The Oka basin is characterized by serious environmental problems associated with the depletion of water resources. Communities of aquatic organisms are the first to respond to negative changes in the aquatic environment, such communities are used as indicators in biological monitoring. The aim of the work was to assess the ecological state of the small river of the Oka basin by bioindication. The quality of the water in the reservoir was assessed by chemical indicators - biogenic and technogenic elements. The saprobity index of the reservoir was determined by the species diversity of periphyton. The results revealed technogenic and organic pollution of the small river. According to the results of biological monitoring, the value of the saprobity index is 2.85, which corresponds to class 4 of the reservoir quality, and the water is assessed as polluted. Monitoring the degree of pollution of the small river by the biodiversity of the periphyton community allows you to determine the degree of water pollution quickly.