<p>This paper presents the effects of the hydroelectric reservoir on the water quality in Bajo Anchicaya, a hydropower plant in the southwest of Colombia. The study implicated the comparison of water quality at five sites: before the reservoir, entering the reservoir, at the intake, in the reduced channel, and downstream of the hydropower plant discharge. The data collected were analyzed with the analysis of variance (ANOVA) statistical tool to investigate the significance of the reservoir, season, and the hydropower plant operation on the water quality. The IPOMO (pollution index by organic matter) showed that contamination by organic matter in all cases was none or low, the IPOSUS (pollution index by suspended solids) showed that there was no contamination by suspended solids, and the WQI (water quality index) showed good results. From the ANOVA results, season (flow) and hydropower plant are all significant about the water quality. ANOVA also showed a positive effect of the hydropower plant operation on water quality in terms of the Settleable solids and electrical conductivity in the water. Studies like these can improve the understanding of the influence of reservoirs and hydropower plants on water quality to implement water resource management strategies that maintain good water quality indices and prevent its deterioration.</p>