From the variable grades, an overall grade is created for each reach. This grade reflects a composite score from each of the assessment variables using a weighted average as covered in the table below. If a reach lacked sufficient data for a given variable or metric, that variable or metric was excluded from the grading.
The health of aquatic life in a river ecosystem is directly shaped by the physical and chemical characteristics of the stream. Healthy aquatic ecosystems sustain diverse biological communities and provide essential services such as nutrient cycling, energy transfer, and fish production. The aquatic ecosystem assessment includes four metrics: microhabitat, trout spawning habitat quality, macroinvertebrate community structure, and trout biomass.
Habitat | Communities | ||
|---|---|---|---|
Macroinvertebrates | Trout Spawning | Macroinvertebrates | Fish |
25 | 25 | 25 | 25 |
The streambed is made up of a mix of fines, sand, gravel, cobble, and boulders. These features provide critical habitat for aquatic insects that form the base of the food web and serve as an essential food source for fish and wildlife. When fine sediment accumulates and fills the spaces between coarser substrate, it limits oxygen exchange and reduces habitat suitability for sensitive species. Stream reaches are graded based on the percentage of substrate particles smaller than two millimeters, with region-specific thresholds established from reference site data to reflect naturally expected conditions.
Trout reproduction depends on clean gravel streambeds where females deposit eggs and embryos develop. Excess fine sediment fills the spaces between gravels, reducing oxygen flow to developing eggs and significantly impairs embryo survival and fry emergence. Stream reaches where spawning is expected are graded based on the percentage of fine sediment smaller than eight millimeters within spawning areas. Consistent with guidance from the State of Colorado in Policy 98-1, a threshold of 20% fine sediment defines the boundary between healthy and impaired spawning habitat.
Grade | Scale | Description | % <8mm fines | Explanation |
|---|---|---|---|---|
A | 90-100 | Optimally functional | 0-5% | Near-pristine habitat and redds, providing excellent oxygen exchange and larval emergence potential. |
B | 80-90 | Highly functional | 5-10% | Minor fines are present with very limited effects on egg development and survival. |
C | 70-80 | Mostly functional | 10-20% | Sediment fines are near the upper end of sediment tolerance which may reduce larval emergence, but the habitat still supports spawning. |
D | 60-70 | Party functional | 20-30% | Levels are beyond the protective sediment threshold, and fry emergence is likely compromised. Watershed review recommended to confirm human-caused sediment sources. |
F | 50-60 | Dysfunctional | >30% | High fines are present which are likely to have severe impacts on embryo survival and emergence. Watershed review recommended to confirm human-caused sediment sources. |
N/A | N/A | Not Graded | No Data |
Aquatic macroinvertebrates, such as caddisflies, stoneflies, mayflies and dragonflies, are found in most streams and rivers and play a vital role in aquatic ecosystems. They spend their lives in the water and are highly sensitive to changes in environmental conditions making them one of the most reliable indicators of long-term stream health. Stream reaches are evaluated using Colorado's Multi-Metric Index (MMI v.5), a standardized tool that combines measures of species richness, community diversity, and tolerance to disturbance into a single score. Reaches with high MMI scores support diverse communities, including sensitive species, and are characteristic of healthy streams. Lower scores are a result of a shift toward more tolerant species and can indicate biological impairment.
Grade | Scale | Description | MMI Median | Explanation | |
|---|---|---|---|---|---|
Transition Biotype | Mountain Biotype | ||||
A | 90-100 | Optimally functional | >80 | >81 | Communities equal to reference sites, undisturbed conditions |
B | 80-90 | Highly functional | 75 - 80 | 78 - 81 | Attainment of standard, similar to reference conditions |
C | 70-80 | Mostly Functional | 45.3 - 75 | 47.6 - 78 | Attainment of standard, acceptable variability from reference conditions |
D | 60-70 | Partly Functional | 33.7 - 45.3 | 39.7 - 47.6 | Gray zone - between the thresholds for attainment and impairment; suspected water quality problems |
F | 50-60 | Dysfunctional | <33.7 | <39.7 | Impairment of standard, failure to meet designated uses, 303d list |
N/A | N/A | Not Graded | No Data | No Data | No Data |
Fish communities reflect the overall availability of food, habitat, water quality, and stream connectivity, making them an indicator of stream health. Diverse fish populations and higher trout biomass signal a well-functioning stream, while reduced abundance often points to stress from altered flows, degraded habitat, or poor water quality. Stream reaches are graded using trout biomass thresholds, with the highest grades reserved for streams that support trout populations well above standard benchmarks and reflecting the high-quality habitat present in Grand County's rivers.
Grade | Scale | Description | Pounds Trout per Acre | Explanation |
|---|---|---|---|---|
A | 90-100 | Optimally functional | >120 | Optimal conditions to support an exceptional fishery |
B | 80-90 | Highly functional | 80-120 | High value fishery with excellent recreational fishing conditions |
C | 70-80 | Mostly functional | 60-80 | CPW designated “Gold medal” waters, functional stream conditions |
D | 60-70 | Party functional | 10-60 | Satisfactory recreational fishing conditions |
F | 50-60 | Dysfunctional | <10 | Limited if any fish >14” present, no spawning |
N/A | N/A | Not Graded | No Data |