Over the last two decades, groundwater systems and surface water bodies are being more and more considered as a same entity which constitutes a single resource within the hydrologic system; understanding groundwater surface water (GW–SW) interactions is necessary to effectively manage the water resources. Concepts, methods of analysis and classifications of these hydrodynamic interactions have thus been developed, mostly in case of porous aquifer. Today, GW–SW interactions are still often poorly understood in many watersheds, particularly in the case of complex aquifers like karst aquifers.This study focuses on the influence of groundwater on the genesis and propagation of surface floods in the case of a highly karstified watershed. GW–SW hydrodynamic interactions are analysed (i) at a local scale focussing on hydrodynamic interactions between the karst aquifer in the vicinity of the river and the river, and (ii) at a larger scale focussing on hydrodynamic interactions between a karst spring and sinkholes in the riverbed. Hydrodynamic time series are described using the karst watershed of the Coulazou River (western Montpellier, France) as a case study. Results are used to adapt GW–SW classifications that are usually devoted to porous media. In case of the studied karst aquifer, it is shown that the initial groundwater level may be used to forecast the type of hydraulic connection between the river and the saturated zone during flood. However, initial water level in the karst aquifer does not give any information about the flow direction (gaining or losing stream) during a flood. The latter is found to be also controlled by the limited discharge capacity of karst conduits. A conceptual model of karst/river interactions during flood is given, with concepts that are derived from the analysis of the Coulazou watershed but that can be easily applied to most of the karst watersheds.