Water samples were analyzed for 42 chemical constituents to determine the water quality of the Susquehanna River at Harrisburg, Pennsylvania from April 1, 1980, to March 31, 1981. The investigation was part of the U.S. Environmental Protection Agency's (EPA) Chesapeake Bay Program's Fall Line Monitoring Project to provide information on the Susquehanna River's freshwater input to the Chesapeake Bay. Streamflow and sediment discharge at Harrisburg during the study were 77 and 72 percent, respectively, of the average annual discharges. Precipitation was 16 percent lower than normal. Streamflow for February 1981 was 140 percent higher than the average monthly flow and transported 61 percent of the total annual sediment discharge. Approximately 2,300,000 tons of suspended sediment and 2,990,000 tons of dissolved solids were transported during the study. About 76 percent of the 42,000-ton nitrogen load was dissolved. Nearly 84 percent of the 2,930-ton phosphorus load and 95 percent of the 111,000-ton iron, aluminum and manganese loads were associated with suspended sediment. The herbicides atrazine and 2,4-Dichloro-phenoxyacetic acid (2,4-D) were the only pesticides measured in significant concentrations during the study period. Concentrations of 2,4-D varies throughout the year, and atrazine varied mostly during the spring and summer. Seasonal variations for other constituent concentrations and loads were directly related to streamflow. The concentrations of many constituents varies with distance along the sampling cross-section. Maximum concentrations of suspended sediment differed between the east and west channels and fluctuated from one channel to the other. Specific conductance, dissolved nutrients, and dissolved major ions were consistently higher along the east and west banks. Dissolved nitrate concentrations were significantly higher in the vertical section closest to the west bank of the river. Diel variations of water temperature, dissolved oxygen, pH, and specific conductance recorded for the period 1974 through 1978 were greatest during the months of June, July, August, and September. However, no trends in the amount of daily variation were determined for the five years of data. Regular diel variation patterns were observed for water temperature, dissolved oxygen, and pH for all streamflow conditions except peak flow and ice melt conditions. Specific conductance showed no regular diel variation, and was inversely related to streamflow. All four constituents were greatly influenced for short durations by melting ice.