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How do tides create partial mixing in a partially mixed estuary?

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Diagram showing four main types of estuaries: salt wedge, partially mixed, vertically homogeneous, and fjord. Each panel illustrates river and seawater mixing patterns, with arrows indicating the direction of saltwater and freshwater flow. — estuary Four main types of estuaries: (A) salt wedge estuary, (B) partially mixed estuary, (C) vertically homogeneous estuary, and (D) fjord (black arrows indicate salt water and white arrows fresh).

In a partially mixed estuary, the vigorous rise and fall of the tide generates strong turbulence, which is the primary mechanism for mixing fresh water and seawater.12

Unlike salt-wedge estuaries where river discharge dominates, the increased tidal effect in a partially mixed system prevents river flow from controlling the environment. This turbulence facilitates the mass transfer of water in both directions across the boundary between the fresh and salt water layers, causing the interface to become less distinct. As a result of this mixing, the river flow entrains a significant volume of salt water—often 10 to 20 times its own volume—which triggers a compensatory landward flow of seawater along the bottom of the estuary.21