Urban water courses are considerably degraded in terms of their hydrology, riparian and channel morphology, substrate heterogeneity and habitat features as well as water and sediment quality. In addition, the combined sewer overflows and the ecotoxicological impacts of its components lead to a change of the physical-chemical and microbial mass balance affecting the biocenoses of higher trophic levels. Combined sewer overflows are therefore an additional stress to the ecological status of the urban course of the River Spree and of its channels, which is damaged already by both preload and background load of the aquatic environment. With regard to the assessment of the ecological water status, the European Water Framework Directives gives priority to the aquatic biocenoses in their capacity as ecological quality parameters. Against this background, an immission-oriented approach for the assessment of combined sewer overflows has to describe also their impacts on the biocenoses of the macrozoobenthos, the fish fauna, the macrophytes and the phytoplancton. These biocenoses are protected against the harmful impacts resulting from CSO only if the modification of their physical and chemical environment is avoided or reduced to an ecologically tolerable level respectively. In case that unfavourable impacts cannot be completely eliminated, the degree of impairment and the number of damaging CSO discharge events, which appear to be acceptable, should be defined. The present study is based on the bibliographic study „ Impact of urban use on the mass balance and the biocoenosis of lowland rivers under special consideration of combined sewer overflows” and deals with the assessment of CSO impacts on the ecological situation of the urban Spree and the channels (Cyprinid water bodies). In general, the immissionoriented assessment of CSO impact on the biocenoses (macrozoobenthos, fish fauna) requires the observation of the intensity, duration and frequency of occurrence of the individual events based on the assumption that, due to the background pollution, top priority is currently given to the acute CSO impacts. Requirements for the protection of aquatic biocenoses are developed with regard to the target parameters oxygen and ammonium/ammoniac and ecological tolerances of the biocenotic subjects of protection, which are strongest influenced by CSO. Initially, it is discussed to what extent the already existing results from laboratory investigations can be transferred to field situations. Next to the commonly accepted threshold values for oxygen concentrations during continuous persistent loads, particular requirements for the oxygen balance in case of peak loads are formulated.