Three beach and spit systems have been studied in northern Galicia, along the northwest coast of Spain, within the regional context of sea level rise and coastal erosion. Each system forms part of an estuary's mouth complex within the inner part of the Ortigueira, Barqueiro, and Viveiro rias. Erosive processes along the coast have been characterised and been found to affect cliffs, beaches, and archaeological sites. To compare both long-term (decadal) and short-term (annual) erosion and accretion trends among estuaries, we analysed the morphodynamic characteristics, the degree of human occupancy, and the historical evolution of each system and monitored two profiles at each beach. Dynamic parameters—dimensionless fall velocity and relative tidal range—were calculated for the characterisation of beaches, and maritime climate data—significant wave height (H_s), peak period (T_p), wave fetch direction, and wave period were obtained, and wave height reduction was estimated for all estuaries. Detailed analysis of the three systems shows that, having originated under similar geomorphic and natural dynamic conditions, they have evolved in totally different ways: balance or slight accretion at Ortigueira, erosion at Barqueiro, and strong modification of dynamics at Viveiro. Anthropogenic activity has been identified as the most influential factor, particularly because structures were made without considering the dynamic characteristics of the natural environment in the sites studied.