Coastal Processes, Hazards, and Society




What is an estuary? How do estuaries form? What sort of processes takes place in estuaries?

Definition and Morphology of Estuaries

Coastal geologists define an estuary as a semi-enclosed body of water with an open connection to the ocean and one or more rivers flowing into it. They represent a transitional environment between the solid mainland and the sea and because of the inflow of rivers are partially diluted with fresh water so that they do not contain normal salinity marine water. As a transitional environment, they are influenced by marine processes, such as waves and tides, as well as river processes, such as the delivery of sediment and fresh water.

The Importance of Estuaries

Some of the world's most productive ecosystems are located within estuaries and host a wide range of organisms. In fact, many species of commercially important fish and shellfish spend part of their life cycle within estuaries before reaching maturity. They are, however, an environment that, like many other coastal environments, faces a wide range of environmental problems arising from land-use practices, dumping of sewage and other pollutants, and the introduction of excess nutrients because of poor agricultural practices.

Historically, estuaries have been classified a number of ways, including how they formed and their morphology, the circulation patterns that are present within the estuary, and the relative importance of waves and tides within an estuary. In this module, we will only be discussing estuary formation and circulation patterns of classification.

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Picture from space of the Humber Estuary on the north-east coast of Britain. The inland drainage basin covers one-fifth of Britain and is the single largest source of freshwater input to the North Sea from the English coastline. It is the most densely populated European estuary, with approximately 500 people/km2.
Credit: NASA
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Land-based picture of the Humber Estuary in Britain showing extensive internal mudflats that are the result of large tidal range, 5.9 m at the mouth of the estuary, and 7.2 km approximately 35 km farther inland. These mudflats provide important feeding and roosting grounds for numerous migratory and wintering fowl, as well as nursery habitat for numerous fish. Humans have been modifying the lands in and around the estuary for more than 2000 years, and it is estimated that approximately 50% of the intertidal lands in the estuary have been lost.