Agricultural land cover in the U.S. Midwest is a major source of nutrient and sediment pollution that has led to impairment of stream water quality. This study examines the impact of a forested state park on nutrient and sediment concentrations within an agriculturally dominated watershed. Water samples were collected over a 2-year study period from eight stream sampling sites along four creeks and processed for total nitrogen (TN), nitrate, total phosphorus (TP), orthophosphate, and total suspended solids (TSS). Hydrology, channel morphology, and remotely sensed land cover and vegetation data were also collected and analyzed within the study area. Results indicate that water quality responses to a forested state park vary between water quality variables, and the variables examined are uniquely influenced by watershed and stream characteristics. The greatest water quality benefits most frequently occurred within the two smallest study streams with the greatest residence times and proportion of watershed areas within the forested state park. Overall, the greatest improvements to water quality occurred during periods of low stream discharge and when riparian vegetation was greenest. The results of this study suggest that conservation of forested areas within agriculturally dominated watersheds can provide water quality improvements in the U.S. Midwest. Targeting watersheds that drain small streams with long residence times for conservation may be most beneficial to improving water quality.