Estuaries are bio-diverse ecosystems. They provide nutrients to native plants and animals, as well as a habitat for many of these. They also are filters for some pollution that may travel down rivers. There are several different types of estuaries, with excellent examples to check out throughout the world.
This type of estuary was formed toward the end of the last ice age. Sea levels rose as the ice melted, and the water invaded river valleys. The gentle sloping bottoms of these river valleys, plus the fact that they are shallow, make these locations ideal for estuaries to form. This is the most common form of estuary and can be found in places like Narragansett Bay in Rhode Island, Thames River in England, Seine River in France, Si-Kiang River in Hong Kong, Murray River in Australia and the Ems River in Germany.
San Francisco Bay is a good example of a tectonic estuary. This type occurs when the earth's crust movement causes cracks, or faults and folds, to form. When this movement occurs, the sand sinks or subsides and estuaries fill in the space or basin that is left.
When sandbars build up close to coastlines they block the water that is behind them from entering the sea. This type of estuary is known to be shallow, and does not have much tidal action. The mixing of the fresh and salt water is generally completed by wind. These estuaries are found along the Gulf Coast of Texas and Florida, in areas of North Carolina and in the Netherlands.
Norway is known for its fjords; however, many do not realize that fjords are actually estuaries. They are formed when glaciers cut deep valleys and then the seas move into the area. There are shallow barriers or ledges at their mouths. The deeper waters of the fjord and the sea are limited in their exchange due to these barriers. Fjords are known to be narrow and have steep, straight and long sides. These estuaries also have deep elongated bases that are U-shaped. Fjords can be seen along the coasts of Chile, Canada, Greenland, Alaska, New Zealand, Siberia, Scotland and, of course, Norway.
In a salt wedge estuary the circulation of the salt and fresh water is controlled by the river's flow. When this occurs, a sudden boundary stops the upper layer which is less salt from invading the salty bottom layer, which is a wedge shape. Good examples of salt wedge estuaries are the mouths of the Mississippi, the Columbia and the Hudson rivers.
Puget Sound is a good example of a partially mixed estuary. The tidal flow in these bodies of water erases the salt wedge, allowing the salt water to rise and the fresh water to move downward.
When there is a strong tidal movement and a low river flow, the sea water is allowed to mix through the entire estuary. Salinity levels are constant from top to bottom and, as the ocean moves to the river, it decreases. An example of a well-mixed estuary is the Delaware Bay.
Travelers along coastlines have access to many different estuaries. By keeping a keen eye open, they can see and easily compare these estuaries.