Bays and Estuaries

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Bays and Estuaries


The terms bay and estuary are commonly taken to mean the same thing, namely a body of water (usually sea water or water that is part of a large lake) that is surrounded on all sides by land, but open to the sea or adjoining large lake on one side. The distinction made by some people between a bay and an estuary is that an estuary has an existing connection to one or more flowing rivers, whereas a bay does not necessarily have that connection. There are many famous and well-known bays, including San Francisco Bay and the Bay of Biscay. Famous and well-known estuaries include Chesapeake Bay and Puget Sound.

Historical Background and Scientific Foundations

Bays and estuaries are common features of marine shorelines that have been affected by rising sea level. Because sea level has been rising on average since the end of the last glacial stage (or ice age), most of the world’s marine shorelines have been affected in this way. Shorelines affected by rising sea level are commonly referred to as submerged shorelines. The term submerged refers to the fact that shoreline configuration generally follows the shape of pre-existing landscapes that have been slowly submerged by rising sea level. For example, a river or glacial valley that existed near the shoreline in the past would now be filled by seawater because of global sea level rise since the end of the last glacial stage. A submerged river valley may form a bay or an estuary, whereas a submerged glacial valley is referred to as a fjord (a type of bay or estuary).

Other bays and estuaries are formed by processes unrelated to sea-level rise, including tectonic effects (for example, fault movements), meteorite impacts, volcanic processes, and local subsidence of the land. Intensive geological investigations are required to understand the origin of bays and estuaries, including surface mapping, subsurface sampling (for example, core drilling), and geophysical studies (including seismic and gravity surveys). Some bays and estuaries have had a long and complex geological history, while others’ histories are rather simple for scientists to determine.

Impacts and Issues

At the boundary between marine waters and the land, most marine bays and estuaries have special conditions, are home to specialized species of organisms, and require special protection as unique environments. The special conditions of bays and estuaries are related to water chemistry and turbidity. Bays and estuaries contain waters that are mixtures of marine and fresh in terms of salinity and acidity (pH). Turbidity, fine sediment in river water that comes from land, is mixed with non-turbid marine waters in bays and estuaries. This mixing occurs over time, but initially, the less dense fresh but perhaps turbid water from land moves over more dense marine water from the sea. This results in a density layering that mixes over time. Special conditions of the local environment may promote or delay the mixing. Also, high evaporation in the bay or estuary may cause limited or no river input into the bay or estuary and higher than normal salinity in surface waters, thus setting up a situation where marine waters are less dense than those in the bay or estuary. In that situation, marine water enters and flows over more dense water already in the bay or estuary.

Specialized species of organisms inhabit the shorelines in general and bays and estuaries in particular. These species must be able to thrive on changing salinity and pH, as well as turbidity changes and tidal changes in sea level on a daily basis. Many of these species have


EFFLUENTS: Waste materials, often as outflow from septic or water treatment systems, that are discharged into the environment.

SUBMERGED SHORELINE: A shoreline formed by the submergence of a landmass, characterized by bays, promontories, and other minor features.

TURBIDITY: A measure of the degree to which water loses its clarity due to the presence of suspended particles.

niches in the bay or estuarine environment that may be imperiled by natural or human-made changes in the ecology of the bay or estuary. Special protections are required for use of bays or estuaries. For example, circulation in general is typically more complex within bays and estuaries than in other types of shorelines, and for that reason, it must be carefully studied before allowing human-made effluents (for example, sewage and storm water discharge) to enter a bay or estuary. As human populations increase, there is increasing use and modification of bays and estuaries for habitation, recreation, and places to discharge solid waste and wastewater.

See Also Coastal Ecosystems; Oceans and Coastlines



Christopher P. White. Chesapeake Bay: Nature of the Estuary. Ithaca, New York, Cornell/Tidewater Publishers, 1989.

Web Sites

Narragansett Bay. “Estuarine Science.” (accessed March 26, 2008).

David T. King Jr.