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Estuaries & Estuarine Circulation

Estuaries are coastal bodies of water have mixed fresh and salty water. The water in estuaries is mixed because they are fed by rivers but also connection with oceans. Estuaries have a barrier to the sea, similar to lagoons, that protects them from ocean energy, but, unlike lagoons, estuaries still connect with the sea. Because of this connection with the sea, estuaries are strongly influenced by tides (NOAA).

Estuaries’ unique environment makes them a great place for humans and animals. Humans benefit from the beauty of estuaries and the food and resources they provide. Estuaries make great tourism attractions or harbors and ports. The mix of fresh and saltwater makes a great feeding ground and habitat for many types of wildlife (EPA).

This water mixture also results in a mixture of types of sediment. Though estuaries have sediment from both upstream rivers and from the ocean, most of the sediment is relatively fine grained (McNally, pg. 3). The type of sediment, and the abundance of it, affects human and animal involvement with estuaries. Too much sediment makes it hard for plants to photosynthesis and limits human use of the estuary since sediment can be deposited, making the estuary unsuitable for ship traffic. If there is too little sediment in the estuary then the land bordering it may be eroded away and increase predation rates in some species (McNally, pg. 2).

There are four main types of estuaries: coastal plain estuaries, tectonic estuaries, bar built estuaries, and fjords. Coastal plain estuaries form from the rise of sea level. They rising sea level then fills an already existing river valley with water, creating an estuary. Tectonic estuaries for on faults, where tectonic activity has created a space that can be filled in with water. The San Francisco Bay is an example of a tectonic estuary. Bar built estuaries are behind some sort of natural bar between the estuary and the ocean, such as a spit. Fjords are valleys that were, at one time, carved out by glaciers and were then filled in with water (ONA).

Estuaries face many anthropogenic threats. Population growth is one of the main threats because 50% of the world's population lives on or in estuary systems. There are few policies in place to protect estuaries from population growth as more people seek the land and resources of estuaries. Another threat to estuaries related to population growth is sewage discharge and pollution runoff from urban areas. Since estuaries are fed by rivers and therefore streams,any pollutants that enter the system farther upstream end up in estuaries. An example of this is the annual "dead zone" that occurs in the Gulf of Mexico due to agricultural runoff (estuaries).