Understanding the relation between dam release characteristics and downstream water quality in the lower Roanoke River, North Carolina, is important for natural-resource management and ecosystem protection. Data from four raingages, four water-quality monitoring sites, and one streamflow-measurement site were used to identify statistical relations and discernible quantitative or qualitative patterns linking Roanoke River instream dissolved-oxygen (DO) levels to releases at Roanoke Rapids Dam for the period 1998-2005. The time-series DO data, complicated by the occurrence of major hurricanes in the short period of hourly DO data collection at the dam, present a mixed picture of the effects of hydropower peaking (a technique used by hydropower dam operators to produce electricity when consumption is high by passing a large volume of water through the dam turbines, which dramatically increases the volume of flow below the dam) on downstream DO. Other than in 2003 when dissolved-oxygen concentrations in the Roanoke River were likely affected by runoff from Hurricane Isabel rains, there were not consistent, statistically significant differences detected in the annual medians of hourly and(or) daily DO values during peaking versus nonpeaking periods. Along the Roanoke River, downstream of Roanoke Rapids Dam at Oak City, North Carolina, using a 95-percent confidence interval, the median value of the May-November daily mean DO concentrations for each year was lower during peaking periods for 2 years, higher for 2 years, and not significantly different for 4 years. Downstream at Jamesville, North Carolina, also using a 95-percent confidence interval, the median value of the annual May-November daily mean DO concentrations during hydropower peaking was lower for 4 years, higher for 2 years, and not significantly different for 2 years. In summary, the effect of hydropower peaking on downstream DO was inconsistent. Conversely, large precipitation events downstream from the dam resulted in consistent, statistically significant decreases in DO in the mainstem of the Roanoke River at Oak City and Jamesville.