Introduction: The U.S. Geological Survey, in cooperation with the U.S. Army Corps of Engineers, and aided by the Susquehanna River Basin Commission, investigated the water quality of the Tioga River basin, Pennsylvania. The study, made from September 1973 to September 1978, was designed to evaluate water quality at various sites prior to impoundment. Results of the investigation will aid the Corps of Engineers in designing the operation of three reservoirs that were in the final stages of construction in late 1979.