The Savio River catchment in northern Italy has recently experienced a series of damaging flood events, most notably in May 2023, which caused widespread infrastructural disruption and economic losses across the Emilia-Romagna region. These occurrences have underscored the urgency of understanding the evolving flood regime in this Mediterranean basin under changing climatic conditions. This study aimed to calibrate the lumped GR4J rainfall–runoff model for the Savio catchment and reconstruct historical monthly streamflow from 1981 to 2025 using CHIRPS precipitation and ERA5 temperature data. The calibrated model, validated over the 2020–2025 period, demonstrated high performance (NSE = 0.88; KGE = 0.83; RMSE = 2.47 m3/s). Subsequently, a comparative analysis between the baseline (1981–2002) and recent (2003–2025) periods revealed significant hydrological changes, including a 12.8% increase in annual maximum flow (AMF), a 69.2% rise in flood frequency, and a seasonal shift in peak flows from November to March. Additionally, Mann–Kendall and Sen’s slope tests confirmed statistically significant increasing trends in both AMF (p = 0.032) and high-flow frequency (p = 0.004). Correlation analysis further highlighted precipitation as the dominant driver of flood behavior (r = 0.62–0.70), with temperature exerting secondary influence (r = 0.37–0.43). The observed trends establish a robust link between climate variability and flood intensification in the Savio catchment over the past four decades and support the urgent need for updated risk assessments and forward-looking flood management strategies.