The soils and rocks of the Upper West Branch Susquehanna River basin, from its headwaters downstream for 150 miles, are laden with pyritic materials that have the potential to produce acid mine drainage. The effects of mine drainage are severe, particularly in the reach between Curwensville and Renovo where present water quality cannot support viable populations of benthic macroinvertebrates or fish. During base-flow periods in May and July 1984, streamflow and water quality were measured at four sites on the West Branch Susquehanna River and near the mouths of 94 tributaries. Water-quality constituents determined were temperature, specific conductance, pH, acidity, alkalinity, and concentrations of dissolved sulfate and total and dissolved forms of iron, manganese, aluminum, and zinc. The data collected for the study indicate that the predominant influence on water quality of the tributaries is land use. An area where few or no coal deposits or disturbed area were present was found to have relatively good surface-water quality (median pH was nearly 5.5 units), whereas areas where coal mining was active in the basin, or where large areas of unreclaimed mines were present, were found to have poorest water quality (median pH was generally less than 4.0 units). In general, Moshannon, Sinnemahoning, Clearfield, and Kettle Creeks were found to be the largest tributary sources of acidity and total-recoverable iron to the river. During the May sampling, Moshannon, Sinnemahoning, and Clearfield Creeks contributed 63 percent of the 365 tons/day of acidity, and Moshannon and Clearfield Creeks contributed 76 percent of the 44.8 tons/day of total-recoverable iron that were discharged to the river. During the July sampling, Moshannon, Kettle, and Clearfield Creeks contributed 60 percent of the 131 tons/day of acidity, and Moshannon and Kettle Creeks contributed 51 percent of the 6.5 tons/day of total-recoverable iron discharged to the river . The West Branch Susquehanna River was found to have pH ranging from 5.4 to 6.5 units and specific conductance ranging from 267 to 310 microsiemens per centimeter at the most upstream site at Curwensville. The water quality was most degraded at the site at Karthaus (pH 3.9 to 4.1 units, specific conductance 330 to 610 microsiemens per centimeter). Quality gradually improved downstream to the site at Renovo (pH 3.8 to 4.6 units, specific conductance 200 to 392 microsiemens per centimeter), although the quality did not recover to that found at Curwensville.