediment in spawning gravels increases above about this density (Cedarholm et al. 1981). |
- Agricultural cover: Agriculture represents the dominant disturbed upland sediment-source pathway.
- Soil erodibility K-factor: K-factor represents inherent soil erodibility, the susceptibility term no other metric screens.
- Road density: Roads are both a sediment source and a hillslope-to-channel connectivity pathway.
- The worse input governs. Source indicators are screened separately and are not summed.
- Intrinsic soil erodibility (K-factor) can lower the rating in undisturbed watersheds. It indicates susceptibility rather than an active source.
- The road-density bands derive from salmonid watershed criteria applied here nationally.
- Agricultural cover and road density are also rated by other metrics and are therefore correlated evidence.
- All three inputs are required. Missing inputs are not converted to zero.
- Bank-derived sediment supply is not represented. The method screens watershed sources only.
- EPA StreamCat Metrics and Definitions
- Allan 2004, Landscapes and riverscapes: the influence of land use on stream ecosystems
- Wang et al. 1997, Influences of watershed land use on habitat quality and biotic integrity in Wisconsin streams
- RUSLE soil erodibility factor classes (Michigan State University Institute of Water Research)
- NMFS 1996 salmonid watershed road-density criteria (as compiled in KRIS)
Substrate Condition (grain size/embeddedness/fines/consolidation)
Stream function Supports aquatic habitats through streambed material and bedform dynamics. Enhances habitat complexity and streambank stability.
Watershed agricultural cover as a pressure proxy for bed condition. It does not measure streambed grain size, embeddedness, fines, or large wood.
| Boundary | What it marks |
|---|---|
| 30% agriculture | Same Good/Fair boundary as the Catchment hydrology agriculture input. |
| 50% agriculture | Same Fair/Poor boundary as the Catchment hydrology agriculture input. |
- Watershed agricultural cover: Agricultural cover represents a separate soil-disturbance, drainage, and runoff pathway that impervious cover does not capture.
- Watershed agriculture share: Crop plus hay cover from the active watershed evidence engine.
- Watershed agriculture is a pressure proxy; it does not measure bed composition, embeddedness, or large wood.
- Disclosed correlated pair with Catchment hydrology: the agriculture input and 30/50 percent bands are shared. The two functions have the same class on about 95 percent of analyzed reaches (rank correlation 0.94).
- The substrate route is unvalidated. Natural slope and stream power strongly influence bed condition; field verification remains necessary.
- National Rivers and Streams Assessment 2018-19 Technical Support Document
- Schueler 1994, The importance of imperviousness
- Allan 2004, Landscapes and riverscapes: the influence of land use on stream ecosystems
- Wang et al. 1997, Influences of watershed land use on habitat quality and biotic integrity in Wisconsin streams
- King et al. 2011, Stream community thresholds at exceptionally low levels of catchment urbanization
- EASI regional reference criteria, September 2026
Physicochemistry
Stream Temperature
Stream function Shade, solar input, and groundwater keep temperature and light within natural ranges for stream metabolism and dissolved oxygen.
Vulnerability to warming, scored on the more limiting of woody riparian shade and watershed impervious cover. It does not estimate stream temperature.
Reference: national curve, 53,175 reference reaches; crossings are approximate
| Boundary | What it marks |
|---|---|
| 10% impervious | Lower impervious-cover pressure transition. |
| 25% impervious | Upper impervious-cover pressure transition. |
| Reference index 0.39 | Fair begins at an interpolated reference index of 0.39. |
| Reference index 0.69 | Good begins at an interpolated reference index of 0.69. Physical crossings depend on the reference stratum. |
- Woody riparian cover: Woody cover represents potential shade and thermal buffering. Grass and herbaceous wetland do not receive thermal-shade credit.
- Watershed impervious cover: Impervious cover represents heated runoff and altered runoff routing.
- This screens thermal vulnerability from corridor shade and watershed impervious cover; it does not rate measured stream temperature.
- Woody cover uses NARS-9 reference expectations with a national fallback. Impervious cover retains its 10/25 percent bands.
- Both inputs are required. Natural prairie and desert channels can meet regional woody-cover expectations with little cover.
Detrital Processing (CPOM retention / shredders)
Stream function Organic matter is captured and broken down, fueling food webs, balancing production/ respiration, moderating pH/ redox, and supplying nutrients.
