This study aims to assess surface water-karst groundwater interactions during floods in the Mediterranean karst watershed of the Coulazou River (southern France) using a conceptual semidistributed model at 5 min time steps. The kinematic wave routing approximation is used for the transfer of surface flow, while overflows from a linear underground reservoir account for karst flows along the riverbed. After calibration, values of parameters and simulated time series are compared to independent physical measurements. Results show that direct runoff can be neglected on the karst terrains. In addition, this study demonstrates that, in some cases, karst watersheds can be considered as relatively poor systems of regulation but strong systems of amplification or generation of floods and flash floods, depending on rainfall characteristics and also on groundwater level conditions prior to the flood event. Considering that the flood peak is the most important factor defining flash flood hazard, it is shown that the flood hazard regulation effect of the karst is relatively limited for low water table conditions prior to the flood, while the aggravating effect for high water table conditions may be higher than 80% with respect to expected values from surface runoff only. These results show that understanding groundwater-surface water interactions is crucial for describing the flash flood dynamics in karst terrains.