Most functions are rated from a single indicator per tier. Where a Screening-tier metric reads more than one indicator, the more limiting one governs the rating. A low level of pressure on one indicator cannot offset a high level on another, because the indicators are not averaged. EASI's Catchment hydrology metric is the clearest example, rating the worse of watershed impervious cover and agricultural cover. The EASI metric reference lists each indicator's boundaries and which one governed.
| Screening | Rapid | Detailed | |
|---|---|---|---|
| Metric Scores | Qualitative (e.g., Good / Fair / Poor) | Semi-quantitative (Likert scale) | Quantitative (measured values) |
| Function scores | User-assigned using recommended ranges | User-assigned using lines of evidence | Computed via reference curves |
Outcomes are observable or quantifiable results that are linked to how one or more stream functions operate.
Physical Outcomes
Physical outcomes are measurable results of hydrologic, hydraulic, geomorphic, and habitat-forming processes. They describe the physical structure of the stream and how water and sediment move through the system.
Examples: floodplain inundation frequency, channel stability, sediment transport balance, habitat unit distribution, substrate composition, and large wood abundance.
Chemical Outcomes
Chemical outcomes are measurable results of water chemistry and biogeochemical processes. They describe how chemical conditions support or limit aquatic life and ecosystem processes.
Examples: dissolved oxygen, nutrient concentrations, temperature regime, pH, contaminant levels, and organic matter decomposition rates.
Biological Outcomes
Biological outcomes are measurable characteristics of aquatic and riparian communities. They describe the presence, abundance, diversity, and functional roles of organisms in the system.
Examples: fish assemblage composition, macroinvertebrate diversity, species richness, presence of sensitive taxa, and functional feeding group distribution.
Ecosystem Condition
The overall state of a stream system, based on the combined performance of physical, chemical, and biological outcomes relative to expected or reference conditions. It indicates how well the system sustains ecological function and resilience over time.