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Water-levels and salinity were measured in seven shallow (ca. 2 m deep) wells installed at distances proximal, medial, and distal to the source of tidal flooding between 2017 and 2019 in a warm-season grass meadow adjacent to a salt marsh. Water-table fluctuations greater than 10-cm were associated with seawater, precipitation, or a combination of the two. When the field was flooded by tides (> 0.5 m above predicted), groundwater salinity increased; when the field was flooded by precipitation (> 2.5 cm), the salinity of the groundwater decreased. The increased head gradient that accompanied the rise in the water table appeared to be sufficient to allow the freshwater from precipitation to push the salt water down and towards the marsh creek, resulting in a freshening of the groundwater that persisted until the next saltwater flooding event. Thus, the relative frequency between saltwater flooding, salinization, freshwater flooding, and flushing controlled the groundwater salinity. These findings indicate the importance of high-tide events in the process of salinization of the groundwater and the ameliorating effects of rainfall events whose magnitude is sufficient to increase groundwater elevation at least ten centimeters. Further, they contribute to a growing body of evidence in support of the interaction between fresh- and saltwater flooding events to enhance the salinity of groundwater and drive ecosystem transition from uplands to salt marshes.