In Barkley Sound, British Columbia, Canada, the wide-bladed red alga Mazzaella splendens occurs intertidally at low to intermediate wave exposure sites but not at adjacent high wave exposure intertidal sites that are occupied by the narrow-bladed sister species Mazzaella linearis. This study used morphological and biomechanical characteristics of both species to determine whether or not drag and acceleration forces could prevent blades of M. splendens from surviving at high exposure sites and hence account, in part, for their distributions For each species, these hydrodynamic forces were calculated for gametophyte and sporophyte phases and, when possible, short and long thalli. The most frequent break location when thalli were pulled by a spring-scale was the junction between the stipe and holdfast. The following predictions were made by a model that compared hydrodynamic forces which a blade would experience to the measured force required to break the stipe/holdfast junction: (1) long blades of M. splendens should not occur at high wave exposure sites, (2) within the range of wave exposure occupied by M. splendens, sporophytes should be more abundant than gametophytes when and where wave exposure is greatest and (3) long blades of M linearis are predicted to occur at high wave exposure sites. All 3 of these predictions agree with other studies of natural populations. Two predictions do not agree with field observations: (1) long M. linearis gametophytes are predicted to have greater survivorship at higher water velocities than long sporophytes but, in natural populations, sporophytes are actually more abundant when wave action is greater and (2) short blades of M. splendens are predicted to survive very high water velocities but, in reality, are absent from high wave exposure sites in Barkley Sound. The latter contradlct~on suggests that recruitment of M. splendens at high wave exposure sltes is prevented at a life history stage prior to the development of short blades.