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National Estuary Program (NEP)

Basic Information about Estuaries

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What is an Estuary?

ariel photo of an estuaryPhoto Credit: Marc Hinz

An estuary is a partially enclosed, coastal water body where freshwater from rivers and streams mixes with salt water from the ocean. Estuaries, and their surrounding lands, are places of transition from land to sea. Although influenced by the tides, they are protected from the full force of ocean waves, winds and storms by land forms such as barrier islands or peninsulas.

Estuarine environments are among the most productive on earth, creating more organic matter each year than comparably-sized areas of forest, grassland or agricultural land. The sheltered waters of estuaries also support unique communities of plants and animals specially adapted for life at the margin of the sea.

Many different habitat types are found in and around estuaries, including shallow open waters, freshwater and saltwater marshes, swamps, sandy beaches, mud and sand flats, rocky shores, oyster reefs, mangrove forests, river deltas, tidal pools and seagrass beds.

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Why are Estuaries Important?

a had holding softshell clamsPhoto Credit: Casco Bay Estuary Partnership

Estuaries provide us with a suite of resources, benefits and services. Some of these can be measured in dollars and cents, while others cannot. Estuaries provide places for recreational activities, scientific study and aesthetic enjoyment. Estuaries are an irreplaceable natural resource that must be managed carefully for the mutual benefit of all who enjoy and depend on them. Below are additional ways in which estuaries are important (click to expand):

  • Estuaries Are Critical Natural Habitats

    Thousands of species of birds, mammals, fish and other wildlife depend on estuarine habitats as places to live, feed and reproduce. And many marine organisms, including most commercially-important species of fish, depend on estuaries at some point during their development.

    Because they are biologically productive, estuaries provide ideal areas for migratory birds to rest and refuel during their long journeys. Because many species of fish and wildlife rely on the sheltered waters of estuaries as protected spawning places, estuaries are often called the "nurseries of the sea."

  • Estuaries Have Economic Value

    a woman holding a large fish out of the waterPhoto Credit: Nanette O'HaraEstuaries have important commercial value and their resources provide economic benefits for tourism, fisheries and recreational activities. The protected coastal waters of estuaries also support important public infrastructure, serving as harbors and ports vital for shipping and transportation.

    The economy of many coastal areas is based primarily on the natural beauty and bounty of estuaries. When those natural resources are imperiled, so too are the livelihoods of those who live and work in estuarine watersheds. Over half the U.S. population lives in coastal areas, including along the shores of estuaries. Coastal watershed counties provided 69 million jobs and contributed $7.9 trillion to the Gross Domestic Product in 2007 (National Ocean Economics Program, 2009).

  • Estuaries Perform Environmental Services

    Estuaries also perform other valuable services. Water draining from uplands carries sediments, nutrients and other pollutants to estuaries. As the water flows through wetlands such as swamps and salt marshes, much of the sediments and pollutants are filtered out. This filtration process creates cleaner and clearer water, which benefits both people and marine life.

    Salt marsh grasses and other estuarine plants also help prevent erosion and stabilize shorelines.

  • Estuaries Act as Protective Buffers

    Wetland plants and soils also act as natural buffers between the land and ocean, absorbing flood waters and dissipating storm surges. This protects upland habitat as well as valuable real estate from storm and flood damage.

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How are Estuaries Threatened?

dense waterside communityPhoto Credit: Maryland Coastal Bays Program

Coastal counties are growing three times faster than counties elsewhere in the nation. Unfortunately, this increasing concentration of people:

  • upsets the natural balance of estuarine ecosystems;
  • threatens their integrity; and
  • imposes increased pressures on vital natural resources like estuaries.

What happens on the land affects the quality of the water and health of the organisms that live in an estuary. For example, if a river or stream flows through an agricultural area, it picks up fertilizer, manure and pesticides from farming operations that run off the land after a rainstorm. As it passes urbanized and suburbanized areas, it gathers substances such as:

  • fertilizers or pet waste that wash off lawns;
  • untreated sewage from failing septic tanks;
  • wastewater discharges from industrial facilities;
  • sediment from construction sites; and
  • runoff from impervious surfaces like parking lots.

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