Potential supply of organic matter from riparian vegetation (forest, shrub, grassland, and wetland combined). It does not measure litter retention or shredder populations.
Reference: national curve, 53,175 reference reaches; crossings are approximate
| Boundary | What it marks |
|---|---|
| Reference index 0.39 | Fair begins at an interpolated reference index of 0.39. |
| Reference index 0.69 | Good begins at an interpolated reference index of 0.69. Physical crossings depend on the reference stratum. |
- Riparian forest: Forest contributes woody and leaf-litter inputs.
- Riparian shrub: Shrub cover contributes litter and low woody material.
- Riparian grassland: Grassland contributes organic material in naturally non-forested settings.
- Riparian wetland: Wetland vegetation contributes organic matter and avoids a forest-only bias.
- This estimates organic-matter supply potential, not CPOM retention or shredder condition.
- All four source classes are required; missing classes are unknown. The capped sum is compared with NARS-9 reference expectations, with a national fallback.
Nitrogen & Phosphorus Concentrations
Stream function Nitrogen and phosphorus transformations feed primary production, regulate excess loads, and store or release nutrients.
Whether nutrient (N and P) concentrations stay near reference levels rather than driving enrichment.
Rated against the NRSA regional benchmarks for the site’s aggregate ecoregion. Each cell gives the Good boundary (at or below) and the Poor boundary (at or above). The worse available analyte governs.
| Region | Total nitrogen (mg/L) | Total phosphorus (mg/L) |
|---|---|---|
| CPL | 0.624 / 1.081 | 0.0559 / 0.103 |
| NAP | 0.345 / 0.482 | 0.0171 / 0.0326 |
| NPL | 0.575 / 0.937 | 0.064 / 0.107 |
| SAP | 0.24 / 0.456 | 0.0148 / 0.0244 |
| SPL | 0.581 / 1.069 | 0.0558 / 0.127 |
| TPL | 0.7 / 1.274 | 0.0886 / 0.143 |
| UMW | 0.583 / 1.024 | 0.0363 / 0.0499 |
| WMT | 0.139 / 0.249 | 0.0177 / 0.041 |
| XER | 0.285 / 0.529 | 0.052 / 0.0959 |
| Boundary | What it marks |
|---|---|
| Good/Fair | At or below the applicable NRSA Table 7-1 good/fair value is Good. |
| Fair/Poor | At or above the applicable NRSA Table 7-1 fair/poor value is Poor. |
- Total nitrogen: TN represents observed nitrogen condition relative to the applicable NRSA region.
- Total phosphorus: TP represents observed phosphorus condition relative to the applicable NRSA region.
- Normalized WQP TN/TP observations: Use valid total-fraction observations from stations within five miles and the preceding ten years.
- StreamCat CHEM integrity fallback: Use when no qualifying TN or TP observation can be normalized.
- Nearby monitoring stations are not necessarily reach-specific.
- Each station is reduced to a median before station medians are combined, preventing heavily sampled stations from dominating.
- One analyte may produce a partial rating.
- When observations are unavailable, the CHEM fallback represents landscape integrity rather than measured nutrient concentration.
Regulatory Impairment Status (305b/303d/TMDL)
Stream function Physical and chemical processes limit pollutants, retain beneficial constituents, and protect ecological and human uses.
Whether the reach is on a Clean Water Act impaired-waters list (303(d)/305(b)/TMDL) for water-quality problems.
- ATTAINS integrated-report category: ATTAINS is the authoritative national source for assessed-water reporting categories.
- Conclusive ATTAINS category: Use an intersecting or qualifying nearby Category 1, 2, 4a, 4b, 4c, or 5 assessment.
- StreamCat CHEM condition fallback: Use when ATTAINS is absent or Category 3 is inconclusive. The result is condition context, not a regulatory determination.
- Category 3 remains unscored because evidence is insufficient.
- A nearby unit within 2 km is explicitly labeled nearby and may not represent the selected reach.
- When ATTAINS is absent or inconclusive, the CHEM fallback is labeled water-quality condition context and not a regulatory determination.
- The category mapping reflects remedy status, not severity. A Category 4a water is as impaired as a Category 5 water but has an approved TMDL, so 4a and 4b rate Fair as impaired-with-a-management-pathway. Category 2 means no assessed use is failing, which is weaker than Category 1.
