40 Fun Facts About Estuaries

Which bay, bordered by Maryland and Virginia, is the largest estuary in the United States?

Chesapeake Bay formed about 10,000 years ago, when rising seas flooded the valley of the Susquehanna River.

Which predator of tropical estuaries from India to northern Australia is the largest living reptile on Earth?

The saltwater crocodile is also called the estuarine crocodile, or informally the saltie.

Which ocean predator has swum up the Mississippi as far as Illinois and also lives in Lake Nicaragua?

The bull shark can thrive in both salt and fresh water, so it hunts in estuaries and the lower reaches of rivers.

Which New York river, called the 'river that flows two ways' by the Mohicans, is tidal as far north as Troy?

The Hudson is an estuary for its whole lower half, so its current changes direction with the tides far inland from New York City.

Which 'living fossil' comes ashore to spawn on Delaware Bay beaches in greater numbers than anywhere else on Earth?

The horseshoe crab lays so many eggs there that migrating red knots stop to refuel on them after their long journey.

Estuaries are often called the 'what' of the sea because so many fish and wildlife spawn in their sheltered waters?

Nurseries of the sea is the nickname, and it fits: estuaries make more organic matter each year than same-sized areas of forest or farmland.

Which California bay is a rare tectonic estuary, formed by crustal movement along the San Andreas Fault system?

San Francisco Bay formed when the land shifted and the sea flooded the lower reaches of the Sacramento and San Joaquin rivers.

Which shellfish once filtered the entire volume of America's largest estuary every three to four days?

Oysters are filter feeders, and a single one can clean up to 50 gallons of water a day in ideal conditions.

On which South American river do surfers ride the pororoca, a roaring wave that travels far inland from the sea?

The Amazon's pororoca may take its name from a Tupi phrase meaning 'great roar', and its waves can reach 4 metres high.

What word, from a Middle Dutch root, describes estuary water that is saltier than a river but fresher than the sea?

Brackish water is found most widely in estuaries, where the salt level shifts every day with the tides.

Which fish hatch in rivers, pass through estuaries to grow up at sea, then return upstream to spawn?

Salmon of the Pacific include the Chinook, coho, chum, pink and sockeye, all of them famous for the round trip.

Which fish live in rivers and estuaries for years, then swim to the Sargasso Sea to spawn?

Eels from Europe face a journey of about 6,500 km to reach their spawning grounds in the Atlantic.

Which type of estuary, common in Norway and Alaska, is a steep-walled valley carved by glaciers and flooded by the sea?

A fjord is usually shallowest at its mouth, where rock bars or glacial debris form a sill that restricts the flow of water.

Which landform builds up where a river drops its sediment on reaching the sea?

A delta got its name because the mouth of the Nile resembles the triangular capital form of the Greek letter delta.

What name is given to shallow water separated from the open sea by a barrier island, reef or narrow peninsula?

A lagoon and an estuary can be one and the same: many shallow bays behind barrier islands are classified as both.

Which salt-tolerant trees, propped above the mud on stilt-like roots, line sheltered estuaries across the tropics?

Mangroves grow mainly between latitudes 30° N and 30° S, and the red mangrove takes in air through pores in its bark.

Why does river water entering an estuary tend to float on top of the seawater moving in beneath it?

It is less dense because it carries far less dissolved salt, so the two layers can stay apart until tides and wind stir them together.

Most coastal plain estuaries in North America formed when rising seas drowned river valleys at the end of what?

The last ice age gave way 10,000 to 18,000 years ago, when melting ice raised the sea and flooded the lower ends of river valleys.

Which European capital stands where the Tagus meets the Atlantic, its estuary crossed by the Vasco da Gama Bridge?

Lisbon's Vasco da Gama Bridge is the longest in the European Union, built to span the wide mouth of the Tagus.

Which glacier-carved estuary in Washington state has Seattle, Tacoma and Olympia on its shores?

Puget Sound is the second-largest estuary in the United States, scoured by a glacier lobe about 3,000 feet thick near Seattle.

Which English river, whose estuary is home to thousands of seals, has also been found to shelter colonies of seahorses?

