Data were collected in the American River basin from February to October 1979 for use in assessing the water quality in the basin and developing land-use/water-quality relations. The basin covers 2,163 square miles of the western slope of the central Sierra Nevada. Basin headwaters are located primarily between Donner Summit and Echo Summit. Water-quality data were collected at 14 stream sites and at 3 sites on Folsom Lake and include selected measurements and analyses for physical, chemical, and biological properties and constituents.