A study of the Buffalo National River in north-central Arkansas was conducted between July 28-30 and August 13-15, 2003, to characterize the base-flow and water-quality characteristics and streamflow gain and loss in the Buffalo River. The study was separated into two time periods because of a precipitation event that occurred on the afternoon of July 30 causing appreciable storm runoff. Streamflow was separated to identify base-flow and surface-runoff components using the Base Flow Index hydrograph separation computer program. Base-flow separation analyses indicated annual variability in streamflow throughout the Buffalo River Basin. Based upon these analyses, total and base flow were below average for the mainstem of the river and Richland Creek during the 2003 water year. Waterquality samples were collected from 25 surface-water sites on the Buffalo River and selected tributaries. Most nutrient concentrations for the mainstem of the Buffalo River were near or below the minimum reporting level and were less than the median flow-weighted concentration for relatively undeveloped stream basins in the United States. Streamflow measurement data were collected at 44 locations along the mainstem of the Buffalo River and at points of inflow (prior to confluence with the mainstem) to identify gaining and losing reaches. Seven gaining and five losing reaches were identified for the Buffalo River. Additionally, surface flow on the mainstem of the Buffalo River was diverted to subsurface flow on the mainstem at two locations (river miles 73.6 and 131.6) where the mainstem was found to be dry. Reaches throughout the length of the river had calculated gains or losses that were less than the sum of measurement errors for the respective reaches of river.