AbstractDye‐release and numerical modeling experiments are conducted to investigate horizontal dispersion in a partially mixed estuary. Longitudinal dispersion of a dye patch shows marked flood‐ebb asymmetry in the first two tidal cycles after a dye release, with most of the dispersion occurring during ebb tides. There are large differences in the dispersion rate between spring and neap tides. Due to increased vertical mixing during spring tides, a dye patch quickly extends from the bottom to the surface and is exposed to the vertical shear in the entire water column, enhancing longitudinal dispersion. In contrast, most of the dye patch is limited to the bottom few meters during the neap tides. Although decreased vertical mixing during neap tides facilitates longitudinal dispersion, the vertical shear across the thin dye patch in the bottom layer is much weaker than the full water column shear in spring tides. Decreased shear leads to reduced longitudinal dispersion. After four tidal cycles, the second moment of the dye patch increases with time in the along‐channel direction at a power of between 2 and 3. The longitudinal dispersion rate varies as the four thirds power of the dye patch size, indicating scale‐dependent diffusion.