Scientists Say: Estuary

This is where a freshwater river meets a salty sea

A river snakes through an estuary landscape on the coast of Maine until it meets the sea.

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Estuary (noun, “EST-chew-AIR-ee”)

This word describes an ecosystem where a freshwater stream or river meets a salty ocean. Estuaries sometimes go by other names: bays, lagoons, coastal marshes and fjords. But all these places have similar features. An estuary is partially enclosed by land. Rivers or streams carry freshwater through this landscape. At the same time, tides draw saltwater in from the ocean. This mix of water is called “brackish,” meaning that it’s somewhat salty. An estuary’s water level and salinity (the saltiness) can change with the tides and seasons. Lots of rain or runoff from melting snow adds freshwater to an estuary. This makes it less salty. At drier times, its salinity rises. The water that moves through estuaries also carries nutrients. These nutrients help plants thrive.

Sometimes people call estuaries “nurseries of the sea.” That’s because many animals lay eggs or have young there. The shallow, calm waters provide a haven for many animals. These include mollusks, such as mussels and clams, and crustaceans, such as shrimp and crabs. Many kinds of birds and fish also live in estuaries. Some live there year-round. Others get food and take shelter when migrating through. People, too, benefit from estuaries. Some catch fish and crustaceans there. And estuaries can protect areas farther inland from flooding caused by hurricanes or other coastal storms.

In a sentence

Alligators have been spotted in estuaries snacking on sharks.

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Carolyn Wilke is a former staff writer at Science News for Students. She has a Ph.D. in environmental engineering. Carolyn enjoys writing about chemistry, microbes and the environment. She also loves playing with her cat.

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