The effect of dilution on lake water quality is analyzed using steady-state mass-balance phosphorus models. The critical (that is, no-effect) phosphorus (P) concentration for dilution water exceeds the lake's predilution concentration by the factor 1/(1 – R), if the retention coefficient R is constant. On the other hand, if the P settling velocity is constant, then the critical dilution water concentration equals the lake's predilution concentration. Dimensionless response surfaces are presented for these alternative models. Data from many lakes support the first model. Applied to the Minneapolis Chain of Lakes, this model investigated water quality effects of augmentation for water level maintenance. Surprisingly, augmenting with Mississippi River water—for which facilities already exist—benefits water quality almost as much as would high-quality ground water.