In the most general terms, an estuary is an ecosystem, comprising both the biological and physical environment, that has developed in a region where rivers meet the sea and fresh-flowing river water mingles with tidal salt water to become brackish, or partly salty. However, several types of wholly freshwater ecosystems have many characteristics similar to what we think of as traditional brackish estuaries.
For example, along the Great Lakes, river water with very different chemical and physical characteristics mingles with lake water in coastal wetlands that are affected by tides and storms just as estuaries along the oceanic coasts are. These freshwater “estuaries” also provide many of the ecosystem services and functions that brackish estuaries do, serving for example as natural filters for runoff and as nursery grounds for many species of birds, fish, and other animals.
Whether salt- or freshwater, estuaries have certain properties that make them distinctive and extremely valuable to the larger natural system, as well as to human health and well-being.
Estuaries are connected by water to many different surrounding environments (oceans, lakes, forests, grassy plains) as well as nearby human communities, and therefore they are affected by what takes place in those environments, whether they be natural processes or human activities.
Rivers carry nutrients, organic matter, and sediments to estuaries from freshwater streams, including small rocks and silt, and leaves and other vegetation debris.
All of these inputs combine to make estuaries extremely productive with a great abundance of plants and animals. Estuaries can also serve as important buffer zones by filtering some pollutants. Estuarine vegetation, both submerged and emergent, helps to filter and trap silt from runoff. However, too great an amount of sediment or nutrients can upset the balance of an estuary and cause the health of the ecosystem to decline.
Estuaries are constantly changing transition areas, so animals and plants that live in an estuary need to be able to adapt to living in a wide range of conditions.
Estuaries act like huge sponges, buffering and protecting upland areas from crashing waves and storms and preventing soil erosion. They soak up excess water from floods and stormy tidal surges driven into shore from strong winds.
Estuaries provide a safe haven and protective nursery for small fish, shellfish, migrating birds, and coastal shore animals. In the U.S., estuaries are nurseries to more than 75 percent of all fish and shellfish harvested.
People enjoy living near estuaries and the surrounding coastline. They sail, fish, hike, swim, and enjoy bird-watching. An estuary is often the center of a coastal community.
Types of Estuaries
Most estuaries are less than 10,000 years old. Estuaries are typically classified by their existing geology or their geologic origins (in other words, how they were formed). The five major types of estuaries—classified by their geology—are coastal plain, bar-built, delta system, tectonic, and fjord.
Thousands of years ago, as ancient glaciers melted, some coastal streams and rivers became covered with water as sea levels rose. The Chesapeake Bay in Maryland and Narragansett Bay in Rhode Island are examples of coastal plain estuaries that were once river valleys.
Sandbars or barrier islands built up by ocean currents and waves in coastal areas have created protected areas fed by small streams or rivers. The barrier islands off the Atlantic coastline of North Carolina and Massachusetts enclose typical bar-built estuaries.
Deltas are formed at the mouths of large rivers from sediment and silt depositing instead of being washed away by currents and waves. When the river flow is restricted by the delta, an estuary may form. The estuaries at the mouths of the Nile River in Egypt and the Mississippi River in Louisiana are examples of delta systems.
Tectonic estuaries were created when a major crack or a large land sink in the Earth, often caused by earthquakes, produced a basin below sea level that filled with water. These types of estuaries usually occur along fault lines. San Francisco Bay in California is an example of an estuary created by tectonics.
Advancing glaciers ground out long, narrow valleys with steep sides. Then when glaciers melted, seawater flooded in. Kachemak Bay in Alaska is an example of a fjord.
Although freshwater estuaries do not contain saltwater, they are unique combinations of river and lake water, which are chemically distinct. Unlike brackish estuaries where mixing processes are typically caused by tides, freshwater estuaries are most often storm driven. In freshwater estuaries the composition of the water is often regulated by storm surges and subsequent seiches (vertical oscillations, or sloshing, of lake water).
While the Great Lakes do exhibit tides, they are extremely small. Most changes in the water level are due to seiches, which act like tides, exchanging water between the river and the lake. Stratification and mixing of water in freshwater estuaries is also caused by temperature differences between stream water and lake waters. The shallow waters of streams respond more quickly to temperature changes than deeper lake waters. These changes affect the water temperature, pH, dissolved oxygen, and salinity of the two water bodies, thus influencing the chemistry of this type of estuarine system.
An example of a freshwater estuary is Old Woman Creek National Estuarine Research Reserve on Lake Erie, near Huron, Ohio. Old Woman Creek Reserve is part of the national network of coastal reserves established as living laboratories for long-term scientific research and estuarine education.