Rivers are biodiversity hotspots and central to human activities. However, in urban areas water quality is degraded by faecal contamination from combined sewer overflows (CSOs) during heavy rainfall. The relationship between rainfall and CSO discharge is complex and poorly understood because of underlying dynamics of the sewer system and a lack of high-frequency CSO measurements. We present a methodology to model CSO discharge from rainfall characteristics measured across a network of gauges. The method is applied to 22 CSOs outlets in the Seine River in Paris monitored over four summer seasons (June–September) from 2020 to 2023 using the data of 5 rain gauges. Results show that (i) most overflow events are caused by a peak of rain (ii) overflow peaks are well represented by a triangular hydrograph, and (iii) a ramp function can predict peak size (total volume and maximum overflow) from rainfall characteristics. Further results show a map of CSOs response to the spatial distribution of rain, providing hints about functioning of the sewer system, that can support CSO discharge operational forecasting and the development of data-driven models by guiding their architecture design.