The Thames has also turned up tope and starry smooth-hounds, even though it flows through the heart of London.

Which river's mouth, where about 2,000 ships have been wrecked since 1792, is known as the 'Graveyard of the Pacific'?

The Columbia runs over a system of bars and shoals at its mouth, one of the most dangerous bar crossings in the world.

What name is given to oxygen-starved waters, such as those in the northern Gulf of Mexico, that marine life flees?

A dead zone is most often blamed on chemical fertilizers, which wash off farmland and feed blooms of algae that use up the oxygen as they rot.

Which river, the outflow of the Great Lakes, ends in one of the deepest and largest estuaries in the world?

The St. Lawrence estuary is protected in part by the Saguenay–St. Lawrence Marine Park, known for its endangered whales.

Which estuary between Argentina and Uruguay, up to 220 km across, is often called the widest river in the world?

The Río de la Plata means 'river of silver', and its sediment turns the water brown as far out as Montevideo.

What name is given to the wave that rushes upriver when an incoming tide funnels into a narrowing estuary?

A tidal bore reverses the direction of the river's current as it passes, and several are ridden by inland surfers.

Which funnel-shaped inlet between South West England and South Wales has a tidal range of roughly 50 feet?

The Severn Estuary's tides are so large that it sits at the centre of British debates over tidal energy schemes.

Which grassy coastal wetland, flooded and drained by the tides, fringes estuaries in temperate and high latitudes?

A salt marsh is dominated by thick stands of salt-tolerant grasses, herbs and low shrubs that give small animals a refuge.

Which Scots word for a wide estuary comes before 'of Forth', 'of Clyde' and 'of Tay' on maps of Scotland?

Firth comes from an Old Norse word for a sea inlet, so it shares an ancient Germanic root with Scandinavian words for the same thing.

Which nutrient from farm runoff and sewage is most often the lead cause of algal overgrowth in temperate estuaries?

Nitrogen overload comes easily because estuaries are naturally rich in nutrients already: land runoff feeds them even without farms or sewage.

Which rooted underwater plants form wide 'meadows' that feed and shelter marine animals in shallow estuaries and bays?

Seagrasses evolved from land plants that moved back into the ocean 70 to 100 million years ago, and their meadows rank among Earth's most productive ecosystems.

Which process makes 'inverse' estuaries, found in dry climates, grow saltier than the open sea?

Evaporation removes far more water than rivers bring in, as happens in South Australia's Spencer Gulf and Senegal's Saloum River.

Which curved estuary inlet of the Bosphorus separates the historic centre of Istanbul from the rest of the city?

The Golden Horn has sheltered Greek, Roman, Byzantine and Ottoman trading ships for thousands of years.

In which Gulf Coast bay does a 'jubilee' send shrimp, flounder and other seafood swarming into the shallows?

Mobile Bay is the only place where a jubilee is documented to recur regularly, and each one draws crowds after easy seafood.

What name is given to a drowned river valley that remains open to the sea?

A ria takes its name from ría, a word from Spain related to río, 'river', and Sydney Harbour is a famous example.

What term describes fish such as striped bass and sturgeon that live in the sea but enter fresh water to spawn?

Anadromous fish also include the sea lamprey, while species that make the opposite journey are called catadromous.

What name do oceanographers give the layer in an estuary or ocean where saltiness changes sharply with depth?

Halocline combines the Greek words for 'salt' and 'to slope', and the layer forms where river water or meltwater meets the salty ocean.

What name is given to the layer of seawater lying beneath a fast-flowing river where ocean tides are weak?

A salt wedge forms beneath the Mississippi, with a sharp boundary between the fresh water on top and the seawater below.

Which Chinese river is famed for the 'Silver Dragon', a wave up to 9 metres high that surges in from Hangzhou Bay?

The Qiantang wave moves at up to 40 km/h, and people who stray too close are killed each year despite warning signs along the shore.

Which estuary, formed where the Garonne and Dordogne rivers meet below Bordeaux, is the largest in western Europe?

The Gironde covers around 635 square kilometres and is navigable, though it is often referred to as a river.