The generation of tidal asymmetries in estuaries is analyzed by examining the differences in celerity at high and low tide. An analytical expression for the difference between ebb and flood durations is introduced and applied to a cross section composed of a channel flanked by tidal flats. The expression depends on the cross-sectional average of the celerity, for which two approximations are proposed and verified with a numerical model. Results confirm that tidal flats enhance ebb dominance, whereas large tidal amplitudes promote flood dominance. Maximum ebb dominance occurs for tidal flats at or above mean water level, depending on tidal amplitude and the extent of the tidal flats. Finally, qualitative arguments indicate that friction also enhances flood dominance.