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A systematic approach was used on monthly data from three hydrometric stations located on the Gheshlagh River (west of Iran) to investigate the flow–concentration relationship and for river base flow analysis. According to the results of this study, the water quality of the Gheshlagh River has been affected by changes in river flow as well as human activities. Using the analysis of flow–concentration relationships obtained for Khalife Tarkhan station, the higher concentration of dissolved solids in groundwater (with a base flow rate from 0.1 to 0.7 m3/s) rather than surface runoff was estimated. Also, in this station, with the increase of river flow and improvement of river self-purification capacity, water quality parameters showed a decreasing trend compared to the groundwater. In Hossein Abad's upstream station, due to low average river flow and discharge of saline drainage from agricultural lands, an increasing trend in the dissolved solids both for groundwater and surface runoff was shown. The trend of spatial changes in river water quality also indicates the high impact of human activities such as fish farms and wastewater discharge of Sanandaj wastewater treatment plant, on increasing the concentration of dissolved solids in Dareh Kolleh downstream station. [ABSTRACT FROM AUTHOR]