This paper presents a study that aims to evaluate the impact of periurban development on the flood regime of a small river crossing a densely urbanised area, the city of Lyon (France). This research was conducted on the catchment of the Yzeron River in western Lyon. Statistical tests showed that both flood frequency and severity have increased in this catchment, between two distinct periods: the 1970s and the 1990s. Evaluation of the specific impact of urban development on the flood regimes requires the paying of attention to all possible contributing factors. For that purpose, we used a diachronic approach, with hydrologic and land-use data from the two periods. We used these data to calibrate a distributed hydrologic model and then to simulate the urban, periurban, and rural hydrologic contributions in the past but also in the future. We also compare the floodable vulnerability amount with the flood hazard increase between the two periods to assess what components mainly affect the flood risk during this land-use evolution. Finally, the basin scale efficiency of a generalised on-site rainwater retention technique is evaluated to know how that technique could contribute to mitigate flood risk.