Effects of waves on aspects of tides which determine the degree of stress to which intertidal organisms are subjected are assessed and quantified. The difference between actual and predicted time of tidal flooding and ebbing under different wave height conditions was measured on 3 distinct, adjacent topographies along a wave-exposed shore on the southwest coast of Vancouver Island. B. C., Canada. Tidal curves which accurately reflected submersion/emersion events were then generated for the 3 transects. Vertical distributions of the conspicuous intertidal algal species were determined along the transects, and related to the empirically determined tide curves. Wave action caused changes in submersion/emersion events from those predicted by tidal data; the extent of these changes depended upon topography and season. Algal distributions on the three transects over 6 yr showed a significant correlation with some aspects of their submersion/emersion histories, indicating that these events are important in influencing the vertical distribution of intertidal seaweeds.