Detailed velocity measurements made with laser Doppler velocimetry have shown that, except very close to the bed, the production of turbulence within a stand of emergent vegetation is dominated by the stem wakes rather than by the bottom-boundary shear, as in open-channel flows. This observation formed the basis for a modified randomwalk model that describes the contribution of stem wakes to the turbulent diffusivity within marsh grasses. The model was validated by comparison to observed diffusivity over a range of population and flow conditions within a simple plantlike array of circular cylinders. The diffusion model was also evaluated for a more complex morphology that included a flexible canopy. Laser-induced fluorescence and image-processing techniques were used to measure the diffusivity as well as to examine turbulence structure within the experimental system. The latter analysis documented changes in turbulcncc scale that arise as larger eddies are broken apart by the stems and smaller eddies (comparable to the stem diameter) are produced within the wakes.Field observations demonstrate that submerged and emergent aquatic vegetation can baflle local currents and dampen wave energy by providing a new source of drag associated with the plant stems and branches (Jackson and Winant 1983;Ward et al. 1984;Gambi et al. 1990;Leonard and Luther 1995). When the flow is reduced, the production of turbulence is also reduced, leading to diminished turbulent diffusivity within the canopy (Raupach and Thorn 1981;Worcester 1995). These changes in advection and diffusion influence the dispersal and settlement of seeds and larvae and thus can affect recruitment, reproductive success, and genetic diversity (Okubo