To gain better understanding of the impact of combined sewer overflows (CSO) on the chemical and ecological status of lowland rivers and to evaluate the effect of CSO control measures a planning instrument for impact-based CSO management is being developed in Berlin, Germany. After completion the model-based planning instrument will be used by the Berlin water and wastewater utility and the water authority for scenario analysis of CSO management strategies. To adapt the planning instrument to their respective needs and to guarantee an efficient transfer of the results a specific project structure was established. Through direct participation in project management, technical and scientific work as well as demonstration the end-users can influence the development and provide technical input on local issues. First project results show the relevance of CSO impacts compared to the background condition of the Berlin river system and the need for additional measurements to provide data for model adaptation, calibration and validation.