ESTUARIES DESCRIPTION AND FORMATION OF ESTUARIES

An estuary is a partially enclosed body of brackish water at the tidal mouth of a river where the saltwater of the tide meets the freshwater of the river current. An estuary may also be called a bay, lagoon, sound, or slough. What is an estuary?

How do they form? The formation of all types of estuary were wholly or partly due to the effects sea-level changes following the end of the last glacial period. These sea-level changes caused the submergence of river valleys either from eustatic (where global sea levels rise), or isostatic (where the local land sinks) changes.

There are four different types of estuary based on how they were formed:  Coastal plain estuaries  Tectonically-produced estuaries  Bar-built estuaries  Fjord estuaries Types of Estuary

Coastal-plain Estuaries Drowned river valley or coastal plain estuaries were formed by rising sea levels during the last interglacial period (about 15,000 years ago). This flooded the lower parts of existing river valleys that were cut into the landscape when sea level was lower. The result is often a very large estuary at the mouth of a relatively insignificant river. Unglaciated river valleys which were flooded in this way are sometimes called rias. Chesapeake Bay, Delaware Bay and Hudson Bay on the U.S. east coast; Galveston Bay and Tampa Bay on the U.S. Gulf coast; and River Exe estuary in England are all coastal plain estuaries

Chesapeake Bay and Delaware Bay

Hudson River estuary waterways around New York City, USA: 1. Hudson River, 2. East River, 3. Long Island Sound, 4. Newark Bay, 5. Upper New York Bay, 6. Lower New York Bay, separated from Upper New York Bay by the Narrows strait, 7. Jamaica Bay, and 8. Atlantic Ocean.

Tampa Bay

River Exe Estuary, Somerset, England

Tectonically-produced Estuaries These estuaries are formed by: – subsidence causing isotatic sea-level rise; – land cut off from the ocean by land movement associated with faulting, volcanoes and landslides; – inundation from eustatic sea level rise during the Holocene Epoch There are only a small number of tectonically- produced estuaries One example is the San Francisco Bay, which was formed by the crustal movements of the San Andreas fault system causing the inundation of the lower reaches of the Sacramento and San Joaquin rivers.

Tectonic estuaries are created by tectonic activity, the process of the Earth's crust rifting apart and shifting together. The San Francisco Bay is a tectonic estuary surrounded by faults, illustrated by red lines on this map. The complex tectonic activity of the area results in downwarping, the process of an area of the Earth sinking to a lower elevation. The Pacific Ocean filled in the downwarped valley southwest of the mouths of the San Joaquin and Sacramento Rivers, visible in the upper right of the image.

Bar-built or Lagoon-type Estuaries The movement of sand and formation of sandbars along the coastline can enclose bodies of water (such as river mouths) and form lagoon- type or bar-built estuaries. These estuaries are semi-isolated from ocean waters by barrier beaches (barrier islands and barrier spits and barrier bars). Formation of barrier beaches partially encloses the estuary, with only narrow inlets allowing contact with the ocean waters.

Bar-built or Lagoon-type Estuaries Barrier beaches form in shallow water and are generally parallel to the shoreline, resulting in long, narrow estuaries. The average water depth is usually less than 5 m, and rarely exceeds 10 m. The barrier beaches that enclose bar-built estuaries have been developed in several ways: – building up of offshore bars by wave action, in which sand from the sea floor is deposited in elongated bars parallel to the shoreline, – reworking of sediment discharge from rivers by wave, current, and wind action into beaches, overwash flats, and dunes, – engulfment of mainland beach ridges (ridges developed from the erosion of coastal plain sediments around 5000 years ago) due to sea level rise and resulting in the breaching of the ridges and flooding of the coastal lowlands, forming shallow lagoons, and – elongation of barrier spits from the erosion of headlands due to the action of longshore currents, with the spits growing in the direction of the littoral drift.

Bar-built or Lagoon-type Estuaries Bar-built estuaries typically develop on gently sloping plains located along tectonically stable edges of continents and marginal sea coasts. They are extensive along the Atlantic and Gulf coasts of the U.S. in areas with active coastal deposition of sediments and where tidal ranges are less than 4 m. The Outer Banks, a series of narrow barrier islands in North Carolina and Virginia, create 3 sandy, bar-built estuaries. Other examples of bar- built estuaries are Barnegat Bay, New Jersey; Laguna Madre, Texas.

Bar-built estuaries are created as sandbars and barrier islands protect river mouths from the open ocean. The Outer Banks, off the coast of North Carolina and Virginia, protect three large estuaries called sounds: Pamlico Sound, Currituck Sound, and Albemarle Sound.

Pamlico Sound – a bar-built estuary

Fjord-type Estuaries This is a type of estuary created by glaciers. They occur when glaciers carve out a deep, U- shaped valleys with steep sides. When the glaciers retreat during warmer climatic periods, the ocean rushes into fill the narrow, deep depression. Unlike coastal plain estuaries, these have steep sides, rock bottoms, and underwater sills contoured by glacial movement. The estuary is shallowest at its mouth, where terminal glacial moraines or rock bars form sills that restrict water flow. In the upper reaches of the estuary, the depth can exceed 300 m.

Fjord-type Estuaries Fjord-type estuaries can be found along the coasts of Alaska, the Puget Sound region of western Washington state, British Columbia, eastern Canada, Greenland, Iceland, New Zealand, and Norway. Puget Sound is a series of fjord estuaries in the U.S. state of Washington. Like fjords found in Alaska and Scandinavia, the fjord estuaries of Puget Sound are very deep, very cold, and very narrow. Unlike many of those fjords, Puget Sound's fjord estuaries also have inflows from local rivers and streams. Many of these streams are seasonal, and fjord estuaries remain mostly salty.

Puget Sound in Washington State, USA

Freshwater Estuaries Some estuaries are not located near oceans. These freshwater estuaries are created when a river flows into a freshwater lake. Although freshwater estuaries are not brackish, the chemical composition of lake and river water is distinct. River water is warmer and less dense than lake water. The mixing of the two freshwater systems contributes to lake turnover—the mixing of the waters of a lake. Freshwater estuaries are not affected by tides, but large bodies of water do experience predictable standing waves called seiches. Seiches, sometimes nicknamed sloshes, rhythmically move back and forth across a lake. The Great Lakes, in the United States and Canada, experience seiches and have many freshwater estuaries.

Freshwater estuaries are a unique ecosystem. They form as a freshwater river or creek empties into a large lake. The different chemicals and temperatures of the river and lake waters create the estuary. Old Woman Creek National Estuarine Research Reserve, near Huron, Ohio, is the only reserve that monitors a freshwater estuary: where Old Woman Creek empties into Lake Erie.

Features of estuaries Water continually circulates into 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 creates 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 creates 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.

Features of estuaries 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.

River Nith Estuary, SW Scotland

Rio de la Plata Estuary, Argentina and Uruguay

River Amazon Estuary, Brazil