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How does river discharge shape 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, river discharge is the dominant force, while tidal effects remain negligible. This high volume of river flow shapes the estuary by creating a distinct, well-defined interface between fresh and salt water, with very little mixing occurring between the two layers.1

The river discharge forces the salt water into a wedge-shaped body that is thickest at the seaward end and tapers to a thin layer toward the continent. The penetration of this wedge is directly controlled by the river's flow:21
* Low flow conditions: The salt wedge extends farther upriver.2
* Flood conditions: The increased river discharge pushes the wedge back toward the sea.2

Because river discharge dominates, it creates a significant salinity gradient and acts to block the entrance of seawater into the system. While mixing is minimal, the movement of fresh water over the salt wedge causes the surface layer to entrain some salt water, which is compensated for by a slow, landward movement of salt water at depth. This circulation pattern is ecologically vital, as it traps nutrients and stimulates biological production within the estuary.123