Methodology

Water quality is a critical component of river health and reflects the ability of the water to support aquatic life, recreation, and human use. Healthy water quality sustains ecosystems, biodiversity, and community benefits, while degradation from pollution or runoff can negatively affect aquatic life. Water quality data and monitoring support informed, long-term environmental decision-making. The water quality assessment includes three metrics: nutrients, iron and physical characteristics.

WATER QUALITY WEIGHTING

pH

pH measures the acidity or alkalinity of water and directly influences the availability of nutrients and metals in freshwater systems. Values that are too high or too low can impair the physiological functions of aquatic organisms, reduce fish reproductive success, and increase the toxicity of contaminants such as ammonia and heavy metals. Stream reaches are graded against established thresholds from the Colorado Department of Public Health and Environment (CDPHE), with the minimum acceptable pH range defining the boundary between passing and failing grades.

DISSOLVED OXYGEN

Dissolved oxygen (DO) is essential for the survival of fish, macroinvertebrates, and other aquatic organisms, making it one of the most fundamental indicators of stream health. Low DO levels, often caused by algal growth or warm temperatures, can result in fish kills, shifts in community composition, and overall loss of biodiversity. Stream reaches are graded against established thresholds from the CDPHE, with separate more stringent thresholds in place during fish spawning periods

NUTRIENTS

Nitrogen and phosphorus are essential nutrients for aquatic ecosystems, but excessive concentrations can trigger algal blooms, oxygen depletion, and loss of aquatic biodiversity. Both total nitrogen (TN) and total phosphorus (TP) are measured to provide a comprehensive picture of nutrient loading and help identify pollution sources such as agricultural runoff, wastewater discharge, and urban stormwater. Stream reaches are graded against interim nutrient criteria from CDPHE for cold-water streams, with optimally functional reaches showing concentrations below detection limits representing the highest grade.

METALS

Metals are present in water naturally in trace amounts but can increase through runofff, mining activity, and atmospheric deposition. Elevated metal concentrations can be toxic to aquatic life, impair biological functions, and accumulate in sediments and organisms. Total iron was selected as the primary metal indicator for this assessment, as it is frequently detected at measurable levels and serves as a reliable signal of changes in water chemistry and potential contamination. Stream reaches are graded against CDPHE aquatic life standards for total recoverable iron, with optimally functional reaches showing concentrations below detection limits representing the highest grade.