The importance of hydroelectricity to the world energy grid motivates the implementation of dams and formation of reservoirs. This implies changes in the ecosystem, and therefore those waters must be constantly monitored. Given the relevance of applied scientific research to monitoring data, this study aims at the spatiotemporal characterization of the water quality of Capivari-Cachoeira hydroelectric power plant reservoir located in Southern Brazil, as well as the water supplying it and the water that is restored to the river. Historical monitoring data obtained between 2005 and 2016 were used for the analysis. The factor analysis used for the study of these data resulted in two factors that explain 59.7% of the total variability. The first factor represents the influence of anthropic activities and land use, signaling the existence of polluting sources upstream of the reservoir. The second fator represents seasonality. Kruskal-Wallis tests applied in factor scores and in the variables with higher factor weights resulted in significant spatial and temporal difference. Regarding the first factor, the reservoir station and those located downstream differed spatially from those located upstream. In the reservoir, reduction in concentration of phosphorus and total solids suggests sedimentation, consequently reducing its values downstream. Considering the seasonal factor, the results obtained during winter showed the lowest temperatures and had the highest values of dissolved oxygen, with the exception of the samples from the reservoir. Thus, the control of the nutrient supply in dammed river demonstrated to be an important tool for managing water resources.

The study of the stratification processes in reservoirs is important because they happen along with several physical and chemical changes in the water, as in the distribution of dissolved gases and nutrients. The aim of this work was to study the stratification process of the Capivari-Waterfall Hydroelectric Power Plant, based on the data of the vertical profiles of dissolved oxygen and temperature, as well as the comparison of them with the calculation of Froude's Densimetric Number, and reservoir residence time. The data used are part of the self-monitoring program carried out by COPEL. These data were collected quarterly between 2005 and 2016. The hydraulic and operational data from the reservoir were used to calculations. According to the measurements of dissolved oxygen and temperature, it was considered to be stratified during almost all the year, with a trend of circulation in the colder months, this behavior characterizes lakes with warm monomictic circulation pattern. Therefore, the result of Froude's Densimetric Number, corresponded correctly with the observed. Regarding the residence time, this was not consistent with the results obtained, from the models. The use of different methodologies in evaluating circulation in reservoirs should be used in order to make the best management decision.