Spatial heterogeneity plays a crucial role in species coexistence. Despite the importance of self-organization in creating environmental heterogeneity in otherwise uniform landscapes, little is known about the effects of self-organized pattern formation on species coexistence and facilitation. We investigated the effects of large-scale pattern formation on species interactions using the aquatic macrophyte Callitriche platycarpa in streams as a model system. Specifically, we focused on the effects of Callitriche on other macrophyte species through flow velocity modification and turbulence generation. Our theoretical model predicted that feedback interactions between plant growth and water flow could promote species coexistence by creating spatial heterogeneities in hydrodynamic conditions. Field observations and transplantation experiments in naturally vegetated rivers supported this hypothesis: we found a significant spatial aggregation of two macrophyte species around C. platycarpa and a facilitative effect on their growth through hydrodynamic stress reduction. Next to flow velocity modification, we explored the effects of turbulence generation by macrophytes on species interactions. Flume experiments showed that the dense patches of Callitriche had a strong hydrodynamic effect, creating high-turbulence regions that facilitated nutrient uptake by a sparser species with weaker hydrodynamic effects. Our results emphasize the importance of self-organized patchiness in promoting landscape-scale species coexistence and influencing ecosystem functioning. Understanding the link between spatial pattern formation and facilitation is therefore essential for successful management of ecosystems.