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How do spits create lagoons and partially enclosed 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).

Spits are narrow coastal landforms composed of sand or shingle that form through the longshore movement of sediment. They typically develop where the coastline abruptly changes direction, often extending across the mouths of estuaries or from headlands.1

Spits create lagoons and partially enclosed estuaries by acting as a physical barrier that separates a body of water from the open sea. As sediment is transported along the shore, a spit may grow until it forms a bay-mouth barrier, effectively enclosing the water behind it. This sheltered area becomes a lagoon or a partially enclosed estuary, where fresh water from rivers mixes with seawater. Because these features trap nutrients and facilitate the mixing of water, they often function as highly productive estuarine environments. In these instances, the definitions of lagoons and estuaries frequently overlap.23