Routine freshwater monitoring data collected by the The WA State Department of Ecology's River and Stream Monitoring Program are summarized by a technique called the "Water Quality Index" (WQI). The WQI ranges from 1 (poor quality) to 100 (good quality). WQI scores above 80 are consistent with meeting water quality goals for conventional pollutants. However, the different color coding for scores less than 80 is a visual aid; breakpoints other than 80 are not ecologically significant.The WQI summary does not include non-standard elements like metals. For temperature, pH, oxygen, and fecal coliform bacteria, the WQI is based on criteria in Washington’s Water Quality Standards, WAC 173-201A.
For nutrient and sediment measures where standards are not specific, results are based on expected conditions in a given region. Multiple constituents are combined and results aggregated over time to produce a single score for each station and each year.