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A method was developed to estimate the lateral effects of a single drainage ditch on wetland hydrology. The method can be used to calculate the distance of influence of a single ditch constructed through a wetland, where the distance of influence is defined as the width of a strip adjacent to the ditch that is drained such that it would no longer satisfy wetland hydrologic criteria. Simulation analyses were conducted with DRAINMOD to define the minimum, or threshold, drainage intensity that would result in failure of a wetland site to satisfy the wetland hydrologic criterion. Analyses were conducted for five hydric soils spanning a wide range of profile hydraulic transmissivities. The analyses were conducted for climatological conditions for three locations in eastern North Carolina. Results for Wilmington, North Carolina showed that the threshold drainage intensities would result in water table drawdown from an initially ponded surface to a depth of 25 cm in approximately 6 days. That is, drainage intensity sufficient to lower the water table from the surface to a depth of 25 cm in a threshold time of about 6 days would result in hydrologic conditions that would just barely satisfy the wetland hydrologic criterion for that location. The threshold time required to lower the water table to 25 cm depends somewhat on drain depth, but it was the same for all five of the soils examined. Similar results were obtained for the other two locations but, because of differences in weather and in the growing season, the threshold time required for the 25 cm drawdown was dependent on location. Water table drawdown can be predicted as a function of distance from a single ditch using methods previously published (Skaggs, 1976). Therefore it is possible to estimate the lateral effect of a single drainage ditch by a relatively simple analysis based on previously published solutions to the Boussinesq equation.