• An estuary is an area where a freshwater river or stream meets the ocean. In estuaries, the salty ocean mixes with a freshwater river, resulting in brackish water.Brackish wateris somewhat salty, but not as salty as the ocean. An estuary may also be called a bay, lagoon,sound, or slough.
• Water continually circulatesinto and out of an estuary. Tides create the largest flow of saltwater, while river mouths create the largest flow of freshwater.
  • When dense, salty seawater flows into an estuary, it has an estuarine current. High tides can create estuarine currents. Saltwater is heavier than freshwater, so estuarine currents sink and move near the bottom of the estuary.
  • When less-dense freshwater from a river flows into the estuary, it has an anti-estuarine current. Anti-estuarine currents are strongest near the surface of the water. Heated by the sun, anti-estuarine currents are much warmer than estuarine currents.
• In estuaries, water level and salinity rise and fall with the tides. These features also rise and fall with the seasons. During the rainy season, rivers may flood the estuary with freshwater. During the dry season, the outflow from rivers may slow to a trickle. The estuary shrinks, and becomes much more saline.
• During a storm season, storm surges and other ocean waves may flood the estuary with saltwater. Most estuaries, however, are protected from the ocean’s full force. Geographical features such as reefs, islands, mud, and sand act as barriers from ocean waves and wind.

Types of Estuaries

• There are four different kinds of estuaries, each created a different way:
• Coastal plain estuaries are created when sea levels rise and fill in an existing river valley. The Chesapeake Bay, on the East Coast of the United States, is a coastal plain estuary.
• Tectonic activity, the shifting together and rifting apart of the Earth’s crust, creates tectonic estuaries. California’s San Francisco Bay is a tectonic estuary.
• When a lagoon or bay is protected from the ocean by a sandbar or barrier island, it is called a bar-built estuary. The Outer Banks, a series of narrow barrier islands in North Carolina and Virginia, create sandy, bar-built estuaries.
• Fjord estuaries are a type of estuary created by glaciers. Fjord estuaries occur when glaciers carve out a deep, steep valley. Glaciers retreat and the ocean rushes into fill the narrow, deep depression. Puget Sound is a series of fjord estuaries in the U.S. state of Washington.

Estuarine habitats

Estuaries enclose a diverse range of habitats from subtidal areas to intertidal areas. These include:

• sheltered upper estuary mangroves, seagrass beds and marshes
• highly energetic beaches on the ocean side of the estuary
• rocky reefs
• wave built bars in estuary mouths
• deep estuarine channels where swift tidal currents flow
• shallow open salt water and fresh water
• river deltas
• tidal pools
• muddy fringing marshes 
• mid-estuary sand banks
• intertidal flats
• estuarine beaches

Importance of Estuaries

Ecological value

• Estuaries are one of the most productive ecosystems on earth.
• They maintain water quality through natural filtration as microbes break down organic matter and sediments bind pollutants. Wetlands that fringe many estuaries also have other valuable functions.
• They help maintain biodiversity by providing a diverse range of unique habitats that are critical for the survival of many species.

Economic value

• They are tourist attractions.
• Used for transport and industry.
• They have ecological importance to commercial fisheries.
• Maintain water quality which benefits both people and marine life
• Natural buffer between the land and ocean, absorbing floodwaters and storm surges.
• They help maintain biodiversity by providing a diverse range of unique habitats that are critical for the survival of many species.
• Many species of commercially important fish and shellfish use estuaries as nurseries to spawn and allow juveniles to grow.

Cultural value

• People value estuaries for recreation, scientific knowledge, education, aesthetic and traditional practices.
• Estuaries are the preferred site for human settlement.

Threats to the Estuary

• Urban development: Housing development, recreational activities and fishing will change the environment and have an impact on estuaries.
• Agricultural activities: Runoff water and chemicals from agricultural land can affect the health of estuaries and the organisms living in and around it.
• Industries like fishing and oyster farming can impact the water quality and also the movement of water in estuaries.
• Over-fishing and habitat loss reduces the amount of fish in the ocean and disrupts the food chain.
• Structures such as sea walls, bridges, drains, jetties, marinas, levy banks, rock walls and breakwaters can all affect the passage of water and can change the conditions in an estuary.
• Erosion and the build-up of sediments and pollutants can cover seagrass and other vegetation. This reduces the amount of food sources and the available habitats for marine life and wildlife to live in.
• Dams and power stations can change the water temperature in estuaries, as well as affecting aquatic ecosystems.
• Litter on beaches and floating in the salt and fresh water pollutes estuaries