We conducted a field and modeling study in northwestern Osaka Bay, near the Akashi Strait, to investigate the mechanisms that entrain Yodo River freshwater across the tidal front that separates the stratified and well-mixed halves of this bay. We find that an eddy, generated by the topography of the Akashi headland during the eastward tidal current, horizontally mixes freshwater across this front. Along with advection of Yodo River water to the Harima Sea through the Akashi Strait during the westward tidal current and mixing induced by the tidal jet, the mass balance of freshwater in eastern Osaka Bay is approximately closed. This indicates that the region near the Akashi Strait is where most of the Yodo River's freshwater leaves eastern Osaka Bay. Throughout the rest of Osaka Bay, therefore, very little freshwater likely crosses the tidal front, substantiating the theory of previous researchers that eastern Osaka Bay is in geostrophic equilibrium.
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