The U.S. Geological Survey (USGS) has worked cooperative with the Unified Sewerage Agency of Washington County (USA) since 1990 to assess water quality in the Tualatin River Basin. The objectives of this study were to: assess the occurrence and magnitude of trace element and selected organic contaminants in streams of the Tualatin River Basin; compare contaminant levels found in Tualatin River Basin streams with published guidelines for the protection of aquatic life; place the contaminant levels found in the Tualatin River Basin streams in context, regionally and nationally, by comparing them with those found in the Willamette Basin and by the USGS National Water-Quality Assessment (NAWQA) Program; compare contaminant concentrations in bed sediment with those in fish tissue; and identify contaminant patterns that would help managers make decisions regarding future monitoring and the implementation of pollution prevention and abatement programs.