An estuary is a partially enclosed body of water where saltwater from the sea mixes with freshwater from rivers, streams and creeks. These areas of transition between the land and the sea are tidally driven, like the sea, but sheltered from the full force of ocean wind and waves, more like a river. Estuaries are generally enclosed in part by the coastline, marshes and wetlands; the seaward border may be barrier islands, reefs and sand or mud flats.

Every estuary is unique; each individual ecosystem has different components that complete the estuarine habitat. One estuary may be enclosed by marshes and barrier islands, while another estuary's borders are the coastline and reefs. Bodies of water that may be estuaries are: sloughs, bays, harbors, sounds, inlets and bayous. Some familiar examples of estuaries are: Chesapeake Bay, San Francisco Bay, Boston Harbor, Tampa Bay and Puget Sound. There are also wetlands in the Great Lakes with estuarine-like functions. These ecosystems have a strong tidal force and are protected from the open water of the Great Lakes by a natural barrier, such as a mud flat.

What types of animals live in an estuary?

A plethora of organisms can be found in estuaries, organisms specially adapted to the "brackish" estuarine waters. Estuaries are homes to all kind of terrestrial or land-based plants and animals, such as wood storks, pelicans, coniferous and deciduous trees and butterflies. Estuaries are also homes to unique aquatic plants and animals, such as sea grass, sea turtles and sea lions.

Why are estuaries important?

Estuaries are important for many reasons. Estuaries are among the most biologically productive ecosystems on the planet. More than two thirds of the fish and shellfish we eat spend some part of their lives in estuaries. These ecosystems also provide many other important ecological functions; they act as filters for terrestrial pollutants and provide protection from flooding. Estuaries also have economic importance. These dynamic bodies of water provide us with an important source of food, but are also a popular tourist destination. Millions of people visit the nation's estuaries each year to boat, swim, bird watch and fish.

Why do we need to protect estuaries?

The fragile balance of these productive estuarine environments may be easily destroyed by human activities. Changes in water quality or alterations, by dredging and construction, to the multiple components of estuaries can result in harmful changes in the ecosystem. The purpose of National Estuaries Day is to promote the need to protect these important areas and to learn how we can safeguard these irreplaceable resources.

What is a watershed?

A watershed is the land area that drains into a body of water. For example, imagine that a bowl is a watershed. The rim of the bowl represents the highest peaks of the mountains surrounding a valley with a river below, the bottom of the bowl. The inner walls of the bowl represent all of the smaller hills and mountains in the valley. Only water that falls on the rim and the inner walls of the bowl will enter the river, all other water will flow in a different direction, or to a different watershed. Watersheds are so important because water that is polluted anywhere in the watershed, even if it is hundreds of miles away, will eventually flow into the estuary. It is crucial to keep the water in the estuary clean, as well as all of the water that will eventually flow into the estuary, to maintain a healthy environment for the plants and animals that live there.

- Adapted from "Where the Rivers Meet the Sea," NOAA, 1990