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How does the boundary between fresh water and salt water differ in salt wedge estuaries and partially mixed estuaries?

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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).

The boundary between fresh water and salt water varies significantly based on the degree of mixing within an estuary.

In a salt wedge estuary, river discharge dominates while tidal effects are negligible. This results in a well-defined, distinct interface between the two water types. Fresh water flows as a surface layer that thins toward the ocean, while a wedge-shaped body of seawater underlies it, thinning toward the continent. Very little mass transfer or mixing occurs across this boundary.12

In contrast, a partially mixed estuary experiences stronger tidal currents that generate significant turbulence. This turbulence promotes prominent mixing between the fresh water above and the salt water below. Consequently, the boundary between the two water types becomes much less distinct than in a salt wedge system, with mass transfer occurring in both directions across the interface.32