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Why do the definitions of lagoons and estuaries sometimes overlap?

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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).

The definitions of lagoons and estuaries overlap because both terms can describe coastal bodies of water where fresh water and seawater interact, particularly when depositional features like spits or barrier beaches enclose an area.12

An estuary is fundamentally defined by the mixing of river water and seawater, creating a gradient of salinity. When barrier islands or spits restrict the exchange between the open ocean and the continental margin, they create partially enclosed bays or lagoons. If these lagoons receive sufficient freshwater runoff, they develop the chemical characteristics of an estuarine environment, leading to the overlap in terminology.12

However, the two features are not always synonymous. While estuaries are defined by their mixing dynamics, lagoons can vary significantly in salinity—ranging from brackish to hypersaline—depending on the balance between evaporation and freshwater inflow. In some arid regions, lagoons may even exhibit circulation patterns that are the reverse of those found in typical estuaries.3