A peculiar feature of tidal channel networks is that their geometry (e.g. channel length, width, density) scales better with watershed area than tidal prism. Platform elevation has little control on network morphology, suggesting that relative platform deepening following sea level acceleration or sediment supply reduction will not cause significant changes to the channel network, despite field observations to the contrary. We explain this paradox with a numerical model of platform and channel network evolution that incorporates feedbacks between vegetation and physical sediment transport processes. In the model, platform deepening stimulates vegetation growth, leading to more stable channel edges. In this situation, channels deepen rather than widen, leading to channel morphologies that do not reflect tidal prism. In an environment where sea level or sediment supply changes have significantly altered the platform elevation, vegetation disturbance will lead to rapid network expansion as channels return to a form determined by tidal prism.