Estuaries
Estuaries are the thin zone along a coastline (such as lagoons, bays, sounds or sloughs) where freshwater systems and rivers meet, and then mix with a salty ocean, becoming brackish.
The five major types of estuaries are coastal plain, delta system, bar-built, tectonic, and fjords. Not every estuary contains brackish waters. But, occasionally, freshwater from a river will mix with a large freshwater body creating a “freshwater estuary” that functions like a typical brackish estuary. An estuary occurs where a massive freshwater system, such as the Great Lakes in the United States, is diluted by river or stream waters draining from an adjacent land.
Although a freshwater estuary does not contain saltwater, it is a unique combinations of river and lake water, which are chemically distinct. Unlike a brackish estuary which is tidally driven, a freshwater estuary is storm-driven. In a freshwater estuary the composition of the water is frequently regulated by storm surges and subsequent seiches (sloshing, or vertical oscillations, of lake water). Estuaries are classified on two characteristics: their geology and how fresh and saltwater mix in them. In this section, we will be discussing the classification based on the geology of an estuary.
Geologic time measures change over thousands to millions of years, not in days. Most estuaries are less than 10,000 years old. An estuary is typically classified by its existing geology or their geologic origins. The five major types of estuaries are coastal plain, bar-built, delta system, fjord, and tectonic.
- Coastal Plain - Millions of years ago, as ancient glaciers melted, some coastal rivers and streams became covered with water as sea levels rose. The Narragansett Bay in Rhode Island and Chesapeake Bay in Maryland are both examples of a coastal plain estuary that was once a river valley.
- Bar-built - Sandbars or barrier islands built up by ocean waves and currents in coastal areas created a protected area fed by small rivers or streams. The barrier islands off the Atlantic coastline of North Carolina and Massachusetts both enclose a bar-built estuary.
- Delta system - Deltas are formed at the mouths of large rivers from silt and sediment depositing instead of being washed away by waves and currents. When the river flow is restricted by the delta, an estuary can form. The estuaries found at the mouth of the Nile River in Egypt and also the Mississippi River in Louisiana are both examples of delta systems.
- Tectonic - A Tectonic estuary is created when a major crack or a substantial land sink in the Earth, mostly caused by earthquakes, produced a basin below sea level that then fills with water. This type of estuary usually occurs along fault lines. The San Francisco Bay in California is an example of an estuary that was created by tectonics.
- Fjords - Advancing glaciers that ground out long, narrow valleys with steep sides. Then when the glaciers melted, seawater flooded in. An example of a fjord is Glacier Bay in Alaska.