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How do internal waves affect the boundary between fresh water and salt water in an 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 an estuary, the boundary between fresh water and salt water is defined by a density gradient, often forming a distinct interface in salt-wedge systems or a more mixed zone in partially mixed estuaries. Internal waves occur on these internal surfaces where water density changes rapidly with depth.1234

These waves manifest as a regular rising and sinking of the water layers around which they center, rather than affecting the sea surface. By causing internal deformation of these density-stratified layers, internal waves contribute to the dynamics of the boundary. While the primary mixing in estuaries is often driven by tidal turbulence and river flow, the presence of internal waves reflects the sluggish, gravity-driven movement of these distinct water masses. In some cases, such as when a shallow, brackish upper layer exists, these waves can be associated with phenomena like "dead water," which influences the movement and interaction of the fresh and salt water layers.345