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How does river flow limit seawater from entering 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 a salt wedge estuary, river flow acts as a primary control on the penetration of seawater. Because fresh water is less dense, it flows over the top of the denser seawater, forming a distinct layer. The volume and velocity of this river discharge determine the extent of the salt wedge: during high-flow or flood conditions, the force of the river pushes the wedge toward the sea, causing it to retreat.12

Conversely, during periods of low river flow, the seawater wedge is able to extend farther upriver. In systems where river discharge dominates, the freshwater flow effectively blocks or limits the inland movement of seawater, maintaining a sharper salinity gradient between the two water types. This dynamic interaction is fundamental to the classification of estuaries, as more substantial fluvial activity generally results in weaker mixing and a more restricted landward reach of marine waters.12