Mussels rely on a strong byssal attachment to persist in a range of habitats with differing rates of water flow. Recent studies, however, suggest that the ability of one mussel species to sense and respond adaptively to the flow in its environment is limited under even modest flow conditions because the process of byssal thread formation is disrupted. This study extends these findings to four mussel species, Mytilus trossulus, M. galloprovincialis, M. californianus, and Modiolus modiolus. Collectively, the response of byssal thread formation decreased with rates of flow above ∼25 cm/s and the critical flow threshold was estimated to be <50 cm/s. How can mussels persist on shores where flow is an order of magnitude higher? Using a combination of techniques for measuring flow, velocity profiles were obtained above and within mussel aggregations in the laboratory and in the field. Flow was greatly reduced within mussel aggregations, ranging from 0.1% to 10% of free-stream velocity. These results suggest one key to the success of mussels in habitats with high rates of flow is the ability to form aggregations that ameliorate flows to a level that is conducive to byssal thread formation.