One interesting fact about estuaries is that they are among the most productive ecosystems in the world. Estuaries occur when saline water mixes with freshwater, forming brackish water. These estuaries are home to diverse plants and animal species adapted to salty conditions that rely on the micro-organisms and plant matter found only in brackish bodies of water. Some other names for estuaries are sounds, sloughs, lagoons and bays.
Four different types of estuaries exist, including coastal plain, tectonic, fjord and bar-built. Creation of a coastal plain estuary, the most common type of estuary, occurs when the seawater level rises and fills in a nearby river valley. The Chesapeake Bay, located along the coasts of Maryland and Virginia, is the largest coastal plain estuary in the United States. The San Francisco Bay is a tectonic estuary, which is created by the drifting apart and shifting together of the Earth's crust. A fjord estuary is typically a narrow and long valley with steep sides carved out by advancing glaciers. When the glacier retreats, the ocean water fills in the deep, narrow impression. Puget Sound is an example of a series of fjord estuaries in the state of Washington. Bar-built estuaries occur when a barrier island protects a bay or lagoon from the ocean. The Outer Banks in Virginia and North Carolina are a series of bar-built estuaries.