An estuary is a type of ecosystem found when a river flows into the ocean. The fresh water of the river combines with the salt water of the ocean, creating a condition described as brackish. This unique environment is home to all sorts of marine life. There are several different ways to measure the size of an estuary, including the length of the estuary itself, the length of the shoreline, or the area of the estuary. It could also be measured by the watershed, which is the larger land area that collects the water that flows into the rivers and streams. Taking these metrics into account, the following list includes 10 of the world’s largest estuaries.
10. San Francisco Bay – California
The largest estuary on the West Coast of North America, the San Francisco Bay covers approximately 1,600 square miles — larger than the state of Rhode Island. The estuary system actually consists of four small bays. Two major rivers, the Sacramento and the San Joaquin, flow into the estuary. They come together near Sacramento, forming a delta region that continues through the Bay Area into the Pacific Ocean. Overall, the San Francisco Bay has a watershed area of more than 75,000 square miles. Many endangered wildlife species make their home in the bay’s ecosystem, and it is an important area for trade and agriculture.
9. Yenisey Gulf – Russia
The Yenisey is one of the longest rivers in Russia and Asia, flowing more than 2,000 miles through Siberia into the Kara Sea, an extension of the Arctic Ocean. It discharges roughly 160 cubic miles of water into the ocean each year. The river’s watershed (about 966,000 square miles) has a sparse human population and few mammal or bird species because of the cold climate. However, the waters of the Yenisey are a fertile source of fish, including sturgeon, and beluga whales live in the Yenisey Gulf. The estuary is a part of the Great Arctic Nature Reserve, the largest nature reserve in Eurasia.
8. Amazon River Estuary – South America
If the size of an estuary is judged by the amount of water discharged into the ocean, no other estuary on Earth can compete with the Amazon River estuary. Roughly one-fifth of the world’s river discharge flows from the Amazon into the Atlantic Ocean. That equates to a staggering 219,000 cubic meters per second. You can be more than 100 miles out to sea and still find fresh water from the Amazon. The ecosystem surrounding the Amazon is divided into four separate bioregions, including mangrove forests, rainforest, coastal islands, and a long strip of coastline. As is the case throughout the Amazon region, pollution and deforestation threaten the ecosystem.
7. Río de la Plata – Argentina and Uruguay
Another of the world’s largest estuaries also feeds into the Atlantic Ocean from the South American continent. The Río de la Plata is formed by two rivers, the Uruguay and the Paraná. They join and form into this funnel-shaped estuary, which is about 180 miles long. The estuary also serves as the border between Uruguay and Argentina. The cities of Montevideo and Buenos Aires both lie along the estuary and serve as major ports for the southern part of the continent. A large portion of both nations’ populations live within the watershed area, while the Reserva Ecologica Costanera Sur is a protected wildlife reserve.
6. Chesapeake Bay – Eastern U.S.
The National Ocean Service lists the Chesapeake Bay as the largest estuary in the United States. Twelve rivers flow into the bay, and its watershed covers 64,000 square miles in six different states and the District of Columbia. The estuary is about 200 miles long, but it has a shoreline of 11,684 miles. The Chesapeake Bay is an area of enormous biodiversity — more than 300 types of migratory birds and more than 250 species of fish can be spotted in the region. In addition to being an important area for fishing and shipping, the Chesapeake Bay is also a vibrant tourist area, with about 9 million visiting the Maryland area of the bay each year.
5. Cook Inlet – Alaska
While Chesapeake Bay is recognized as the largest estuary in the U.S., there are some experts who argue that there is a much larger competitor for the title farther north. Alaska’s Cook Inlet has an area that fluctuates between 7,307 and 7,930 square miles, depending on the tides, significantly larger than Chesapeake’s 4,480 square miles. However, the Cook Inlet does have a smaller watershed area, bordering Lake Clark National Park and the Kenai National Wildlife Refuge. The inlet is a key source of fish for the region’s fisheries industry.
4. Bay of Bengal – South Asia
Thirty-nine rivers flow into the Bay of Bengal, making it the largest bay in the world and one of the planet’s largest estuaries, too. Among the rivers that end here are the Ganges, the Irrawaddy, and the Brahmaputra. The Bay of Bengal covers an area of approximately 839,000 square miles in the countries of Bangladesh, India, Indonesia, Myanmar, and Sri Lanka. The estuary creates a thriving ecosystem that includes the extensive Sundarban Mangrove Forest, a UNESCO World Heritage Site and the largest mangrove forest in the world, home to the endangered Bengal tiger. For centuries, the region has been an important shipping hub, and that role continues today.
3. St. Lawrence River Estuary – Canada
Beginning at the eastern end of Lake Ontario and flowing to the Atlantic Ocean, the St. Lawrence River is the main connection between the Great Lakes and the coast. The river’s estuary begins near Quebec’s Ile d’Orleans and extends roughly 150 miles into the Cabot Strait. The estuary’s watershed spans both the U.S. and Canada. In New York, many rivers and streams from the Adirondacks flow into the St. Lawrence. This has long been an important international trade route, with large amounts of iron and steel moving along its waters.
2. Gulf of Ob – Russia
The second Russian estuary on the list is the Gulf of Ob. Like the Yenisey River, this inlet is located in Siberia on the Kara Sea. It is also considered the world’s longest estuary, extending for around 500 miles. Since the 16th century, the gulf has been a hub for trade with Europe. In the 20th century, natural gas, oil, and hydrocarbon reserves were found in the area, greatly increasing the region’s economic importance.
1. Hudson Bay – Canada
While some might argue that Hudson Bay in Canada is not an estuary but an inland sea, NASA describes it as “an important cold-water estuary ecosystem of the Arctic Ocean and the Atlantic Ocean.” It is also the largest bay in the world, based on a shoreline length of 7,623 miles, according to the Guinness Book of World Records. The Hudson Bay’s watershed is the largest in Canada and covers five provinces, plus the Northwest Territories and Nunavut. A number of rivers flow into the bay, which in turn flows into two oceans via the Hudson Strait and the Foxe Channel.
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