When planning the installation of a tidal farm, the disturbances on the marine environment associated with its operation must be studied in detail. The objectives of this paper are to assess the impacts on the hydrodynamics (water level and flow velocity) and to determine how these impacts can alter the tidal resource. For this purpose, a high-resolution model of Ria de Ribadeo (NW Spain) is used to describe the potential effects resulting from the operation of two prospective tidal farms. Two different scenarios of extracted power from the flow (high and low) are analyzed. Overall, it is found that the impact on the water level is negligible, but that on the flow velocity is significant. The velocity is reduced upstream and downstream the farm, and increased beside it. These effects are enhanced in the scenario with the higher power extraction. Finally, these modifications in the flow pattern alter the available energy density at the tidal turbine, with a reduction of 21% and 12% for the high and low levels of power extraction, respectively.