Geography of Estuaries

Learn Information about the World’s Estuaries

Chesapeake Bay
U.S. Coast Guard HH-60J Jayhawk helicopter passes over the Chesapeake Bay Bridge-Tunnel while on patrol in Portsmouth, Virginia. Getty Images

An estuary is defined as a place where freshwater such as a river or stream meets the ocean. As a result of this meeting estuaries are unique because they are a mixture of freshwater and saltwater. This is known as brackish water and although it is salty, it is less salty than the ocean so many different types of plants and animals can live in estuaries that cannot live in rivers, streams or the ocean.

It should also be noted that the level of salinity and water level of an estuary varies throughout the day because water continually circulates into and out of them with the tides.

There are many estuaries throughout the world and some of them are very large. Some of the largest are located in North America and they have different names such as a bay, lagoon, sound or slough. Some examples of large estuaries in North America include Chesapeake Bay (along the coasts of Maryland and Virginia in the United States), the San Francisco Bay in California and the Gulf of St. Lawrence in eastern Canada.

Types of Estuaries

Along with varying in size, estuaries also vary in type and they are classified based on their geology and water circulation. Estuary classifications based on geology include coastal plain, bar built, delta, tectonic and fjord estuaries. NOAA) Those based on water circulation are salt-wedge, fjord, slightly stratified, vertically mixed and freshwater estuaries ( NOAA).

Geologic Estuaries

A coastal plain estuary is one that formed thousands of years ago at the end of the last ice age.
During this time, sea levels were lower than they are today so more coastal land was exposed. As the large ice sheets on land began to melt around 10,000 to 18,000 years ago sea levels began to rise and fill in low-lying river valleys to create coastal plain estuaries.

Bar built estuaries, also called restricted mouth estuaries, are created when sandbars and barrier islands are formed after ocean currents push sediment toward the shore in areas fed by rivers and streams (NOAA).

Generally the rivers flowing into these types of estuaries have low water volume and lagoons form between the barrier island or sandbar and the coastline.

Deltas are a type of geologic estuary that form at the mouth of a large river where sediment and silt carried by the river are deposited where the river meets the ocean. In these areas the sediment accumulates and overtime wetlands and marshes form as part of an estuary system.

Tectonic estuaries form over time in areas with fault lines. During an earthquake depressions can occur when land sinks along the fault lines. If the land sinks below sea level and it is near the ocean, the seawater pours into the depression. Over time other faults and depressions allow rivers to do the same and eventually the freshwater and seawater meet to form an estuary.

Fjords are the final type of geologic estuary and they are created by glaciers. As these glaciers move toward the ocean they carve long, deep valleys in the coastlines. After the glaciers later retreat, seawater fills in the valleys to meet freshwater coming in from the land to form estuaries.

Water Circulation Estuaries

In addition to being classified as a geologic estuary, fjords are also a type of water circulation estuary. As advancing glaciers move toward the ocean creating their valleys they also deposit sediment that creates a sill at the mouth of the valley near the ocean. As a result when the glaciers retreat and the ocean water moves in to meet the freshwater coming off of the land water circulation is restricted so the water does not mix well.

Another type of water circulation estuary is a salt wedge estuary. This type of estuary occurs when fast flowing freshwater enters the ocean where ocean currents are weak. In this these areas the freshwater pushes the saltwater back to sea. Because the freshwater is less dense than saltwater it then floats on top of the saltwater creating a layered estuary.

Slightly stratified, also called partially mixed, estuaries form when saltwater and freshwater mix at all depths.

The salinity of these estuaries does vary; however, it is greatest at the mouth of the estuary. Estuaries that are mixed even better than slightly stratified estuaries are called vertically mixed. These estuaries occur in areas where the river flow is low and ocean currents are strong when the two meet.

The final type of water circulation estuary is a freshwater estuary that occurs in areas where freshwater does not meet the ocean. Instead it forms an outlet into another body of freshwater such as a lake so all of the water in the estuary remains fresh.

Importance of Estuaries

Major cities all over the world are located on estuaries. Places like New York City and Buenos Aires have grown and become major cities on estuaries. As a result estuaries are extremely important economically. In the United States for example, estuaries provide habitat for over 75% of commercial fishing and contributes billions to the economy ( NOAA). The city of New Orleans, Louisiana depends on fishing profits from the Mississippi River delta and estuary. Estuaries also provide recreational activities boating, fishing and bird watching that also contribute to local economies via tourism.

In addition to providing economic benefits, estuaries are also extremely important to the environment because they provide critical habitat for species that must have brackish water to survive. Salt marshes and mangrove forests are two types of ecosystems that exist because of estuaries. These areas are home to species such as oysters, shrimp and crab as well as nesting species like pelicans and herons.

Because of the changing salinity and water level of estuaries many species living in them have also developed different adaptations to survive making them unique to those areas. Estuarine crocodiles for example are specially adapted to living in brackish water but they can also survive in saltwater or freshwater by feeding on a variety of species and swimming out to sea during dry times (National Geographic).

Estuary Examples

Chesapeake Bay and the San Francisco Bay in the United States and Canada's Gulf of St. Lawrence are all very large and important estuary examples. All of them have large cities with economies that are tied to them along their banks. They are also all important environmentally.

Chesapeake Bay is a coastal plain estuary and it is the largest in the United States. It has a 64,000 square mile (165,759 sq km) watershed and major cities like Baltimore, Maryland are on its shores (Chesapeake Bay Program). The San Francisco Bay is a tectonic estuary and it is the largest estuary in western North America. Its watershed covers 60,000 square miles (155,399 sq km) and drains 40% of California. It is surrounded by cities such as San Francisco and Oakland and it is home to many plant and animal species such as the Pacific herring and many endangered waterfowl. It is important economically as well because it is a prime fishing area and its freshwater irrigates 4 million acres of agricultural land (San Francisco Estuary Partnership).

Eastern Canada's the Gulf of St. Lawrence is also an incredibly important estuary because it provides an outlet from the Great Lakes to the northern Atlantic Ocean. This estuary is claimed by many to be the largest in the world at 744 miles (1,197 km) long.. The Gulf of St. Lawrence is a salt wedge estuary it is very important to Canada's fishing economy as there are many harbors along it that provide thousands of jobs to Quebec alone.

Pollution and the Future of Estuaries

Despite the importance of estuaries like the Gulf of St. Lawrence and the San Francisco Bay, many estuaries around the world are currently being subjected to serious pollution that is harmful to their delicate ecosystems. For example, many toxic substances like pesticides, oil and grease are polluting estuaries due to run off into storm drains. As a result many cities and environmental organizations like the Chesapeake Bay Program have begun campaigns to educate the public about the importance of estuaries and ways to reduce pollution so that they can thrive for many years to come.