The relevance of the research is caused by the necessity of controlling hydrochemical parameters of large freshwater ecosystem for the objective estimation of waterbody transformation under the anthropogenic impact. 
The main aim of the research is to examine the spatial and temporal variability in major ion concentrations in Angara River water before regulating its flow by the Boguchany hydropower plants dam, as well as in Boguchany reservoir water during filling and water level stabilization. 
Objects of the research are the Angara River before regulating its flow by the Boguchany HPP dam (2007), and Boguchany reservoir during filling (2013, 2014) and water level stabilization (2015, 2017). 
Methods. Chemical analyses in the water samples were determined in the Centre for Collective Use «Isotope-Geochemical Research» of the IGC SB RAS (Irkutsk, Russia). Flame emission spectrophotometry was used in the analysis of Na⁺ and K⁺, atomic absorption spectrometry was applied to analyse Ca²⁺ and Mg²⁺, the turbidimetric method was used to measure SO₄^2–, the titrimetric method was employed in the analysis of HCO₃^– and the mercurimetric method was applied to study Cl^–.
Results. The spatial and temporal variability in the concentrations of cations (Ca²⁺, Mg²⁺, Na⁺, and K⁺) and anions (HCO₃^–, SO₄^2–, and Cl^–) was examined. The results of the study show that the increase in the water level led to changes in the major ion concentration ratio. In comparison with the Angara River, in the Boguchany reservoir operation, SO₄^2–, Cl^–, Na⁺, and Mg²⁺ concentrations increased, while HCO₃^– and Ca²⁺ concentrations decreased in the initial period of reservoir operation. The ionic composition of water in the Boguchany reservoir resulted primarily from the contribution from the area located upstream (runoff from the Ust-Ilimsk reservoir). Anthropogenic factors include wastewater and reservoir creation, which affect the hydrochemistry of major ions.