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). 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.