Estuaries
0Loading crowd target
0Loading crowd target
7correct to beat the crowd
40 free Estuaries trivia questions with answers — science & nature quiz, new questions added Oct 2026.
An estuary is where a river runs out of land and the sea pushes back. The two kinds of water meet, mix and move with every tide, and the result is some of the most productive habitat on Earth, along with the sites of many of the world's great port cities. This quiz covers the science and the geography: how estuaries form and how they are classified, why their water behaves the way it does, the plants and animals built for a place that changes by the hour, and the famous river mouths of North America, Europe, Asia and beyond. It suits earth science and geography students revising coastal landforms, teachers after a ready-made review, and anyone who likes knowing why a river can flow backwards. Every answer comes with a short explanation.
30 of 40 questions with answers and explanations. Play the quiz
Q 01Which bay, bordered by Maryland and Virginia, is the largest estuary in the United States?
Chesapeake Bay
Chesapeake Bay formed about 10,000 years ago, when rising seas flooded the valley of the Susquehanna River.
Q 02Which predator of tropical estuaries from India to northern Australia is the largest living reptile on Earth?
Saltwater crocodile
The saltwater crocodile is also called the estuarine crocodile, or informally the saltie.
Q 03Which ocean predator has swum up the Mississippi as far as Illinois and also lives in Lake Nicaragua?
Bull shark
The bull shark can thrive in both salt and fresh water, so it hunts in estuaries and the lower reaches of rivers.
Q 04Which New York river, called the 'river that flows two ways' by the Mohicans, is tidal as far north as Troy?
Hudson
The Hudson is an estuary for its whole lower half, so its current changes direction with the tides far inland from New York City.
Q 05Which 'living fossil' comes ashore to spawn on Delaware Bay beaches in greater numbers than anywhere else on Earth?
Horseshoe crab
The horseshoe crab lays so many eggs there that migrating red knots stop to refuel on them after their long journey.
Q 06Estuaries are often called the 'what' of the sea because so many fish and wildlife spawn in their sheltered waters?
Nurseries
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.
Q 07Which California bay is a rare tectonic estuary, formed by crustal movement along the San Andreas Fault system?
San Francisco Bay
San Francisco Bay formed when the land shifted and the sea flooded the lower reaches of the Sacramento and San Joaquin rivers.
Q 08Which shellfish once filtered the entire volume of America's largest estuary every three to four days?
Oysters
Oysters are filter feeders, and a single one can clean up to 50 gallons of water a day in ideal conditions.
Q 09On which South American river do surfers ride the pororoca, a roaring wave that travels far inland from the sea?
Amazon
The Amazon's pororoca may take its name from a Tupi phrase meaning 'great roar', and its waves can reach 4 metres high.
Q 10What word, from a Middle Dutch root, describes estuary water that is saltier than a river but fresher than the sea?
Brackish
Brackish water is found most widely in estuaries, where the salt level shifts every day with the tides.
Q 11Which fish hatch in rivers, pass through estuaries to grow up at sea, then return upstream to spawn?
Salmon
Salmon of the Pacific include the Chinook, coho, chum, pink and sockeye, all of them famous for the round trip.
Q 12Which fish live in rivers and estuaries for years, then swim to the Sargasso Sea to spawn?
Eels
Eels from Europe face a journey of about 6,500 km to reach their spawning grounds in the Atlantic.
Q 13Which type of estuary, common in Norway and Alaska, is a steep-walled valley carved by glaciers and flooded by the sea?
Fjord
A fjord is usually shallowest at its mouth, where rock bars or glacial debris form a sill that restricts the flow of water.
Q 21Which English river, whose estuary is home to thousands of seals, has also been found to shelter colonies of seahorses?
Thames
The Thames has also turned up tope and starry smooth-hounds, even though it flows through the heart of London.
Q 22Which river's mouth, where about 2,000 ships have been wrecked since 1792, is known as the 'Graveyard of the Pacific'?
Columbia
The Columbia runs over a system of bars and shoals at its mouth, one of the most dangerous bar crossings in the world.
Q 23What name is given to oxygen-starved waters, such as those in the northern Gulf of Mexico, that marine life flees?
Dead zone
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.
Q 14Which landform builds up where a river drops its sediment on reaching the sea?
Delta
A delta got its name because the mouth of the Nile resembles the triangular capital form of the Greek letter delta.
Q 15What name is given to shallow water separated from the open sea by a barrier island, reef or narrow peninsula?
Lagoon
A lagoon and an estuary can be one and the same: many shallow bays behind barrier islands are classified as both.
Q 16Which salt-tolerant trees, propped above the mud on stilt-like roots, line sheltered estuaries across the tropics?
Mangroves
Mangroves grow mainly between latitudes 30° N and 30° S, and the red mangrove takes in air through pores in its bark.
Q 17Why does river water entering an estuary tend to float on top of the seawater moving in beneath it?
It is less dense
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.
Q 18Most coastal plain estuaries in North America formed when rising seas drowned river valleys at the end of what?
The last ice age
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.
Q 19Which European capital stands where the Tagus meets the Atlantic, its estuary crossed by the Vasco da Gama Bridge?
Lisbon
Lisbon's Vasco da Gama Bridge is the longest in the European Union, built to span the wide mouth of the Tagus.
Q 20Which glacier-carved estuary in Washington state has Seattle, Tacoma and Olympia on its shores?
Puget Sound
Puget Sound is the second-largest estuary in the United States, scoured by a glacier lobe about 3,000 feet thick near Seattle.
Q 24Which river, the outflow of the Great Lakes, ends in one of the deepest and largest estuaries in the world?
St. Lawrence
The St. Lawrence estuary is protected in part by the Saguenay–St. Lawrence Marine Park, known for its endangered whales.
Q 25Which estuary between Argentina and Uruguay, up to 220 km across, is often called the widest river in the world?
Río de la Plata
The Río de la Plata means 'river of silver', and its sediment turns the water brown as far out as Montevideo.
Q 26What name is given to the wave that rushes upriver when an incoming tide funnels into a narrowing estuary?
Tidal bore
A tidal bore reverses the direction of the river's current as it passes, and several are ridden by inland surfers.
Q 27Which funnel-shaped inlet between South West England and South Wales has a tidal range of roughly 50 feet?
Severn Estuary
The Severn Estuary's tides are so large that it sits at the centre of British debates over tidal energy schemes.
Q 28Which grassy coastal wetland, flooded and drained by the tides, fringes estuaries in temperate and high latitudes?
Salt marsh
A salt marsh is dominated by thick stands of salt-tolerant grasses, herbs and low shrubs that give small animals a refuge.
Q 29Which Scots word for a wide estuary comes before 'of Forth', 'of Clyde' and 'of Tay' on maps of Scotland?
Firth
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.
Q 30Which nutrient from farm runoff and sewage is most often the lead cause of algal overgrowth in temperate estuaries?
Nitrogen
Nitrogen overload comes easily because estuaries are naturally rich in nutrients already: land runoff feeds them even without farms or sewage.