The King County Streams and Rivers Water Quality Monitoring Program collects and evaluates information on water chemistry and microbes monthly at 75 sites on streams and rivers throughout the county. The purpose of the program is to provide an understanding of spatial and temporal changes in water quality and evaluate whether streams are meeting their designated uses. This report documents the current conditions (2016-2020), status, and long-term trends of water quality at the county’s routine stream monitoring sites. Additionally, it documents current conditions and long-term trends for discharge at gages monitored by King County and the U.S. Geological Survey.

In general, we found urban and agricultural streams had lower water quality than suburban, rural, and forested streams. Despite widespread impairments for temperature, dissolved oxygen, and fecal bacteria, most of the sites across the county are categorized as having “excellent” or “good” water quality according to scores based on Ecology’s water quality index. Trend analysis indicated worsening trends for temperature and dissolved oxygen but improving trends for nutrients and fecal bacteria. Across gaged streams, flows have been stable over time; only one site exhibited a meaningful increase in stream flashiness.

Recommendations include performing additional analyses to identify causal factors for the observed trends and filling existing data gaps to inform decision-making. Specifically, we recommend adding additional co-located stream gages and biological monitoring (i.e. periphyton and benthic macroinvertebrates) to ambient monitoring sites.