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The Estuary System

An estuary is a partially enclosed body of water where fresh water from rivers and streams mixes with salt water from the ocean. Estuaries are areas of transition between the land and the sea. They are often called bays, harbors, inlets or sounds.

The Chesapeake Bay: the largest estuary in the United States

The Chesapeake Bay is the largest of more than 100 estuaries in the United States. About half of the Bay’s water volume comes from salt water from the Atlantic Ocean. The other half drains into the Bay from its enormous 64,000-square-mile watershed. Because of this mix of fresh and salt water, the Bay’s salinity gradually increases as you move from north to south.

Why are estuaries important?

Estuaries are among the most productive environments on earth, creating more organic matter each year than similarly-sized forests and agricultural areas. Estuaries also provide diverse habitats for wildlife and aquatic life, protect our communities against flooding, reduce pollution to waterways, and support local economies through commercial and recreational activities.

Estuaries provide critical habitats

Estuaries provide many different types of habitats, including shallow waters, open waters, marshes and wetlands, sandy beaches, mud flats and oyster reefs.

Thousands of species of wildlife and aquatic life, including fish, crabs, birds and mammals, depend on estuarine habitats for their survival.

  • Migratory waterfowl stop and feed in the Bay’s marshes, wetlands, open waters and shallow waters during their seasonal flights along the Atlantic Flyway.
  • Anadromous fish such as American shad and striped bass migrate up the Bay each spring to spawn in freshwater rivers and streams.
  • Underwater grasses growing in the Bay’s shallows protect juvenile fish and blue crabs, as well as molting blue crabs, from predators.

Estuaries protect communities and clean water

The wetlands that fringe estuaries perform extremely valuable functions that protect our waterways and communities. Wetlands act as natural buffers between the land and the water, guarding against erosion, flood waters and storm surges. As water runs off the land, wetlands absorb and filter out pollution before it can make its way into the Bay.

Estuaries support local economies

Estuaries have important commercial value. The Chesapeake Bay and other estuaries serve as spawning and nursery grounds for two-thirds of the nation’s commercial fish and shellfish species. Without estuaries, we wouldn’t have many of the seafood dinners we enjoy!

Many major shipping ports are located in estuaries. The Chesapeake Bay is home to Baltimore and Hampton Roads, two of the North Atlantic’s five major ports.

Estuaries are important for people, too. Estuaries support many popular educational and family activities, including boating, fishing, swimming, jet skiing, photography and bird-watching.


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