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In 2017, a spillover dam was constructed in the middle course of the Amazar River of Russia, forming a reservoir to provide water to the Amazar Pulp and Saw Mill project known as Polyarnaya. The dam uses an integrated approach, combining hydrochemical, hydrobiological, and ichthyological methods, as well as echo sounding. Comprehensive studies of the transformation of the Amazar River into a reservoir demonstrate the initially low biodiversity of hydrobionts characteristics for a semi-mountain river under the conditions of the sharply continental climate of the Trans-Baikal region. During the initial stage of formation, the reservoir was similar to the original watercourses in physical and chemical parameters and in the composition of the flora and fauna. It featured extensive shoals that were gradually turning into silt-covered and plant-filled shallow bays. These bays will eventually be locations of maximum concentration and diversity of hydrobionts and future nursery and spawning grounds for fish. The construction of the dam has significantly changed the hydrology of the Amazar River downstream of the dam. These findings reveal problems related to fish migration to the Thymallidae and Salmonidae spawning areas, as well as reductions in the biodiversity and quantity of the macrozoobenthos typical for run-of-river reservoirs.