Arithmetic averages of daily temperature and current surveys during July--August 1972 are used to define a mean summer coastal circulation pattern in Lake Ontario. Some of the important features o this mean flow pattern may be explained by simple equilibrium models of wind-driven flow. Discrepancies between these models an coastal zone observations appear to be due mainly to advection of momentum during transient flow episodes, especially those involving the intermittent appearance of full upwelling along the north shore. The south shore flow pattern consists of divergent coastal jets, much as the theoretical steady response of a frictionless two-layer basin to an eastward wind. At the eastern end of the lake a deep current is generated by interface friction below the strong coastal jets. Along the north shore there is no mean coastal jet pattern, and the return flow of warm water must be supposed to be accomplished some distance offshore.