In the eastern temperate region of North America, treed headwater swamps are a familiar watershed feature. These low-gradient wetlands commonly exist at groundwater discharge sites and represent a link between the underlying groundwater system and the surface drainage system. In contrast to the extensive literature pertaining to the hydrologic modeling of agricultural and forest land classes, little attention has been focused on the development and testing of numerical simulation models for predicting the hourly stormflow response from headwater wetland sites. If required to predict the rate of outflow from a wetland-dominated catchment, the hydrologist or engineer has few numerical tools and little data available to assist in the prediction. The objective of this research was to investigate the feasibility of applying a numerical model to simulate the rainfall-runoff response from a treed headwater wetland site. The wetland model utilizes a hydrology model coupled to a hydraulic stream-routing model. A depth-averaged laminar flow model is used to simulate the horizontal movement of stormwater both through and over the wetland sediments. The development and testing of the wetland model were completed in conjunction with a data collection program in which hydrometric and meteorologic data were obtained at a 400-ha first-order headwater swamp located within the Teeswater River watershed in southern Ontario, Canada. An analysis revealed that the simulated wetland streamflows were sensitive to the antecedent saturation of the wetland sediments, the storage and flow transport characteristics of the wetland sediments, and the conveyance capabilities of the wetland channel system. ¿ 1999 American Geophysical Union