Biology
In-stream Habitat Complexity & Cover
Stream function Channel and floodplain structure supply depth, velocity, substrate diversity, and vegetation to support native organisms through all life stages.
Woody riparian corridor cover as a stand-in for habitat support (cover, wood recruitment, bank structure). It is not a field inventory of pools, wood, or bedforms. Sinuosity is shown for context.
Reference: national curve, 53,175 reference reaches; crossings are approximate
| Boundary | What it marks |
|---|---|
| Reference index 0.39 | Fair begins at an interpolated reference index of 0.39. |
| Reference index 0.69 | Good begins at an interpolated reference index of 0.69. Physical crossings depend on the reference stratum. |
- Woody riparian cover: Woody riparian cover represents potential cover, wood recruitment, and bank-zone structure, the strongest desktop indicator of habitat support.
- Reach sinuosity: context only Sinuosity indicates planform variability and is shown for context. It is not rated because sinuosity measured over the fixed assessment reach understates planform sinuosity.
- Corridor woody cover is a habitat-support proxy, not a field inventory of pools, wood, cover, or bedforms.
- NARS-9 reference curves account for regional cover expectations; a national curve is used when the regional curve is unavailable.
- Naturally open prairie and desert channels may rate Good with little woody cover where reference expectations are low. Sinuosity is context only.
Biological Integrity (IBI / community condition)
Stream function Physical and chemical conditions enable spawning, juvenile growth, migration, and stress survival of target taxa.
Aquatic-community condition inferred from EPA's benthic-condition model where available, otherwise from complete landscape-integrity products. This is not a field biological assessment.
| Boundary | What it marks |
|---|---|
| 0.25 | Lower EASI probability-integration tier. |
| 0.50 | Upper EASI probability-integration tier. This is a modeled probability, not a measured class. |
- Predicted probability of Good BMMI condition: The published model predicts the probability that benthic condition is Good. It is not a measured IBI class.
- Published BMMI probability: Use prg_bmmi0809 from the other area of interest within the published model frame.
- Published ICI/IWI landscape products: When the BMMI model has no value, use the lower of the two complete integrity products.
- This is a modeled probability of Good benthic condition, not a measured MMI or multi-community IBI.
- The 0.25 and 0.50 cutoffs are EASI integration tiers. The model is unavailable for about 59 percent of analyzed reaches; the disclosed ICI/IWI fallback remains.
- The integrity fallback reuses landscape evidence scored by other functions and requires all twelve components.
Invasive / Non-native Species Presence
Stream function Species interactions sustain biodiversity, restrain invasives, and build resilience to disturbance.
Extent to which invasive or non-native species are present in the watershed and may displace natives.
| Boundary | What it marks |
|---|---|
| 1 taxon | Transparent STAF transition from no established taxa recorded to recorded pressure. |
| 3 taxa | Transparent STAF transition to the upper count tier. |
- Established NAS taxa count: Established taxa records provide a transparent national indicator of invasive-species pressure.
- A zero count means no established taxa were recorded. It does not confirm absence.
- HUC8 fallback results carry lower confidence than HUC12 results.
- Count bands are STAF screening judgments.
- Establishment status often cannot be confirmed from occurrence records, and uneven sampling effort inflates richness where effort is high (Mangiante et al. 2018).
Fish Passage & Barrier Effects (longitudinal connectivity)
Stream function Continuous pathways for free movement of water, sediment, and organisms supporting recolonization, maintaining species diversity, and enabling recovery.
Mapped dams within a mile of the reach, as a sign of possible barriers to fish movement. Proximity alone does not establish whether a barrier is passable.
| Boundary | What it marks |
|---|---|
| 1 dam | A single mapped dam within one mile changes the automated result from Good to Fair. |
| 2 dams | Two or more mapped dams within one mile rate Poor. |
- Mapped NID dams within one mile: Mapped dam proximity identifies potential longitudinal-connectivity pressure.
- A mapped dam count is a proximity screen and cannot confirm passability or the severity of a barrier.
- NID carries about 91,000 regulated dams while the National Aquatic Barrier Inventory maps over 500,000 barriers of all sizes, so zero mapped dams is frequently a false negative for passage.
- Query failure is unscored, not Good.