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Outcome: Protect water quality and quantity

About this indicator

This indicator focuses on the condition of lakes, streams and rivers within King County's watersheds.

To assess the condition of lakes in King County, King County DNRP uses Carson's Trophic State Index (TSI). A lake's trophic state is defined as the total weight of living biological material in its waters and includes measurements of water clarity, phosphorus levels and algal levels. These attributes provide a good indication of a lake's biological activity, which is influenced by a variety of factors, both natural (including watershed size, lake depth and climate) and man-made (including land development, increases in impervious land surfaces and the introduction of sewage to a lake). The increase in a lake's biological activity is referred to as eutrophication. Natural eutrophication occurs over centuries and is often not observable in a single human lifetime, but human activity can accelerate these natural processes.

Through the Stream Monitoring Program, DNRP routinely monitors the quality of a number of the county's streams and rivers. Water samples are collected during routine base flow conditions and are analyzed for a variety of parameters including: temperature, dissolved oxygen, turbidity, total dissolved solids, pH, conductivity and nutrient content. The parameters are aggregated into a single value - the Water Quality Index (WQI) - which allows for comparative analysis over time and across sampling locations. Based on its WQI value, a stream location is identified as being of low, moderate or high concern with regard to its water quality. The sites reported here are found in Water Resource Inventory Areas (WRIA) 8 and 9. WRIA 8 roughly combines the Lake Washington/ Cedar River and Lake Sammamish/ Sammamish River Watersheds; WRIA 9 roughly combines the Green/ Duwamish Watershed and South Puget Sound Drainage Basin.

Key findings

  • Percent of sampled small lakes rated "good" or "moderate": 76% (2007)
  • Percent of stream sampling stations rated "low" or "moderate" concern: 76% (2008)

Graphs and tables

Last updated March 2010


Technical notes

Water quality data provided by King County DNRP, Water and Land Resources Division (WLRD). Large lakes and small lakes testing samples are taken in summer months. While each major division (10, 20, 30, etc.) of the Trophic State Index (TSI) represents a doubling or halving of algal biomass and is related to nutrients and water clarity, the TSI values are a continuum and some lakes may be in a borderline range, exhibiting some qualities of upper and lower classifications. Subsequently, small lakes with TSI values of 39.9 to 40.1 are included in the oligotrophic range and those with TSI values of 49.9-50.1 are included in the mesotrophic range. Eutrophication is not interpreted here as a statement of water quality but an indication of the conditions existing in lakes.

Trophic State Index

Graphs and tables

Click image to enlarge

Phosphorous TSI values for major lakes in King County

Distribution of small lakes in King County by phosphorous TSI values

Streams in King County WRIA 8 and 9 rated as low, moderate, and high concern