River runoff into Ría de Viveiro presents great seasonal variability due to the region's climate—but always in the context of discharge values that are small relative to the size of the estuary. This work has two main objectives: (i) to implement the first numerical model of ría hydrodynamics and (ii) to determine whether the river discharge variability has significant effects on the system and, if so, which. The numerical model, which takes into account all driving agents (tides, river discharge, and winds), is successfully validated based on sea level and flow velocity observations, with very high correlation coefficients. It is then applied to different study cases representative of typical summer and winter scenarios. Three main conclusions can be drawn: (i) The flow is primarily tide driven. (ii) However, the river discharge is not without significance—it causes ebb–flood asymmetry with ebb dominance. (iii) The main consequence of the seasonal variability of river discharge is the seasonality of the ebb–flood asymmetry itself. Weak and essentially confined to the inner ría in summer, it gains strength in winter and extends to the outer ría, where different flood and ebb patterns arise.