Streamflow and surface-water-quality data were collected from November 1995 through April 1998 (water years 1996-98) from a surface-water network in the Puget Sound Basin study unit of the U.S. Geological Survey National Water-Quality Assessment program. Water samples collected monthly and during storm runoff events were analyzed for nutrients, major ions, organic carbon, and suspended sediment, and at selected sites, samples were analyzed for pesticides and volatile organic compounds. Eleven sites were established in three major watersheds--two in the Skokomish River Basin, three in the Nooksack River Basin, five in the Green-Duwamish River Basin, and one site in Thornton Creek Basin, a small tributary to Lake Washington. The Skokomish River near Potlatch, Nooksack River at Brennan, and Duwamish River at Tukwila are integrators of mixed land uses with the sampling sites locally influenced by forestry practices, agriculture, and urbanization, respectively. The remaining eight sites are indicators of relatively homogeneous land use/land cover in their basins. The site on the North Fork Skokomish River is an indicator site chosen to measure reference or background conditions in the study unit. In the Nooksack River Basin, the site on Fishtrap Creek is an indicator of agriculture, and the Nooksack River at North Cedarville is an indicator site of forestry practices in the upper watershed. In the Green-Duwamish River Basin, Springbrook Creek is an urban indicator, Big Soos Creek is an indicator of a rapidly developing suburban basin; Newaukum Creek is an indicator of agriculture; and the Green River above Twin Camp Creek is an indicator of forestry practices. Thornton Creek is an indicator of high-density urban residential and commercial development. Conditions during the first 18 months of sampling were dominated by above-normal precipitation. For the Seattle-Tacoma area, water year 1997 was the wettest of the 3 years during the sample-collection period. Nearly 52 inches fell (about 14 inches above average) and monthly precipitation was often 200 percent of normal. The wet years kept streamflows generally above normal and contributed to high concentrations of pesticides, nutrients, suspended sediment, and organic carbon in samples. On the basis of chemical concentrations, dissolved oxygen concentrations, and water temperature, the relative quality of water among the 11 study sites ranged from exceptionally high in the North Fork Skokomish and the Green to fair in Springbrook and Thornton. Water in the large rivers (Skokomish, Nooksack, Green-Duwamish) and in two of the small streams in the Puget Sound Lowlands (Big Soos and Newaukum) was characterized by dilute water chemistry with dissolved solids concentrations less than 130 milligrams per liter. Water in three other small streams in the Lowlands (Fishtrap, Springbrook, and Thornton) had dissolved solids concentrations as high as 320 milligrams per liter. Nutrient and pesticide concentrations mostly were higher in the small streams than in the large rivers. Suspended-sediment concentrations, however, were highest in the large rivers, with averages ranging from 85 to 443 milligrams per liter. During storm and flood events, suspended-sediment concentrations in samples from the Nooksack were as much as 2,800 milligrams per liter, and from the Skokomish, 1,500 milligrams per liter. Out of 86 pesticides and 86 volatile organic compounds analyzed, a total of 35 pesticides and 11 volatile organic compounds were detected at concentrations above laboratory reporting levels in samples collected from the four intensively studied sites, the lower Nooksack River, Duwamish River, Fishtrap Creek, and Thornton Creek. Herbicides were detected more frequently than insecticides. The herbicide prometon was detected in 66 percent of all 124 samples collected, followed by simazine (65 percent), atrazine (64 percent), and the insecticide diazinon (50 percent). The detected volatile organic c