The hydrodynamics of tidal exchange and freshwater inflow are the principal physical forces that determine the mangrove/marsh ecotone in the coastal region of Everglades National Park. The effects of these hydrodynamics can be characterized by observing the soil and surface water gradients that exist between the coastal mangrove fringe and inland marshes. Near the Harney river, two surface/ground water wells were established to continuously monitor water levels and salinity. One well is located in the mangrove fringe, 30 meters from the river bank, the other well is 300 meters inland, within the coastal prairie. An elevated boardwalk spans the distance between the two wells and was used to collect intermittent grab samples of soil and surface water along the transect. Preliminary data from the two wells shows salinity differences, thus suggesting a horizontal salinity gradient due to tide and freshwater fluctuations. The mangrove fringe well is primarily influenced by the semi-diurnal tides with salinity values ranging from 35 mS in April 1996 to 10 mS in June 1996. The interior prairie well indicates dampened tidal influence with salinity ranging from 25 mS in April 1996 to 5 mS in June 1996. Intermediate soil and surface water observations have been used to quantify the horizontal salinity gradient along our transect. Small changes in the horizontal salinity gradient are suggestive of future vegetation changes of the mangrove/marsh ecotone. This study provides preliminary data in which future salinity fluctuations can be quantitatively monitored and changes in the mangrove/marsh ecotone can be evaluated.