What is an estuary?

An estuary is a coastal area where saltwater from the ocean mixes with freshwater from rivers, rainfall, and upland runoff.

Within the estuary, salt and fresh water proportions differ daily depending on the season, weather, and tides. Vital coastal ecosystems exist in these constantly changing conditions.

The mixing of salt and fresh water begins offshore, where water, sediment, and nutrients from the Mississippi River mix with the salt water of the Gulf of Mexico. Wind and tides drive this mixture, along with fish and shellfish, into the estuary. Changes on the coast affect water levels and habitats throughout the entire system.

This dynamic combination of physical and biological factors produces an ecosystem unrivaled in productivity and commerce.

Such characteristics are often cited to describe estuaries as the “cradles of civilization.” One of the most expansive and productive estuaries in the world is located in the United States at the interface of the Mississippi River and the Gulf of Mexico. The Barataria-Terrebonne National Estuary Program is chartered with the protection, development and study of this vital ecosystem.

Watercolor Illustration of Estuary by Diane Baker

Watercolor Illustration of Estuary by Diane Baker

Why are estuaries important?

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):

Image: Fishing at Sunset

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 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 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.

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.

Information on the importance of estuaries from epa.gov.