Estuary
An estuary is a partially enclosed coastal body of brackish water where freshwater from rivers or streams meets and mixes with saltwater from the open sea. Estuaries form transition zones between riverine and marine environments and are classified as ecotones, areas where different ecosystems overlap. They are influenced by both marine processes (such as tides, waves, and saline water intrusion) and fluvial processes (including freshwater flow and sediment input). The mixing of seawater and freshwater provides high levels of nutrients both in the water column and in sediment, making estuaries among the most productive natural habitats in the world.
Most existing estuaries originated during the Holocene epoch with the flooding of river-eroded or glacially scoured valleys when the sea level began to rise about 10,000–12,000 years ago. Estuaries can be classified using four fundamental criteria. These include salinity distribution, geomorphology, water circulation and vertical stratification, and system energetics. They can have many different names, such as bays, harbors, lagoons, inlets, or sounds, although some of these water bodies do not strictly meet the above definition of an estuary and could be fully saline.
Many estuaries suffer degeneration from a variety of factors including soil erosion, deforestation, overgrazing, overfishing and the filling of wetlands. Eutrophication may lead to excessive nutrients from nitrogen run off, sewage and animal wastes; pollutants including heavy metals, polychlorinated biphenyls, radionuclides and hydrocarbons from sewage inputs, and diking or damming for flood control or water diversion.
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