Marine Science Chapters


Formation of Estuaries

Geologists describe the formation of estuaries based on a few characteristic features:

Coastal Plains form where the sea level has risen to flood a river valley. The mouth of the river is now inundated with salt water from the ocean to varying degrees. Many estuaries and bays on the west coast of the United States represent this form of estuary.

Bar Built estuaries form when barrier islands or long peninsulas shelter a region with an influx of both fresh and salt water. Some mangroves also establish themselves in these regions. Estuaries such as Pamlico Sound in North Carolina are bar built estuaries.

Delta formed estuaries exist where the deposition of sediment at the river mouth creates a shallow region that successively develops into an estuary environment. The Mississippi River Delta is this type of estuary.

Tectonic formation is typically around a fault region. A fault in the earth's crust may cause the land to subside and form a depression. San Francisco Bay is an example of an estuary formed by tectonic action.

Fjord estuaries are formed in valleys when glaciers retreat. The rivers that drain into the valley mix with ocean water to form estuaries including Puget Sound in Washington, one of the largest estuaries in the world.

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(Revised 10 August 2018)
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