Freshwater mussels are dominant ecosystem engineers in many streams throughout North America, yet they remain among the world’s most imperiled fauna. Extensive research has quantified the ecological role of mussels in aquatic habitats, but little is known about the interaction between mussels and their surrounding physical and hydrodynamic habitat. Here, the biophysical interactions of mussels on near-bed flow are investigated by quantifying the effect of mussel density on near-bed turbulent flow in an experimental channel. The results show that (1) mussels disrupt the distributions and magnitudes of flow velocity and Reynolds stress depending on mussel density, and (2) at densities of 25 mussels m-2 and greater, a hydrodynamic tipping point (skimming flow) is achieved where the maximum Reynolds stress is displaced to the height of the mussel canopy and near-bed shear stresses are reduced by as much as 88%, thus markedly decreasing the dislodgement potential of the mussels. These results provide strong empirical evidence for a positive density-dependent effect related to flow-organism interactions and their ecological success, such as enhancing river bed habitat complexity or reducing the physical forces required to dislodge mussels from the river ed. Such information enhances the understanding of the long-term persistence of mussel beds and helps focus conservation strategies.