In 1988 and 1989, open-marsh water management modifications were implemented at tidal marshes near West Onslow Beach and Hobucken, North Carolina, as part of a pilot program to evaluate the effectiveness of ditching techniques as a mosquito-control method in open marshes. In 1984, before implementation of the modifications, a study was initiated to allow definition of the effects of those modifications on the hydrology of the marshes. Water levels in canals near the West Onslow Beach study marsh are controlled by periodic, gravitational tides. Daily maximum tides exceeded the elevation of the upper marsh surface 30% of the time before and 18% of the time after open-marsh water management. Daily maximum tides at this marsh exceeded the upper marsh surface 34% of the time before and 24% of the time after open-marsh water management. Variation in tidal conditions resulted in varying numbers and duration of floods at the study marshes. Duration analyses indicated relations between tide levels and marsh surface-water levels were unchanged after modifications. Groundwater movement through the marshes varies seasonally and is primarily vertical. Withdrawals are by evapotranspiration and recharge is by infiltration. During nongrowing months saturated conditions prevail. Groundwater flow to the marsh interior from the surrounding tidal canals was not detected during these declines. Changes in the natural variation in withdrawals from and recharge to groundwater were not indicated by the data collected during this study. Water levels in canals adjacent to the Hobucken study marsh are primarily controlled by wind-driven tides.
