An estuary is a partly enclosed coastal body of water with one or more rivers or streams flowing into it, and with a free connection to the open sea. Estuaries form a transition zone between river environments and ocean environments and are subject to both marine influences, such as tides, waves, and the influx of saline water; and riverine influences, such as flows of fresh water and sediment. The inflow of both seawater and freshwater provide high levels of nutrients in both the water column and sediment, making estuaries among the most productive natural habitats in the world.
Estuaries come in all shapes and sizes and are called many different names—bays, lagoons, harbors, inlets, or sounds. The sheltered waters of estuaries are home to countless plants and animals that like to live in water that is part fresh and part salty. Examples include horseshoe crabs, ospreys, manatees, mangroves, and seagrasses. Hundreds of fish and shellfish, such as scallops, shrimp, and salmon, live in estuaries at some point in their life. Estuaries protect water quality by filtering out dirt and pollution. In addition, estuaries and the land surrounding them are places where people live, sail, fish, swim, and bird watch. As a result, estuaries are often the centers of our coastal communities.