The aim of this paper is to better understand the functioning of the River Selle (northern France) during dry weather and storm events, to assess the impact of a town on the surface water quality and to suggest qualitative assumptions on the vulnerability of water quality to weather conditions. Two high-frequency monitoring stations covering the Cateau-Cambrésis town were deployed during 4 months in 2016. River flow, water temperature, pH, dissolved oxygen, turbidity, conductivity, total organic carbon, nitrates and phosphates were monitored every 10 min. The water supply of the River Selle is mainly dominated by groundwater as shown by the behaviour of the river flow, the water temperature and the nitrate concentrations in both surface water and groundwater. The reference station located at the upstream of the river (Saint Souplet) exhibits low anthropogenic pressure during dry weather but is significantly impacted during storm events. At the downstream of the Cateau-Cambrésis town, the water quality is severely impacted by phosphates during dry weather mainly due to wastewater inputs into the river. An additional load of pollution is highlighted during storm events. According to our results, the water quality of the River Selle would degrade if actions to reduce dry-weather and storm events pollution sources are not undertaken rapidly. Moreover, nutrients, particularly phosphates, are clearly in excess in this system. Efforts to combat soil leaching and the revision of sewage systems and urban wastewater treatment in the catchment are two key points to tackle. Finally, this study shows the importance of understanding the current behaviour of a given river towards dry weather and storm events before suggesting local scenarios of the impact of climate change on surface water quality.

本文的目的是更好地了解干燥天气和暴风雨期间塞勒河（法国北部）的功能，评估城镇对地表水水质的影响，并提出关于水质对水体脆弱性的定性假设。天气状况。2016年，在四个月内部署了覆盖Cateau-Cambrésis镇的两个高频监测站。每10分钟监测一次河流流量，水温，pH，溶解氧，浊度，电导率，总有机碳，硝酸盐和磷酸盐。塞勒河的供水主要由地下水控制，这表现为河流流量，水温和地表水及地下水中的硝酸盐浓度。位于河流上游的参考站（圣索普莱特）在干旱天气时人为压力较低，但在暴风雨天气中会受到很大影响。在Cateau-Cambrésis镇的下游，干旱期间的磷酸盐严重影响了水质，这主要是由于废水输入河中。在暴风雨事件中会突出显示额外的污染负荷。根据我们的结果，如果不迅速采取措施减少干旱天气和暴风雨事件的污染源，塞勒河的水质将下降。此外，该系统中的营养素，尤其是磷酸盐显然过量。努力解决流域的土壤淋溶，改正污水系统和城市污水处理是要解决的两个关键问题。最后，