On bimodal microtidal beaches, there are grain-size segregations and the relationship between mean size and foreshore slope is not clear. Dealing with macrotidal beaches composed of gravel and sand, the dynamics vary in response to the tide: reflective conditions at high tide, and a dissipative behaviour during low tide. These dynamic changes are related to grain-size segregations: the upper beach is dominated by gravel (or gravel with coarse sand) and the low-tide terrace is exclusively composed of fine sand. Along the Atlantic coast of Tierra del Fuego, nineteen beaches analysed vary in mean grain size between −4 and 4 phi units. The slope of the foreshore (reflective portion) is linearly related to grain size. The slope of the low-tide terrace (dissipative portion) has no relationship to grain size (fine to very fine sand). These segregations cause temporal changes in the composition of the groundwater sceping at the low-tide terrace. Test experiments performed at the beach permitted to recognise the relationships between coastal and ground water. Salt groundwater similar to the sea (31 practical salinity units; psu) seeps at the initial stages of the dropping tide. Seeping groundwater salinity diminishes progressively, and at low tide it arrives to a minimum of 10 psu. Tidal effects are therefore conditioning (chemically and physically) the water exchange between the beach and the nearshore areas.