This study was aimed at identifying the processes responsible for the major ion composition of pore water from the bottom sediments of the Bratsk water reservoir, which is a part of the largest freshwater Baikal-Angara water system. The pore water ionic composition varies both along the sediment depth profile and across the water area. In pore water, the difference between the highest and lowest values is remarkably large: 5.1 times for K+, 13 times for Mg2+, 16 times for HCO3-, 20 times for

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... +, 23 times for Na+, 80 times for SO42-, 105 times for Cl-. Such a variability suggests that the dominant factors, influencing pore water chemistry, depend on the location. At the first stages of the sedimentation process, the chemistry of the pore water in the Bratsk reservoir is dependent on HCO3-Ca of the overlying water. Later on, due to the interaction with the sedimentary terrigenous material, they changed to SO4, SO4-HCO3, HCO3-SO4, HCO3-Cl-SO4-water types with exchangeable cations, mainly Ca. Some of pore waters may have a complex genesis associated with subaqueous groundwater discharge. The change in the redox potential observed in the pore water is the indicator of early-diagenetic transformations taking place in bottom sediments.