The objective of this study was to separate the individual human impacts on streamflow changes in the Gamcheon watershed in South Korea. The streamflow regime was disturbed by the construction of an upstream dam in 2013 and was continuously affected by the discharge of treated sewage water, as well as withdrawals of stream water and groundwater. To assess each of the impacting factors, a physical-based semi-distributed hydrological model, Soil and Water Assessment Tool (SWAT), was applied to the study area. The model was set up using not only climatic and spatial data on watershed characteristics but also data on human activities, such as dam operation and water use. The model parameters were first calibrated using observed streamflow data over the period of 2013–2020. Naturalized streamflow data were then generated without changing any of the calibrated parameters and without considering any human activities. Several simulations of runoff, each considering only one human activity, were performed, and the results were compared with those of the naturalized streamflow to determine the individual impacts on changes in streamflow. The results showed that the combined effects of human activities on streamflow changes were not significant; however, the individual impacts were evident. The 10th lowest flow rate in a year, Q355, affected by stream water intake, groundwater pumping, dam operation, and treated sewage water discharge, was changed by -13.7 %, -54.0 %, 34.0 %, and 38.4 % relative to the natural flow, respectively. The Gamcheon Watershed was evaluated to balance the influence of human activity, ensuring streamflow was maintained at a natural level without causing a serious decrease.