This report summarizes existing information on water quality and other indicators of the environmental conditions of Lake Washington. The report was prepared in support of King County’s combined sewer overflow (CSO) long-term control planning efforts.  It answers the question, “What are the existing water quality impairments in receiving waters (waterbodies) where King County CSOs discharge?” Similar reports were prepared for the Duwamish River, Elliott Bay, and Lake Union/Ship Canal as part of the Water Quality Assessment and Monitoring Study. 

King County and other agencies have collected data on water quality, sediment chemistry, and fish tissue contamination in Lake Washington. These data were evaluated and summarized as a part of this assessment to describe current conditions, identify long-term trends, and make comparisons to Washington State standards. Water column data included physical and basic chemical parameters (temperature, dissolved oxygen, conductance, pH, turbidity, and total suspended solids), eutrophication parameters (chlorophyll a, nutrients, Secchi transparency, phytoplankton, and zooplankton), indicator bacteria, metals, and various groups of organic compounds. Data on metals and organic compounds in sediments and fish tissue were also analyzed.