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Why does fresh water float above salt water in many 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).

In an estuary, fresh water floats above salt water primarily due to differences in density.

Because seawater contains dissolved salts, it is significantly denser than the fresh water flowing from rivers. In systems such as salt-wedge estuaries, where tidal mixing is minimal, this density contrast creates a stable, stratified water column. The less dense fresh water remains as a distinct surface layer, while the denser, more saline water forms a wedge that extends along the bottom of the estuary toward the land.12

The degree of this stratification depends on the balance between river discharge and tidal energy. While strong tidal currents can generate turbulence that partially or fully mixes these layers, the fundamental physical principle remains that higher salinity increases the density of water, causing it to sink beneath fresher, less dense water.324