Theoretically the Berlin River Spree could be under pressure from depressions in dissolved oxygen (DO) and high concentration of fish toxic ammonia following overflows of the combined sewer system. However, monitoring results indicate that the Spree is only under pressure from depressions in dissolved oxygen (DO). Consequently, a sewer model, a river water quality model and an impact assessment tool were calibrated and validated for representation of DO depressions. The three elements are joined in a planning tool, which will be used to test the effect of CSO management approaches for the current situation and with altered boundary conditions to account for expected climate change.