Agricultural nonpoint source pollution from fertilizer runoff contributes to eutrophication in Midwestern freshwater systems. This study: 1) examines how a forested state park (Hueston Woods State Park) containing forested streams and a hypereutrophic reservoir influence nutrient concentrations in an agricultural watershed in southwest Ohio, and 2) examines relationships between changes water quality and weather (temperature and precipitation). Over a two-year period (2019–2021), biweekly water samples were collected from nine sites: four at agricultural-forested stream boundaries, four at stream-reservoir confluences, and one at the reservoir outflow. Samples were analyzed for total nitrogen (TN), nitrate (NO₃⁻), total phosphorus (TP), and orthophosphate (PO₄³⁻). Stream discharge was calibrated using stage-discharge curves and pressure transducers. Weather data for the watershed was attained from the PRISM Climate Group. Multivariate statistical analyses were performed from streamflow-weighted event mean concentrations to assess changes in nutrient concentrations within the full study area and the individual stream and reservoir sections. Multivariate analyses also revealed relationships between water quality changes and weather. Full study area results showed NO₃⁻ decreased by 29%, while TP increased by 60%. High TN:TP ratios throughout indicated the system was largely P limited. Nutrient changes were more strongly correlated with reservoir changes than with stream inputs, suggesting the reservoir’s longer residence time and increased biogeochemical processes had a dominant influence on nutrient dynamics. Changes in water quality were related to both temperature and precipitation, but varied by water quality parameter. These findings highlight the need to consider both forested streams and reservoirs in water quality management across mixed-use landscapes.