At present, small rivers in the Republic of Moldova are facing a lot of environmental problems caused by inadequate function and management of biological wastewater treatment plants. The terrible conditions of the rivers water result from untreated wastewater in addition to secondary pollution caused by strong mud settling of reservoirs and adjacent pools. The most polluted river is found to be Bac, the most altered segment of which starting from the Chisinau municipality. Pollution increases especially from the place where the partially treated water sheds from the Chisinau Waste Water Treatment Plant in the Bac River to its confluence with the Nistru River. Quality indicators show a disastrous ecological status of water, creating impossible conditions of the fish existence.