Understanding Estuaries: Types, Processes, Impacts

Definition and scope of estuaries

An estuary is a partially enclosed coastal body of brackish water where freshwater from rivers and streams meets and mixes with saltwater from the open sea. It is a dynamic transition zone (an ecotone) between riverine and marine environments, shaped by both marine influences (tides, waves, and saline water intrusion) and fluvial influences (freshwater flow and sediment input). Because seawater and freshwater mixing supplies nutrients to both the water column and sediments, estuaries are among the most productive natural habitats. The scope of estuaries is broad and not limited to a single strict form. A widely used definition describes a semi-enclosed coastal water body with a free connection to the open sea in which seawater is measurably diluted by land-derived freshwater. A more comprehensive definition extends the scope to systems connected to the sea up to the tidal limit or salt intrusion limit and receiving freshwater runoff, even if freshwater inflow is not perennial, the sea connection is seasonal, or tidal influence is weak. This broader view includes features such as fjords, lagoons, river mouths, and tidal creeks, all connected to the sea through which seawater enters with tidal rhythm.

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Estuaries as ecotones between river and sea

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