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How do barrier islands create estuarine environments?

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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).

Barrier islands create estuarine environments by acting as depositional features that enclose large areas of water between the open ocean and the continental margin. By forming a physical barrier, these islands and associated spits restrict the free exchange of water between the sea and the land.1

This enclosure creates lagoons or partially enclosed bays where river water and seawater meet and mix. Because these areas trap fresh water from rivers and salt water from the ocean, they develop the characteristic salinity gradients and chemical properties of an estuary. In these settings, the definitions of lagoons and estuaries often overlap, as the restricted circulation allows for the dilution of seawater with fresh water, which is the defining feature of an estuarine environment. Notable examples of such systems include Galveston Bay in the United States and the Waddenzee in the Netherlands.12