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Why do salt marshes form in temperate and subarctic 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 temperate and subarctic estuaries, salt marshes form primarily due to the accumulation of inorganic sediments that provide an enriched substrate for specialized flowering plants adapted to saline conditions.1

These marshes develop in areas where the rate of sediment accumulation is equal to or greater than the rate of land subsidence and where there is adequate protection from disruptive waves and storms. The process is facilitated by the interaction between river-borne particles and salt water; as these particles encounter salt water, they flocculate and settle on the bottom, creating the necessary environment for salt marsh vegetation.21

For these marshes to persist, the intertidal land must be subject to regular flooding and draining by seawater, as salt marsh grasses cannot survive on permanently flooded flats or in areas where strong currents make the substrate unstable. Once established, fibrous-rooted grasses help bind the mud together, further stabilizing the environment and encouraging the continued growth of the marsh.3