What is an What is an Estuary?Estuary?

By Ms. AldridgeBy Ms. Aldridge

An estuary is the thin zone An estuary is the thin zone along a coastline (such as along a coastline (such as bays, lagoons, sounds or bays, lagoons, sounds or sloughs) where freshwater sloughs) where freshwater systems and rivers meet, systems and rivers meet, and mix with a salty ocean, and mix with a salty ocean, becoming brackish. becoming brackish. 

Freshwater estuaryFreshwater estuary

Sometimes, Sometimes, freshwater from freshwater from rivers mixes rivers mixes with large with large freshwater bodies freshwater bodies creating a creating a "freshwater "freshwater estuary”, such as estuary”, such as the Great Lakes in the Great Lakes in the northern the northern United StatesUnited States

TypesTypes

The 5 major types of estuaries are coastal The 5 major types of estuaries are coastal plain, bar-built, delta system, tectonic, and plain, bar-built, delta system, tectonic, and fjords.  fjords. 

Estuaries are unique places that are valuable Estuaries are unique places that are valuable to the environment and to society. to the environment and to society.

Estuaries are typically classified by Estuaries are typically classified by their existing geology or their their existing geology or their geologic origins (in other words, geologic origins (in other words, how they were formed).how they were formed).

Coastal Coastal PlainPlain

Millions of years ago, Millions of years ago, as ancient glaciers as ancient glaciers melted, some coastal melted, some coastal streams and rivers streams and rivers became covered with became covered with water as sea levels water as sea levels rose. The rose. The Chesapeake Bay in Chesapeake Bay in Maryland and Maryland and Narragansett Bay in Narragansett Bay in Rhode Island are Rhode Island are examples of coastal examples of coastal plain estuaries that plain estuaries that were once river were once river valleys. valleys.

Bar-Bar-builtbuilt

Sandbars or barrier islands built up by Sandbars or barrier islands built up by ocean currents and waves in coastal areas ocean currents and waves in coastal areas created a protected area fed by small created a protected area fed by small streams or rivers. The barrier islands off streams or rivers. The barrier islands off the Atlantic coastline of North Carolina are the Atlantic coastline of North Carolina are enclosed bar-built estuaries.enclosed bar-built estuaries.

Delta Delta systemsystem

Deltas are formed at Deltas are formed at the mouths of large the mouths of large rivers from sediment rivers from sediment and silt depositing and silt depositing instead of being instead of being washed away by washed away by currents and waves. currents and waves. When the river flow is When the river flow is restricted by the restricted by the delta, an estuary may delta, an estuary may form. The Mississippi form. The Mississippi River in Louisiana are River in Louisiana are examples of delta examples of delta systems. systems.

TectoniTectonicc Tectonic estuaries were Tectonic estuaries were

created when a major created when a major crack or a large land sink crack or a large land sink in the Earth, often caused in the Earth, often caused by earthquakes, produced by earthquakes, produced a basin below sea level a basin below sea level that fills with water. These that fills with water. These types of estuaries usually types of estuaries usually occur along fault lines. occur along fault lines. San Francisco Bay in San Francisco Bay in California is an example California is an example of an estuary created by of an estuary created by tectonics. tectonics.

FjordsFjords

Advancing glaciers ground out Advancing glaciers ground out long, narrow valleys with steep long, narrow valleys with steep sides. Then when glaciers melted, sides. Then when glaciers melted, seawater flooded in. Glacier Bay in seawater flooded in. Glacier Bay in Alaska is an example of a fjord.Alaska is an example of a fjord.

Each estuary can Each estuary can make up an make up an individual ecosystem, individual ecosystem, estuaries are also estuaries are also interconnected with interconnected with other surrounding other surrounding environments environments (oceans, lakes, (oceans, lakes, forests, grassy plains) forests, grassy plains) and nearby human and nearby human communities.  communities. 

Estuaries are Estuaries are constantly constantly changing. changing. Some animals Some animals and plants and plants specialize in, specialize in, or adapt to, or adapt to, living in the living in the unique unique conditions of conditions of estuaries. estuaries. 

Estuaries vary widely around the Estuaries vary widely around the world. Earth’s changing geology, world. Earth’s changing geology, flowing water and different weather flowing water and different weather patterns help create many diverse patterns help create many diverse types of habitats.types of habitats.

Rivers provide Rivers provide nutrients, organic nutrients, organic matter, and sediments matter, and sediments to estuaries. Rivers to estuaries. Rivers flow downstream flow downstream delivering fresh water delivering fresh water from streams, small from streams, small rocks and silt, and rocks and silt, and leaves and other leaves and other vegetation debris. vegetation debris. Nutrients support life Nutrients support life in the estuary.  in the estuary. 

