The restriction of tidal flow to salt marshes, a common phenomenon in densely populated coastal areas, leads to marked changes in the hydrologic regime of the marsh, which can in turn pose water quality problems both within the marsh and for the larger estuary. We have found significant differences in marsh chemistry and water quality between a tide-gated (restricted flow) marsh and a nearby, unrestricted reference marsh. During the summer of 1995, the desiccation of the restricted marsh due to operation of the tide gate led to a disconnection among the sediments, the small drainage ditches, and the main channel. This allowed dramatic changes to take place in sediment chemistry, including lowered alkalinity and elevated nutrient levels. Re-connection of the sediments with the surface water as a result of rainstorms led to two episodes of severe acidification (pH 3−4) as well as to increases in surface water nitrogen concentrations. Clean-technique trace metal measurements showed that the acidic conditions led to mobilization of Pb, Cu, Ag, and Cd, with extremely high levels observed in the dissolved phase (>2000 ng/L dissolved Pb). Sulfide, sulfate, and chloride measurements indicated that the acidification was most likely the direct result of desiccation-induced oxidation of the reduced sulfide (e.g., pyrite) that had accumulated in the sediments. The seasonal operation of the tide gate in this marsh may contribute to the potential for acidification. In contrast to observations in other restricted marshes, low dissolved oxygen (DO) does not seem to be a problem in the restricted marsh:  DO concentrations were higher than in the reference marsh. The two marshes differed in the distribution of plant species but not in productivity or C:N ratios for a given species.