The interest in the exploitation of the tidal stream energy has increased significantly over the last years and several tidal farms have been proposed. In spite of this, only a few studies dealt with the potential impacts on the environment resulting from the extraction of this energy, most of them by using 2D numerical models. However, some of the areas of interest for tidal stream exploitation, such as the Galician Rias, present complex transient and residual circulation patterns which in turn result in one of the largest oceanic productivities in the world and whose potential changes cannot be properly studied by means of 2D models. In this work, a 3D numerical model was implemented in Ria de Ortigueira, a promising region for tidal stream energy exploitation, to study the potential flow changes due to the operation of a power plant, including the assessment of the potential impacts on the 3D residual flow in a real estuary. First, the model was validated on the basis of current velocity measurements, then, it was used to describe the potential effects resulting from the operation of a previously proposed tidal stream farm during typical winter and summer conditions. For this purpose, the momentum sink approach was used. Overall, it was found that the resulting transient flow modifications were concentrated in the area occupied by and next to the farm, with nearly negligible effects outside the inner ria. Furthermore, important asymmetry effects were also observed; although the inner part of the estuary is flood dominated, the most important effects occur during the ebb as a result of the complex geometry of this area. Finally, the effects on the residual flow are of the same order, in terms of percentage of velocity variation, as in the case of the transient flow; however, they extend over a wider region, affecting the middle ria, where a complex 3D circulation pattern (a positive estuarine circulation) develops. Nevertheless, the operation of the tidal plant is not capable of modifying the general 3D flow structure in this area.