The authors analyzed the dynamics of benthic communities in the Kuybyshev Reservoir Priplotinnyi Reach, over a long-term observation period 1958-2020. The authors note a certain frequency of changes in the ratio of macrozoobenthos coenoses and biomass of benthic communities at the stations of the former Volga channel and the Priplotinnyi Reach floodplain. During the research period in 2005, 2020 there was a decrease in the biomass of benthos. The stage of destabilization of the Kuybyshev reservoir ecosystem, noted since the mid-1980s, against the background of anthropogenic eutrophication processes, is accompanied by a sharp decrease in the biomass of benthos and structural changes in benthic communities. In 2020, the benthos biomass, compared to 1985, decreased in deep-water areas and in the Priplotinnyi Reach coastal zone by about 8 times and amounted to 2.0 g / m2, which is due to a decrease in the proportion of pelophilic coenosis of oligochaetes Tubifex tubife and Limnodrilus hoffmeisteri. The biomass of forage benthos in the coastal zone decreased from 5.48 g / m2 to 2.94-1.75 g / m2 due to anthropogenic impact. The authors revealed similar processes of development of the chironomid coenosis on the former channel and the flooded floodplain. During the autumn survey, 9 species were recorded in shallow waters, the proportion of which is 19.5%. There was found a low biomass of Procladius ferrugineus larvae -0.05 -0.08 g / m2. The authors discuss the data of hydrobiological monitoring at the stations of the former channel and the flooded floodplain of the Priplotinnyi Reach, including the change in the coenosis of chironomids (Diptera, Chironomidae) as an object of indication of the ecological state of the reservoir.