This work addresses the effects of the construction of a reservoir 19 km from the mouth on the dynamics of the Guadalfeo delta (southern Spain), a Mediterranean delta in a semiarid and high‐mountain basin. The sediment volume transported as bed load and accumulated in the delta was estimated under two scenarios by means of a calibrated hydrological model: a managed scenario, considering the flows drained by the dam, and an unmanaged scenario, considering the absence of such infrastructure. Bathymetric and topographic measurements were analyzed and correlated with the fluvial and maritime forcing agents. Results indicate that the reservoir has significantly modified the dynamics downstream: the coast has lost almost 0.3 hm³ of sediments since the entry into operation of the dam, generating a 1.4 km coastline retreat around the mouth, with a maximum retreat of 87 m (92% of the initial). The beach profile decreased by up to 820 m², whereas the average decrease around the mouth was equal to 214 m². Under unmanaged conditions, more than 2 hm³ of bed load would have reached the coast. Based on the results, three new management scenarios of flows drained by the dam, in combination with bypassed sediment from the reservoir, were proposed to prevent more severe consequences in the delta and the silting of the reservoir. The proposed methodology for new management scenarios can be extended to other worldwide deltas, especially to those in semiarid and Mediterranean basins, and it represents an advanced tool for decision making.