We studied a Great Lakes peatland protected by a barrier dune system to test the hypothesis that changes in the morphology of a barrier beach alter the wetland surface-water and ground-water hydrology such that the plant rooting zone in the wetland interior might be affected. Hydraulic head measured in observation wells located on transects perpendicular to a flow-through channel, uplands, and Lake Ontario were compared to lake-level changes, weather patterns, and the temporal conditions of the barrier beach. Water-table elevations within the wetland complex always remained 0.35 to 0.84 m above Lake Ontario's water level (mean difference = 0.62 m) and did not fluctuate in parallel with changes in the lake's water level. Flooding events occurred when breaches closed and isolated the wetland complex from Lake Ontario; daily average water levels rose between 0.06 m and 0.27 m. When the barrier beach was breached naturally or by human activity, daily average water-table elevations in the wetland receded between 0.16 m and 0.41 m. During the growing season, vertical ground-water-flow directions also changed as a function of changing hydraulics controlled by the barrier beach. Vertical recharge-discharge gradients ranged from −0.2 to 0.1 m·m⁻¹. Water-table elevation and hydraulic head gradients across the study area were controlled primarily by short-term (days to weeks) changes in shoreline geomorphology, whereas seasonal weather patterns and Lake Ontario water-level fluctuations imposed secondary controls on the ground-water-flow regime.