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Why does salt water form a wedge beneath fresh water in a salt wedge 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 salt wedge estuary, the formation of a salt water wedge beneath fresh water is primarily driven by differences in water density and the dominance of river discharge over tidal effects.1

Because salt water is denser than fresh water, it naturally sinks and forms a distinct, wedge-shaped layer along the bottom of the estuary. In these systems, the river flow is strong enough to keep the fresh water moving as a surface layer, while the salt water remains as a bottom layer that thins as it moves landward.12

The interface between these two layers is well-defined because there is minimal mixing between the fresh and salt water. While some salt water is entrained into the surface layer due to mixing at this boundary, the salt wedge is maintained by a slow, compensatory movement of salt water up the estuary at depth. This circulation pattern is a defining characteristic of salt wedge estuaries, where the high volume of river discharge effectively restricts the intrusion of marine water.21