Equations of mass (or volume) continuity and salt continuity are developed for the two- and one-dimensional estuary from the basic forms of these equations in three dimensions. The three cases of vertical, lateral, and sectional homogeneity are treated. Some misuses of the continuity concepts as applied to estuaries have appeared in recent literature. These are discussed. The effect of phase difference between tidal slack water and high tide on the nontidal drift in sectionally homogeneous estuaries is presented.