When rivers are impounded, the reduction in downstream flow can produce important and often adverse effects, especially in the estuarine environment. One or more dams have been proposed for the Olifants River system in the Western Cape, South Africa. This estuary has an extensive area of salt marsh that was examined to see whether it required occasional flooding with freshwater to wash out accumulated salts. The results showed that the dominant salt marsh species, Sarcocornia pillansii agg., occurred where the water table was shallowest and the soil moisture highest. S. pillansii has a wide salinity tolerance range (0 – 70 ppt) and the plants are tolerant of prolonged periods of inundation (> 15 weeks), enabling them to occupy waterlogged and / or hypersaline soils to obtain moisture. Aerial photographs and simulated runoff data showed that no flood had covered the floodplain during the previous 80 years. The conclusions from the data indicate that salt marsh plants use saline groundwater during the dry months of the year in order to survive, but utilise the short season winter rainfall period with low salinity conditions to germinate, grow and reproduce. Live roots of S. pillansii were found at the level of the water table. Plant tissue and soil water potentials, the increase in vegetation cover with a decrease in water table depth and lower electrical conductivity of the groundwater support the conclusion that saline groundwater is the only source of water to the plants during the drier months of the year. Groundwater hydrology and floodplain geomorphology ultimately determine the distribution of S. pillansii by affecting depth to the water table and the salinity of the groundwater. However, freshwater flooding of the river in winter is likely to be critical to survival because it covers the supratidal area with less saline water and reduces the depth to the water table on the floodplain, making the groundwater more accessible to the halophytes growing on the floodplain.