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Why do organic and inorganic particles settle out where river water meets seawater?

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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).

When river water meets seawater in an estuary, organic and inorganic particles carried by the river tend to flocculate, or aggregate into larger masses, upon encountering the salt water.1

This process of flocculation causes the particles to increase in size and weight, leading them to settle out of the freshwater layer and sink toward the bottom. Once these particles reach the bottom, they may be carried upstream by the circulation of the salt wedge, where they contribute to the nutrient-rich sediment that supports various estuarine ecosystems, such as salt marshes and mangrove forests. The deposition of this sediment is also influenced by the dynamics of the system, including the interaction between river discharge and tidal currents, which can diminish water velocity and facilitate the settling process.12