Despite controversy about effects of plantation forestry on streamflow, streamflow response to forest plantations over multiple decades is not well understood. Analysis of 60‐year records of daily streamflow from eight paired‐basin experiments in the Pacific Northwest of the United States (Oregon) revealed that the conversion of old‐growth forest to Douglas‐fir plantations had a major effect on summer streamflow. Average daily streamflow in summer (July through September) in basins with 34‐ to 43‐year‐old plantations of Douglas‐fir was 50% lower than streamflow from reference basins with 150‐ to 500‐year‐old forests dominated by Douglas‐fir, western hemlock, and other conifers. Study plantations are comparable in terms of age class, treatments, and growth rates to managed forests in the region. Young Douglas‐fir trees, which have higher sapwood area, higher sapflow per unit of sapwood area, higher concentration of leaf area in the upper canopy, and less ability to limit transpiration, appear to have higher rates of evapotranspiration than old trees of conifer species, especially during dry summers. Reduced summer streamflow in headwater basins with forest plantations may limit aquatic habitat and exacerbate stream warming, and it may also alter water yield and timing in much larger basins. Legacies of past forest management or extensive natural disturbances may be confounded with effects of climate change on streamflow in large river basins. Continued research is needed using long‐term paired‐basin studies and process studies to determine the effects of forest management on streamflow deficits in a variety of forest types and forest management systems.