The tidal characteristics of Cartagena Bay (Colombia), its morphology and the flow rate fluctuation of the Dique Channel discharge throughout the year, modify the water exchange mechanism of the bay with the open sea through two entrances (Bocagrande and Bocachica). In arder to study the water exchange process, three different scenarios are designed and numerícally modelled using the Regional Ocean Modelling System (ROMS_AGRIF). The results show that fluctuations of the Dique Channel flow rate discharge strongly modify the underlying ebb-and-flood current regime through the entrances of the bay. Considering the annual increment of the river díscharge (from low to high flow rata discharge), the intensity of the average currents flowing out of the bay through Bocagrande can increase up to a 60 %, while currents entering the bay through Bocachica experiment variations between 23 % (in the deep navigational channel sector) and 50 % in the shallower section. During the high discharge rate season of the Dique Chanel, through Bocagrande entrance at the northwest of the bay, the water mass flux is generally directed out of the bay. This is largely dueto the presence of The Escollera (submersed dike), which is located between the Tierrabomba island and the mainland; it acts as a wall that prevents the influx of denser oceanic waters. At the same time, the mixed and less dense water on the surface is pushed out of the bay by the current induced by Dique Chanel. Two water mass exchange regimes were identified at the southwest of the bay (Bocachica). In the shallow water section, closest to the mainland, the regime is similar to that detected in Bocagrande, and also observed in the surface layers (0 to 6m) on the navigational channel in Bocachica. In the deep layers of the channel, the water flow is directed toward the bay.