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loss, which affects the configuration of ground- water flow systems as well as how ground water interacts with surface water. In the parts of coastal landscapes thaf are affected by tidal flooding, the interaction of ground water and surface water is similar to that in alluvial valleys affected by flooding. The prin- cipal difference between the two is that tidal flooding is more predictable in both timing and magnitude than river flooding. The other signifi- cant difference is in water chemistry. The water that moves into bank storage from rivers is gener- ally fresh, but the water that moves into bank storage from tides generally is brackish or saline. . Tidal mangrove wetland in Florida. (Photograph by Virginia Carter.) Estuaries are a highly dynamic interface between the continents and the ocean, where discharge of freshwater from large rivers mixes with saline water from the ocean. In addition, ground water discharges to estuaries and the ocean, delivering nutrients and contaminants directly to coastal waters. However, few estimates of the location and magnitude of ground-water discharge to coasts have been made. In some eSfuaries, sulfate-rich regional ground water mixes with carbonate-rich local ground water and with chloride-rich seawater, creating sharp boundaries that separate plant and wildlife communities. Biological communi- ties associated with these sharp boundaries are adapted to different hydrochemical conditions, and they undergo periodic stresses that result from inputs of water having different chemistry. The balance between river inflow and tides causes estuaries to retain much of the particulate and dissolved matter that is transported in surface and subsurface flows, including contaminants. "Ground water discharges to estuaries and the ocean, delivering nutrients and contaminants directly to coastal waters" 43