Estuaries can filter Estuaries can filter small amounts of small amounts of pollutants and runoff. pollutants and runoff. Vegetation helps Vegetation helps filter and trap silt. filter and trap silt. However, too much However, too much nutrient or sediment nutrient or sediment input will create an input will create an unbalanced situation unbalanced situation causing the health of causing the health of the ecosystem to the ecosystem to decline. decline.

Estuaries act like Estuaries act like huge sponges, huge sponges, buffering and buffering and protecting upland protecting upland areas from crashing areas from crashing waves and storms waves and storms and preventing soil and preventing soil erosion. They soak erosion. They soak up excess water up excess water from floods and from floods and stormy tidal surges stormy tidal surges driven into shore driven into shore from strong winds. from strong winds. 

Estuaries provide a safe haven and Estuaries provide a safe haven and protective nursery for small fish, protective nursery for small fish, shellfish, migrating birds, and shellfish, migrating birds, and coastal shore animals. In the U.S., coastal shore animals. In the U.S., estuaries are nurseries to over 75% estuaries are nurseries to over 75% of all fish and shellfish harvested.of all fish and shellfish harvested.

People enjoy living near estuaries and People enjoy living near estuaries and the surrounding coastline. They sail, fish, the surrounding coastline. They sail, fish, hike, swim, and enjoy bird watching. An hike, swim, and enjoy bird watching. An estuary is often the center of a coastal estuary is often the center of a coastal community.community.

Effects of Effects of humanshumans

Anthropogenic(resulting from Anthropogenic(resulting from the influence of humans on the the influence of humans on the natural world) disturbances to natural world) disturbances to estuaries include coastal estuaries include coastal development, introduction of development, introduction of invasive species, pollution via invasive species, pollution via runoff, over fishing, dredging runoff, over fishing, dredging and filling, dams and global and filling, dams and global climate change. climate change.

Coastal DevelopmentCoastal Development

Hard surface can no longer absorb water Hard surface can no longer absorb water or provide shelter or food for the animals or provide shelter or food for the animals and plants that exist in coastal regions. and plants that exist in coastal regions.

Purple loosestrife, nutria, Purple loosestrife, nutria, hydrilla, snakehead fish, hydrilla, snakehead fish, green crabs, and zebra green crabs, and zebra mussels are just a few mussels are just a few invasive species invasive species

introduced by humans introduced by humans through dumping of through dumping of aquaria containing non-aquaria containing non-native fish. Many exotic native fish. Many exotic species compete for food species compete for food and shelter, prey on na and shelter, prey on na tive species or push native tive species or push native species out of their natural species out of their natural habitat. habitat.

Invasive Invasive SpeciesSpecies

Pollution Via Pollution Via RunoffRunoff

Water is always Water is always moving! Water runs moving! Water runs downstream or through downstream or through the ground from cities, the ground from cities, farms and factories, farms and factories, this can carry many this can carry many things harmful to things harmful to estuaries such as estuaries such as excess nutrients, raw excess nutrients, raw sewage and manure sewage and manure and chemical waste. and chemical waste.

Over FishingOver Fishing Over fishing Over fishing

reduces the reduces the number of number of organisms in organisms in that population that population and, can have a and, can have a negative effect negative effect the local food the local food web, in turn web, in turn effecting other effecting other species. species.

Dredging Dredging and Fillingand Filling

Filling and draining of Filling and draining of wetlands, and dredging wetlands, and dredging deep navigation deep navigation channels through channels through estuaries and wetlands estuaries and wetlands ultimately destroys and ultimately destroys and damages habitat. They damages habitat. They also change water and also change water and sediment flow. sediment flow.

DamsDams Changing river Changing river

water flow can water flow can restrict sediment restrict sediment deposits and deposits and nutrient availability nutrient availability downstream, fish downstream, fish migration, and can migration, and can increase saltwater increase saltwater into underground into underground water tables.water tables.

Global Climate Global Climate ChangeChange

Scientists are Scientists are confident that the confident that the Earth’s climate has had Earth’s climate has had change over the past change over the past 1,000 years. Changes 1,000 years. Changes in temperature of the in temperature of the air and water, air and water, increases in sea level, increases in sea level, more frequent and more frequent and intense tropical storms, intense tropical storms, and changes in coastal and changes in coastal currents, all of these currents, all of these changes can cause changes can cause change!change!

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