Thermal power plants (TPP) belong to the enterprises that accompany the development of any city. These industrial facilities consume and discharge large amounts of water. Therefore, thermal power plants are often located near water bodies that need to assess their ecological state and predict the consequences of anthropogenic impact. The purpose of the work was to assess the ecological state of the floodplain Lake Ivanovskoye (Russia, Kirov region), which receives wastewater from a thermal power plant, using a triadic approach and comparing the results of chemical analysis, bioassay and bioindication. It is shown that the main pollutant in the lake and the canal connecting Lake Ivanovskoye with the nearest river Vyatka is ammonium ion (up to 3.2 mg/dm3). In addition, the Russian standards for biological oxygen consumption are exceeded (analysis period is 20 days). Tests for toxicity of waste water showed inhibition of Paramecium caudatum and Escherichia coli reactions. On the contrary, natural surface water from Lake Ivanovskoye stimulated the test functions of Scenedesmus quadricauda, P. caudatum, and E. coli. The death of Daphnia magna in the samples were not recorded. The totality of the facts pointed to the organic nature of pollution, leading to anthropogenic eutrophication of the lake. The hypothesis was tested by bioindication method and by analyzing the totality of species of coastal aquatic plants. It was shown that the reservoir belongs to the mesotrophic type with a moderate degree of pollution, which decreases with distance from the wastewater discharge points. The increase in the trophicity of the reservoir is facilitated by thermal pollution (up to + 15 ℃) and an alkaline reaction of wastewater (pH up to 9.2). Thus, the joint analysis of the results of chemical analysis, bioassay and bioindication made it possible to carry out an objective assessment of the ecological state of the reservoir and identify the reasons for the revealed facts.